ATDEPARTMENT OF HEALTH AND HUMAN SERVICES
FOOD AND DRUG ADMINISTRATION
CENTER FOR FOOD SAFETY AND APPLIED NUTRITION
DIETARY SUPPLEMENT SUBCOMMITTEE MEETING
A DISCUSSION OF THE TERM "METABOLITE"
AS CONTAINED IN THE STATUTORY DEFINITION
OF A DIETARY INGREDIENT
Tuesday, March 25, 2003
This transcript has not been edited or corrected, but appears as received from the commercial transcribing service. Accordingly the Food and Drug Administration makes no representation as to its accuracy.
1000 Baltimore Avenue
College Park, Maryland
Johanna Dwyer, D.Sc., R.D., Chair
Constance J. Hardy, FDA
Nancy M. Childs, Ph.D.
Harihara M. Mehendale, Ph.D.
Michael Wayne Shannon, M.D., M.P.H.
TEMPORARY VOTING MEMBERS:
Eric P. Brass, M.D., Ph.D.
Paul L. Schiff, Jr., Ph.D.
Edward Blonz, Ph.D., M.S., C.N.S., F.A.C.N.
NON-VOTING ACTING INDUSTRY REPRESENTATIVE:
Annette Dickinson, Ph.D.
C O N T E N T S
Regulatory Perspective and Challenges,
Christine Taylor, Ph.D. 4
Robert J. Moore, Ph.D. 16
Administration Issues, Constance J. Hardy 22
Remarks by Chairperson, Johanna Dwyer, D.Sc., R.D. 33
Biochemical Facts Behind the Definition and Properties of Metabolites, Edward D. Harris, Ph.D. 38
Questions and Discussion by Subcommittee Members
on Dr. Edward Harris' Presentation 65
Philip Harvey, Ph.D. 111
Steven Dentali, Ph.D. 122
of Charge and Questions by the Subcommittee 130
P R O C E E D I N G S
DR. TAYLOR: Good morning and welcome to FDA's Food Advisory Subcommittee for Dietary Supplements. This is the first meeting of this particular subcommittee. I do want to extend my personal appreciation to the members of the committee. They are not only experts of renown, but they come from quite far away from us. Thank you for agreeing to participate in this particular subcommittee.
The subcommittee is, as we have mentioned, under the auspices of the Food and Drug Administration, particularly the Office of Nutritional Products, Labeling and Dietary Supplements.
My welcome is not only on my behalf, Christine Taylor--I am the Director of the Office--but I also am extending a welcome for Mr. Joseph Levitt, who is the Center Director. He ordinarily does attend the first meeting of every subcommittee. Today, he was unable to attend so I bring his apologies but also his best wishes for a very good meeting.
Basically, the session agenda starts this morning with the overview, which is inclusive of some advisory logistics as well as an overview put together both by Mr. Levitt and myself, what he calls "the dietary supplement state of play." Again because this is the first meeting of the subcommittee, we will spend just a few moments on that.
We will then put up the charge to the subcommittee for this particular meeting and, from there, we will move to a presentation from one of the FDA staff on the regulatory process which will consider again those charges. We will then do an official handoff to your Chairperson, Dr. Johanna Dwyer. From there, she will introduce the members or you can introduce yourselves, however Johanna prefers to do it. There will be a "white paper" presentation on the topic today and then you will move to your discussion and your recommendations.
Just a little bit of a reason for the existence, now, of subcommittees. Many of you may remember that for many years the Center for Food Safety and Applied Nutrition had a food advisory committee. That one food advisory committee handled all of the issues within the Center for Food Safety and Applied Nutrition.
A few things have happened in the last couple of years that have made us look at a way of enhancing that particular process. One is that there is clearly an increased need for scientific input on the part of the Center. What is envisioned is a chance for the agency to make the possible regulatory decisions based on scientific input and one way to get that kind of scientific input is through advisory subcommittees.
But also over the last couple of years there has been an increased need for a notion that a more broad food advisory committee could revive that kind of input for a variety or topics that became increasingly difficult. So, several years ago the agency began to work to develop subcommittees of the Food Advisory Committee and there are now six official subcommittees. There is a contaminants and toxicology one. There is a biotechnology one. There will be one for infant formula. There is now one for food additives. There will be one for nutrition and this is the sixth subcommittee, dietary supplements.
From our perspective, the establishment of these subcommittees is extremely useful because you will have the chance to develop what I call a standing relationship. That is, the folks who are members of the subcommittee will come together perhaps once, twice or three times a year, depending on the topics at hand, and begin to develop a relationship with the regulatory issues that they are providing scientific advice for.
There also will an effort to stagger rotations on these subcommittees so that it is not as if a whole new committee meets each and every time, but people have various lengths of their terms and, therefore, people are integrated in and then rotated out.
The general theme for these kinds of advisory committees is that the focus is on scientific input. Regulatory matters, as you would expect are retained by the agency and what we are seeking is scientific input that can better inform our regulatory decisions. It is very important that the FDA staff present here in the room do not influence or lead the discussion. We do kick it off. We do present charges to you, but then we are available at a side table to clarify questions but not to participate in your discussion per se. We are always available to try and answer your questions but we don't want to influence or lead you in any way.
We do have several FDA scientists present and I will introduce them at this point in time. We have Dr. Susan Walker, who is the Acting Director of our Division of Dietary Supplement Programs. She is here, on the end. Very shortly Dr. Beth Yetley will be joining us. She is the Center's lead scientist for nutrition. Then, we have available Dr. Robert Moore, who is the Branch Chief of one of our organizations within the Dietary Supplement Division. As I have mentioned before, I am Christine Taylor and I am the Director of the Office of Nutritional Products, Labeling and Dietary Supplements.
We also have an FDA Executive Secretary in the form of Connie Hardy, who is sitting with the Chair to help with the logistics of the meeting. I do believe our ethics representative, Mary Anne Kelley, might be coming today. We do have an ethics representative with us today.
I will just spend a minute or two talking about our perspective of the dietary supplement state of play. In our parlance, legislation that happened in 1994 is really relatively new legislation. At the time DSHEA occurred there were a number of studies within the agency that looked at implementing legislation, and the rule of thumb was that it takes almost ten to twenty years before a new piece, a major piece of legislation really becomes a comfortable vehicle for the agency to operate in.
I think it is fair with DSHEA to characterize it as a primarily post-marketing set of provisions, and what anyone can tell you at FDA is that a post-marketing paradigm is not easier than a pre-marketing paradigm. There is a great deal of work in either paradigm and it is important to recognize that DSHEA was a switch for the agency and, therefore, it is taking us some time to better understand kind of what the boxes are that we operate in. The government does bear a considerable burden in a post-marketing world and has a considerable amount of responsibility. As you can probably tell, this is leading up to the need for an advisory committee.
Because the statute happened in 1994, was passed by Congress in 1994, we are in the stage of interpreting and implementing the statute. Clearly, the notion of what Congress intended by the statute is foremost on our plate. When the statute was enacted all legislative history, that is, the background discussion of the committees, was struck from the record so we don't have the advantage of legislative history to help clarify some of the phrases and the meanings of the provisions in that Act.
No matter what kind of statute you are dealing with, you do rely heavily on precedent actions and case law, and that is why it is a ten- to twenty-year process to fully implement the statute. We are in that process now.
The feeling that Mr. Levitt wanted us to sound today is that building a foundation for implementation is slow work. From his perspective, he is having to muster resources. DSHEA did not come with resources and, therefore, he is working to put in place resources to implement the statute. Then, there is a whole series of policy questions in terms of what do you address as precedent issues. Where do you want to put your efforts in order to better clarify the intention of the statute? That is highlighted in yellow, here. An important role is clarifying the science that drives that implementation. That is one of the purposes of having the advisory committee, to help with that particular set of issues.
In many ways, some of the plans that were put in place three or four years ago, as you would expect, have been overtaken by other issues and pressures, and dietary supplements are in a state of flux at the moment in terms of implementing the statute, interpreting the statute and defining it.
One important evolving issue is the notion of fraudulent claims. For those of you who have followed FDA's activities in the area of the Dietary Supplement Health and Education Act, given the resource limitations, much of the effort, the focus on the limited resources was what Mr. Levitt termed safety first. He will be perfectly frank about his lack of resources to deal with the issue of fraudulent claims.
This really did change last December when our new Commissioner put in place an initiative. He entitled it "Consumer Health Information for Better Health" and one of the aspects of that initiative was to enhance enforcement against false or misleading claims or otherwise unapproved drug claims. So, you will now see FDA in a much more active mode in terms of not only safety but this whole issue of fraudulent claims and labels on dietary supplements.
As I look to the future, I am sure that there will be some need for guidance on substantiating these claims clearly if you expect the claims to be truthful and not misleading. The agency does have some responsibility to clarify what that basis is. So, I do see us in the future seeking some input.
The evolving safety standard is the whole question of significant and unreasonable risk. The statute is clear that dietary ingredients can be removed from the market if they present significant and unreasonable risk. The big question is how do you determine when something has reached the threshold of a significant and unreasonable risk? What is the standard? What is the role of adverse events or sentinel events versus a cause and effect relationship?
Some of you may have noticed recently that in the Federal Register there were some provisions for some warning statements, as well as a series of I think very important questions. The agency is seeking comment on whether evidence of significant concerns, short of a definitive demonstration of a risk profile, is sufficient to declare significant and unreasonable risk. So, again, the agency is going to have to look for clarification and definition on the standard and I think again, we will be seeking input.
A third one is the whole issue of manufacturing standards that are known as GMPs, good manufacturing standards. Less than two weeks ago the agency published a proposed rule for dietary supplement GMPs which did focus on the quality and purity of the products. The idea here is to ensure that the product is what it says it is and contains amounts of what it is supposed to contain and that manufacturers of ingredients know what they are putting in their particular products.
This particular proposal has a phenomenal amount of technical interpretive aspects, as well as questions about appropriateness. This will be an important process for the agency as it works to a final rule and, again, I think we will be seeking input.
Finally, where we are today--an evolving issue of definitions. Let me tell you that a plan for dietary supplements was put in place in the year 2000. Quite a bit of space was given to the concept that you need to define boundaries for dietary supplements. Congress gave us words as to what dietary supplements are but, as I think you will see from your discussions today, a lot of that evolves around what do these words mean. As I have mentioned, we don't have legislative history which is important for clearly having a good handle on what these definitions are. But it is very important that these definitions be grounded in sound science and that is one of the purposes of having an advisory committee.
Part of today's overall umbrella though is what is the definition of a dietary supplement. So among other things today, you will find out that a dietary supplement is defined as a metabolite of a vitamin, mineral, amino acid, herb or other botanical, or dietary substance for use by man to supplement the diet, and the word that is the issue today is metabolite. We are asking consideration of the meaning and the application of that word, metabolite. What substances are or are not metabolites of other substances is the key question.
In a few moments you will get the regulatory context from Dr. Moore, who will then revisit the notion of the charges to the committee, but I will just highlight them now before I turn over the microphone to Dr. Moore.
There are basically three charges to the committee for your discussion today. The first is, is it possible to identify particular scientific criteria, principles or conventions that enable a determination to be made about when a substance is or is not a metabolite of another dietary ingredient? Again, Dr. Moore will review this.
Second, consider and discuss the scientific strengths and weaknesses of the following concepts with respect to their usefulness in identifying whether a substance is or is not a metabolite of another dietary ingredient. As you will see, there are four components that we have asked you to consider and, again, these will be reviewed in some detail.
The third charge to the subcommittee is to discuss the scientific validity and likely usefulness for identifying when a substance is or is not a metabolite of another dietary ingredient. If so, what characteristics associated with the criterion makes it valid or useful?
Those are basically FDA's charges or questions to the advisory subcommittee, and we very much look forward to your discussion and appreciate the chance to hear from you. Dr. Moore? I know at one point we are going to turn it over to the exec. sec. but I think right now we will move right into Dr. Moore's presentation and then turn it over to Connie.
DR. MOORE: Thank you. What I am going to cover is just briefly, you know, a brief summary of the legal framework that applies.
As Chris has already said, in 1994 the law was amended and among other requirements that DSHEA added was a definition for dietary supplements that really forms the basic element of the statute from which everything else flows.
In general, in implementing any provision the issues that are raised fall into two categories, the nice, simple, straightforward ones, for example with DSHEA that required us to withdraw an advanced notice of proposed notice of rule-making that doesn't require a great deal of thought or insight. Then, there are more complicated provisions that ask us to interpret an ambiguous phrase within the statute in a way that fairly and even-handedly applies that provision to the marketplace.
Today's purpose is one of those ambiguous phrases in the statute. It is important to recognize that what we are considering is not so much defining the term metabolite; we are looking for principles or conventions that allow us to determine when something that may be a metabolite is a metabolite of something, of another substance. I will walk through the definition that is in the statute and will make it more clear why that sort of parsing of the phrase is important.
I don't know if you heard that in the back, but Dr. Dwyer was mentioning that there is sort of a one-page summary of my discussion somewhere in that book.
One of the things that DSHEA did was define what dietary supplements are. The statutory definition defines them as a product, other than tobacco, intended to supplement the diet that contains one or more "dietary ingredients." It then went on to define the universe of substances that would be eligible to be used as dietary ingredients. It defined dietary ingredients as a vitamin, a mineral, an amino acid, an herb or other botanical, a dietary substance for use by man to increase total dietary intake. Then, in the last provision, it included a category that was a metabolite, a constituent, a concentrate, an extract or a combination of any preceding dietary ingredient and that brings us to the point we are at today.
As Dr. Taylor mentioned, the statement of agreement that constitutes the legislative history for DSHEA doesn't directly address the issue of metabolite. It is not defined elsewhere in the statute and the plain language is simply ambiguous. Clearly, it envisions not just any metabolite because it redirects to metabolites related to other substances.
So the question, is in the face of ambiguity, of no legislative history and the lack of a definition in the statute, how does one go about interpreting and adding meaning to this phrase? The basic question then is because only metabolites that are metabolites, if you will, of something previously named in the statute, what we have to do to decide which substances may or may not be lawfully used in ingredients is find out when is a substance or is not a substance a metabolite of something else.
The reason that is important, just to reemphasize what I just said, is that the statute doesn't say any metabolite necessarily is a dietary ingredient but only those substances that are metabolites of something else that has been previously named.
In trying to reach a starting point to parse out a definition when the legislative history is silent, one generally looks to the plain language of the statute. Again, the term metabolite is ambiguous so one looks to the ordinary meaning of the term that one finds in authoritative references, in dictionaries and in other types of texts, and what one comes up with is a series of potential definitions of a metabolite, but all sort of condensed down as largely a substance that is consumed or produced in metabolism.
That sort of general plain meaning of the term then leads you to the conclusion that anything is a metabolite and yet, as I have mentioned, it doesn't address the basic question of when or if a particular substance is a metabolite of some other substance and that is the basic question that we need to consider.
"Anything goes" is meaningless. The agency has an obligation, under general rules of statutory construction, to interpret portions of the Act in a way that gives them meaning and so to interpret metabolite in a way that renders the phrase meaningless or reads it out of the Act simply isn't an option available to us.
More importantly, if it is undefined and is undefined in such a general or meaningless way, it provides no basis or guidance to firms trying to make good faith efforts to decide whether a novel substance may or may not be a dietary ingredient. So, this adds certainty to the marketplace that is obviously a necessary requirement for firms that are trying to make business decisions for the long-term.
If one looks at is there even a need, and I am not going to go through any of these in any detail because that is why Dr. Harris is here, but there are a series of things in the marketplace that when one looks at them, it raises or begs the question as to are they metabolites of some other dietary ingredient. Some of these examples are many of the hormones and bioactive molecules, some of which may be simple one- or two-reaction steps from the parent compound; others may reside at the end of very multi-level branched metabolic pathways.
The question is from the time we start the cholesterol or arachidonic acid and reach steroid hormone or prostaglandin if there are twenty steps in there, is there a way to tell how one approaches that and say this is or isn't a metabolite of all of the things that preceded it? In a very general way that is setting the stage, hopefully, for what Dr. Harris will address.
So, the basic question that we come back to is that the issue at hand isn't defining metabolite per se. What it is, is looking at how a scientist looks at a substance and reaches a conclusion that it is or is not a metabolite of some substance that may have preceded it in that metabolic pathway.
Real briefly since Dr. Taylor mentioned it, what we are really looking for is not so much to reach a conclusion necessarily today, but to explore the thought process and the principles, the scientific principles that biochemists and metabolic physiologists, and the like, would apply to reaching these types of conclusions and then weigh the usefulness and the merits or the arguments against relying on one or more of these principles, and how useful they are in the context of what we are required to do in the statutory framework to decide these things are or are not metabolites of some other substance that preceded them in the pathway. I will leave it at that.
DR. TAYLOR: Thank you, Bob. As I mentioned, we will be available to answer clarifying questions from the committee but at this point we do need to begin our hand-off to the chair of the committee, and it begins with some input from the executive secretary on how the meeting is managed logistically as one aspect, and there are also some ethical and procedural concerns that Ms. Connie Hardy will now present to the committee. At that point we will turn it over to Johanna Dwyer who will introduce the committee and begin the discussion. Thanks.
MS. HARDY: Good morning. I am Constance Hardy, the Executive Secretary of the Dietary Supplement Subcommittee of the Food Advisory Committee. I want to welcome everybody and take a few minutes to go over a few procedural rules in terms of advisory committee operations since this is the first meeting of Dietary Supplement Subcommittee.
Subsequent to that, I will read into the record the conflict of interest statement. Each of the subcommittee members should have received a copy of the committee member guide to the FDA advisory committees and a video. The video's title is "A Panel Member's Responsibility." There are copies of the member guide available at the registration desk for anyone who may be interested. The committee member guide is in need of updating but, by and large, provides a good operational overview.
As a scientific regulatory agency, FDA needs access to highly qualified expert external advisors who can provide scientific and technical advice. Thus, FDA uses advisory committees to supplement the agency's internal expertise to help the agency staff stay current with state-of-the-art technology. Committee members participate in an advisory capacity and final decisions are ultimately made by agency officials.
With respect to the committee members, the chair of the committee presides at and conducts the meeting. She may ask FDA staff for clarification at any time during the meeting. Each committee member contributes their unique scientific and technical background and education experience to the committee process. The standing committee members generally are voting members, including a consumer representative who is voting and an industry representative who is non-voting, as specified in the charter. All members of advisory committees are appointed by the Commissioner. Voting members are appointed as special government employees following a conflict of interest review.
Conflict of interest reviews are also conducted immediately prior to each meeting to determine whether a voting member might have a conflict based on the specific topic to be addressed at a particular meeting.
Other advisory committee participants may include guest speakers. A guest speaker will present pertinent information to the committee and will not participate in the committee discussions or vote. However, we do request that in the interest of fairness, guest speakers report any financial interest that may be affected by the committee's discussions.
Additionally, the advisory committee process encourages public interaction with the agency in arriving at decisions. Not only is there a consumer representative, in this case a voting member, appointed to serve on the committee, but the public is invited to appear before the committee during the open public comment period.
FDA participates in a listening mode and is available to respond to questions and provide clarification if such service is requested by the chair. FDA relies on its advisory committees to provide the best possible scientific advice available to assist us in making complex decisions. Our goal is to do this in as open and transparent manner as possible. Part of that openness carries with it a request for members to try to avoid giving the appearance that issues are being decided or conclusions are being reached outside the actual meeting.
We understand that issues raised during the meeting may well lead to conversations over breaks or during a meal. In fact, we hope the discussions are thought provoking. We have had instances where members have come back from a break and said, "well, you know, we were talking in the break and we would like to request that FDA provide us some additional information so we can better understand thus-and-such." This is a perfectly acceptable thing that can occur. What we don't want is to have a situation where, after a break, members come back and say, "well, we were talking in the break and we decided that the answer to question one is..." From our perspective, that would be particularly troublesome because neither the agency nor the public would have had the benefit of listening to the entire discussion, the questions raised, the responses, etc.
In fact, FDA has recently adopted a policy that only procedural matters can be decided by a show of hands. For example, all comments must be placed on the record, each being attributed to the member making the statement. The policy goes even further. If a member has to leave a meeting early, the member waives the right to vote. You may wonder why a person loses their right to vote and the answer is fairly simple. FDA believes all parts of the meeting and the discussions are important. Consequently, voting on issues without having the benefit of all the discussions would be premature.
The issue of openness is larger than what transpires during the course of a meeting. I would like to call your attention to the section in the member's guide entitled, "member interaction before, during and after a meeting." In essence, this section underscores the fact that all communication with the members should be routed through the committee's executive secretary. No one, not even the FDA staff with the exception of the executive secretary, should be contacting members about upcoming meetings, topics, etc. The same guidance applies to consultations between members prior to a meeting. If a member receives an inappropriate contact, the member should feel free to notify the executive secretary and/or refer the person making the contact to the executive secretary. All contacts should be routed through the executive secretary so that we can minimize any situation that could be misinterpreted.
Appearance issues are always difficult because, as true of many things, appearances can be deceiving. We ask that all members, guest speakers and everyone attending the meeting be mindful of how an interaction between a committee member and anyone might be perceived.
Please let me be clear. It is not my intention to question anyone's motives or integrity but, as you can imagine, we do not want to see respected individuals becoming objects of negative attention based on misperception and we wouldn't want anyone in this room to become such a target. I am confident that everyone here recognizes these issues and can appreciate that my comments are intended as a general reminder.
Moving on to the issue of conflict of interest, the following announcement addresses that issue with respect to this meeting, and is made part of the record to preclude even the appearance of such at this meeting.
By the authority granted under the Food Advisory Committee Charter of July, 2002, the following individuals have been appointed by Joseph A. Levitt, Director of the Center for Food Safety and Applied Nutrition, as temporary voting members for the March 25th meeting on the term metabolite as contained in the statutory definition of a dietary ingredient: Edward Blonz, Ph.D.; Eric Brass, M.D., Ph.D.; Paul L. Schiff, Jr., Ph.D.
The issues to be discussed at this meeting are issues of broad applicability. Unlike issues in which a particular sponsor's product is discussed, the matters on today's agenda do not have a unique or distinct impact on any particular product or manufacturer but, rather, have widespread applications with respect to all dietary supplement ingredient products and/or manufacturers. To preclude even the appearance of a conflict of interest, the subcommittee participants have been screened for interests in dietary supplement manufacturers or dietary supplement suppliers.
As a result of this review, in accordance with 18 U.S.C., Section 208(b)(3), Dr. Johanna Dwyer, Dr. Eric Brass, Dr. Nancy Childs and Dr. Harihara Mehendale have been each granted a particular matter of general applicability waiver that permits each of them to participate fully in the matters at issue.
With respect to screening our guest speaker, Dr. Ed Harris, he has reported that he has no financial interest related to today's meeting topic. Dr. Harris is employed by Texas A&M University.
A copy of the waiver statements for each of these individuals can be obtained by submitting a written requirement to the agency's Freedom of Information Office, room 12A-30 of the Parklawn Building. We would also like to note for the record that Dr. Annette Dickinson is participating in this meeting as the acting industry representative and a non-voting member of the committee.
In the event that discussions involve any other issues not already on the agenda for which FDA participants have a financial interest, the participants are aware of the need to exclude themselves from such involvement and their exclusion will be noted for the record. With respect to all other meeting participants, and this will be our public commentors, we ask in the interest of fairness that they address any current or previous financial involvement with any firm that is involved with dietary ingredients or dietary supplements.
As this meeting is being transcribed, I would request that you please use your microphone when speaking and that you clearly identify yourself for the record.
At this point we will have a three minute presentation by Dr. Catherine McComus who is here doing a survey on FDA advisory committees.
MS. MCCOMUS: Good morning. My name is Catherine McComus and I am a faculty member at the University of Maryland. I am conducting a study with collaborators at the FDA on public understanding of the procedures that the FDA uses to monitor and manage real or potential conflicts of interest of its advisory committee members.
So, I am responsible for the surveys that are on your chairs, and for the AC members I have a special survey that I will be distributing to you also. Your participation is voluntary but, of course, it is going to be appreciated because it will help us to accurately portray what people perceive and understand about the conflict of interest procedures that the FDA uses.
This survey takes about 15 minutes to fill out. It is anonymous. If you complete it today, there will be a box on the registration desk and you can just drop it in there. Otherwise, there is a postage-paid envelope inside and you can drop it in the mail as soon as you can once you leave this meeting.
Again, we really appreciate your assistance in helping us to understand what the public understands about the conflict of interest procedures. It is not for the FDA people to fill out at this time. I will be contacting them for another aspect. I will be around today, as well as my research assistance, to answer any questions or provide any sort of background on this questionnaire and research. Again, anybody is welcome to have a copy of the results. There is a letter with my contact information inside. Feel free to contact me separately and, again, thank you very much for your time.
DR. BRASS: Can you clarify who is the sponsor?
MS. MCCOMUS: Yes, it is being sponsored by the Joint Institute for Food Safety and Applied Nutrition, which is a collaborative research effort between the University of Maryland and the FDA. So, CFSAN actually provides some of the funding for this.
DR. DWYER: You are aware that this is the first meeting?
MS. MCCOMUS: Yes, I am.
DR. DWYER: Fine, as long as you are aware.
MS. MCCOMUS: And I appreciate it whether you have gone to one meeting or fifty, I appreciate your feedback; your perceptions.
DR. DWYER: And you have IRB approval?
MS. MCCOMUS: Yes, this has gone through the University of Maryland Institutional Review Board.
DR. DWYER: Just checking!
MS. MCCOMUS: Definitely so. Thank you very much for your time.
MS. HARDY: That should clear up all the administrative matters at this point and I am going to turn the meeting over to Dr. Johanna Dwyer.
DR. TAYLOR: I would like to begin just by introducing Dr. Dwyer. She is the chair of this particular subcommittee. And, to the extent that we need to discuss what just occurred, it was unknown to me it was occurring, Johanna, so I will be glad to talk to you about that. Dr. Dwyer, the meeting is now yours and you are free to introduce your members and begin the discussion.
Remarks by Chairperson Johanna Dwyer
DR. DWYER: Thank you. Welcome to everybody on the committee. It has taken two years to get this together but it is nice to finally be here. Welcome to everyone who has come all the way out to the far reaches of Maryland for this meeting.
What I would like to do is just tell you that I am a professor at Tufts. I have just come back from a year, about a year and a half, over at USCA where I was on IPA, interagency personnel acquisition, working a fair amount of time there and also at Tufts. I was the assistant administrator for human nutrition at the U.S. Department of Agriculture and have been back at Tufts since the first of the year.
I am a clinical nutritionist and I do a lot of work as well with databases that are very much involved in some of the emerging components, trying to characterize where they are in food and developing databases.
Why don't we just go around the table and people can say why they are here. Annette, would you start, please?
DR. DICKINSON: I am Annette Dickinson. I am president of the Council for Responsible Nutrition, which is a trade association of dietary supplement manufacturers and, as was mentioned earlier, I am the acting industry representative on this subcommittee.
DR. SHANNON: I am Michael Shannon. I am a pediatrician and a toxicologist at Children's Hospital Boston and Harvard Medical School.
DR. SCHIFF: I am Paul Schiff. I am a professor of Pharmaceutical Sciences in the School of Pharmacy, University of Pittsburgh.
DR. BLONZ: My name is Ed Blonz. I am a nutrition scientist. Currently I am the consumer representative on the committee. I am a newspaper columnist as well. I answer consumer questions about nutrition, foods, food science, health, dietary supplements, interpreting science and answering questions correctly for the consumers. This is a syndicated column and I work at home. I have been doing this for twelve years, and I am really pleased to be here.
MS. HARDY: Connie Hardy, the executive secretary. DR. MEHENDALE: Harihara Mehendale, from the University of Louisiana, School of Pharmacy. I am interested in toxicology. I am interested in mixture of toxicology, diet, caloric restriction and toxicities and mechanisms of toxicities in general.
DR. CHILDS: I am Nancy Childs, professor in the Department of Food Marketing, St. Joseph's University.
DR. BRASS: I am Eric Brass, from the Department of Medicine at Harbor-UCLA Medical School.
DR. DWYER: Thank you. Yes, Dr. Dickinson?
DR. DICKINSON: Would you accept one or two comments before Dr. Harris begins his presentation?
DR. DWYER: Yes, I think we have time.
DR. DICKINSON: I thank the FDA representatives for a thorough summary of the definitional issues here, but I would just like to add the notion that there are other aspects, of course, of DSHEA that limit what ingredients can be used in dietary supplements.
I have read Dr. Harris' excellent presentation that was distributed earlier, and I just would like to make a comment so that everybody doesn't assume that everything that is listed as part of the definition or any metabolite is necessarily eligible to be sold as a dietary supplement. I am sure we will get into these other aspects later, but there are requirements about whether the ingredient was grandfathered, which means that it was on the market as a dietary supplement prior to 1994.
There are also provisions that require that if it was not grandfathered, there are certain safety provisions that have to be observed. In most cases this takes the form of a 75-day notice that a manufacturer must submit to FDA providing information as to why the ingredient would reasonably be expected to be safe. In addition, if the ingredient--and some of the examples that Dr. Moore and Dr. Taylor mentioned earlier are clearly pharmaceutical ingredients, that is, the end metabolite in some cases is a pharmaceutical ingredient--there is a provision in DSHEA that that prevents a pharmaceutical ingredient from being marketed as a dietary supplement.
So, there are a number of safeguards, that I am sure we will come to, that limit the scope to some extent of the marketing of ingredients even if they are covered by the definition.
DR. DWYER: Thank you, Annette. Are there any other comments or questions from the committee from the presentation we have heard so far?
I have one, Dr. Moore. It will take a while so you may want to think about it. On one of your slides you mentioned examples and mentioned the number of steps, and I wondered if later on you could give us a rough idea of the steps from cholesterol to steroid hormones, arachidonic acid to prostaglandins, amino acids to catecholamines and there was one more. I just don't have those pathways in my brain and I would like to get whether it is two or 25, what the order of magnitude is.
Biochemical Facts Behind the Definition and
Properties of Metabolites
DR. HARRIS: Good morning to the members of the subcommittee. First let me say I am very happy to be here, and work with you, and help you in arriving at a definition of what in biochemistry is a very common word that we use almost on a daily basis, and that is a metabolite. I say it is a common word, implicating that it is almost invisible to us, much the same as numbers are to a mathematician or words to a grammarian. We simply use the term and never really think about what its implications are.
So, in addressing this assignment I really had to try to step back, keeping my profile as a biochemist and, yet, try to arrive at a definition of what is and what is not a metabolite. In my presentation I hope to bring to you some historical perspective and some examples that will, hopefully, help you see how these general principles have evolved.
I found this quotation, which I found extremely interesting primarily for the date, and that was 1839. This is a quotation that was basically derived right after the realization that life was a process that occurred within cells. We are looking at a quotation by Theodor Schwann where he said, "one must first consider the combination of molecules to form a cell and secondly those which result in chemical change in the cell itself." So, this is the first recognition, you might say, that we are dealing with a process carried out by cells that are involved in chemical changes.
When we think of a metabolite we have to think in a larger context. Certainly, it is an element of metabolism but what do we mean when we say the word metabolism? That, again, is a syllabus for a historical search here and, as you can see, we go down the list here and we can see that the word metabolism was first introduced back around the turn of the nineteenth century. A German, Johann Reil, represented it as something that said that matter is changing or stoffwechsel which is changing components. The French referred to this as le changement du materiel and that has again given us the implication that chemistry is evolving in cells and in living systems. Actually, the word metabolism, which implies something happening after the fact in a changing context, was introduced by Sir Michael Foster in a Textbook of Physiology for the first time.
So, if we are looking at the historical perspective here, we certainly want to be caught up with today's interpretation. Obviously, our earlier ancestors and pioneers in science did not have the opportunity to see the things that we simply know today. Things happen inside cells certainly but what are these things? Can we define them in a more precise manner?
Let's take a look at what textbook markers in biochemistry have to say. I think this will sort of fortify some of the definitions that you have seen in the pamphlet that really came from dictionaries. Here we can see a little more in well-renowned textbooks on biochemistry. In this quotation, metabolites are the basis of metabolism. I will read a quotation from Bohinski who wrote a very authoritative book on biochemistry: "metabolic expression is due to the integration of individual reactions into a dynamic reaction circuitry of intricate design controlled by sensitive regulatory checks and balances." Wow! What does he mean?
Well, he provides us with a picture. When I show this to my class in Texas A&M to show them what metabolism is all about, half of the class decides they want to be finance majors and the other half want to know if it is going to be on the exam.
What is a metabolite? Well, if we look at some of the definitions we see it is a chemical intermediate in the enzyme-catalyzed reactions of metabolism. That was by Lehninger who is probably one of the most authoritative authors in biochemistry.
Here is a more recent quote by Voet, Voet and Pratt: "a metabolite is a reactant, intermediate, or product of a metabolic reaction." Well, they seem to be saying the same thing. Here is a quotation I put in my book: "a molecule considered an intermediate in a series of reactions generally those in a pathway."
From each of these definitions we can probably glean some very key words to help us reach a definition. For instance, enzyme-catalyzed--that means a metabolite has to be recognized by an enzyme. It is a reactant or an intermediate or a product. It means a series of reactions. Finally, a series of reactions that we would consider and define as a pathway. As I mentioned to you, a pathway is simply a convenient way of putting some order and focus on different things that are happening inside the cells.
Let me give you some definitions that follow from this. Metabolites are products of metabolic events occurring within cells. Metabolites are substrates, products and intermediates in defined biochemical pathways; and metabolites arise by the action of enzymes acting on defined precursors. If we want to put this in more simplistic terms, let's just say that a metabolite is something that is made by the body, used by the body, recognized by the body and is essential to body functions and, of course, it is benign in its many ways.
Here is a principle that I am going to elaborate a little bit later at the end of my talk, but I want you, in arriving at this definition, to see it from many different perspectives. The more perspectives I can introduce to you, perhaps the more you can get a feeling of what this is all about. So, let me give you this thought, that only through chemical change can a molecule yield energy to a cell or provide material that will be used for cell structure. Such molecules that are in the act of changing chemically going out to something more global in cell function, these are what we call metabolites.
Let's address the question of when something is and when it is not to be considered a metabolite. These are some factors for you to consider. Most obvious, to be considered a metabolite an unknown must have some semblance of structure to a naturally occurring reference compound. That is obvious; that is intuitive. We can all see the relationship to that.
But we have to be careful. I have to point that out to you because I am going to add a few more statements to that. I am going to say it must also be a substrate, product, or intermediate of a defined biochemical pathway in the cells. It must arise from another metabolite, which itself is a product of cellular activity. It must enter into enzyme-catalyzed reactions. Finally, it must be both a product and a substrate of successive reactions. I will point out to you why I feel that is important in the definition. The last point I want to leave you with is that a metabolite has to undergo metabolic turnover. It cannot accumulate in a cell.
So, if those are the definitions of what we would say would define an acceptable realization of a metabolite, let's address the question of what is not a metabolite and see the other perspective. Obviously, a metabolite is a chemical that is not recognized by the cell or has no connection with the living system; or any substance that is not derived in the biochemical pathway; any substance that is not a product of a substrate within a biochemical pathway; or any molecule that cannot be acted upon by an enzyme; any substance that interacts unfavorably with a living system; and any substance that is unable to undergo metabolic turnover. These, I would tend to feel, would be more excluded from being considered as metabolites.
I am going to address the question of metabolism itself. I am going to bring to your attention that metabolism occurs inside cells. I have stressed the importance of why metabolism requires enzymes and enzymes are only found inside cells. Enzymes are functioning in altering chemical structures. These are cellular events.
Then, I want to just bring something to your attention here just to show you what I am getting at. Here we have a very well-known cycle that occurs in physiology of biochemistry. It is called the Cori cycle. What it represents is an interplay between the liver and the muscle. It is an interplay because it shows an interdependence of these two organs on one another. Liver, we know, can make glucose. It can make it from smaller precursors. It can pour the glucose into the blood and provide the muscle, therefore, we a source of nourishment for energy. Glucose. The muscle will take the glucose once it has received it. It will metabolize it and create lactate or lactic acid. Lactic acid then is released back into the blood where it now again travels back to the liver only to be reformed into glucose. This is a Cori cycle, so defined in about 1935 or so, whose authors won the Nobel Prize for recognition of this.
What does this have to do with metabolism and metabolites? Well, we can see, if we look at this from a more careful perspective, that all the chemical changes we are talking about, for instance the making of the glucose all the way through to these intermediates here, and all of these intermediates are metabolites, derived from preceding intermediates, pouring into the blood. In the blood we see pretty much a reversal of the trend, starting with glucose-6-phosphate and going back up to lactate and so forth. So, we get a little better perspective of just how this Cori cycle is operating and just where the role of metabolites would be in this context.
I am going to show you one more complicated slide here and it gets a little more involved with biochemical pathways, but it basically is addressing the same issues that I pointed out to you previously. We are looking at what happens to a substance once it penetrates the cells and become eligible to be considered a metabolite.
Here we see glucose enter the cell and become glucose-6-phosphate. That is a biochemical intersection because once it has formed glucose-6-phosphate it has many different fates ahead depending upon whichever enzyme-determined need the cells has for it. For instance, if the cell needs to store the glucose it alters the pathway in that direction. If the cell needs energy it will open a pathway down to pyruvate. If the cell needs to make nucleic acids it will open the pathway that way to DNA and RNA. So, you can see metabolites go every which way, every which direction at once. What a biochemist generally refers to when we look at these opportunities for multiple directionality, a biochemist calls this carbon flux, flow. If a cell needs energy the flow is down toward pyruvate and into the mitochondria. If the cell doesn't need energy and needs to store the compound the flow is toward the storage and so forth. My point in pointing this out to you is that all these components that get us from point A to point B, all of these components are metabolites.
Here is another picture of that. Here we can see that we are talking about directionality. You basically have two different enzymes that are involved here in controlling blood glucose. One enzyme takes glucose into the cell and converts it into glucose-6-phosphate. The other enzyme actually takes the glucose-6-phosphate and brings it back to glucose for release. Here I have put in just a few other little things connecting pathways here. You can see L-alanine, an amino acid, flowing to pyruvate, either down to carbon dioxide and water or lactate can flow into pyruvate and then, of course, it can go back the other way as we saw previously.
This is what I mean when I say a defined biochemical pathway. We know precisely every step in that pathway; we know every intermediate, and every intermediate that is part of that pathway we call a metabolite.
Let's look at this in a more focused way. Let's say we have three metabolites that are in order, A, B, C, and for the moment let's just focus on component B, here. To properly define B we would say that it is an intermediate in the pathway from A to C. We would know that there are enzymes that are involved in the conversion of A and B and enzymes involved in the conversion of B to C. But if we think in terms of B, relative to B we would call A a precursor metabolite and we would call C a product metabolite.
The point here is that B is a product of enzyme A and B is a substrate of the starting component or enzyme 1--enzyme 1; enzyme 2. So, you see, in that definition we can see that we formed B from A and gave rise to C from B. That is a metabolic sequence. We would very easily define B as a metabolite.
Let's look at the front of the picture now. Again, we have A, B and C. Under these conditions we see that enzyme 1 again catalyzes formation to B; enzyme 2 to C. But in this case A has no precursor. So, we could consider A something that has just entered the cell or has been in the cell for a while and is now about to be chemically converted. A, therefore, is a substrate of enzyme B and, in this regard, A is a substrate, B is a product and A would be a precursor.
Here is my take-home for you. In a biochemical pathway a metabolite is typically the product of one reaction and the substrate of a second successive reaction.
Take-home, a compound that has no precursor can still be considered a metabolite but only if ensuing metabolic changes occur via a known series of reactions. That is important because we could make compound B, let's say, through some event and then stop at that point. So, would that say then that A is a metabolite? We would say no because it is not a successive series of events.
I am going to give you some examples here, and I am going to draw some from some arbitrary definitions here. Please don't try to connect it with something that you already know, but basically it is to illustrate some of the points that I am making here.
Let's talk about compound X. Let's call it an artificial sweetener. Let's say that X has the unusual property, favorable really, of not yielding calories, which makes it a very attractive sweetener. Compound X has the basic formula of sucrose with the exception that in that structure it contains three chlorine atoms. The question is, is compound X to be considered a metabolite? Well, if compound X is to be considered a metabolite, we know that as mentioned from the previous talk it has to have a precursor and it has to be converted into a product. All right, so we need two enzymes to do that. Reminding you again of that basic principle, to be considered a metabolite, a compound must be the product of one enzyme and the substrate of a second.
So, let's look at what we would consider favorable in calling compound X a metabolite. First of all, it resembles sucrose structurally, and has biological function that emulates sucrose. So, in this regard it appears to have the functionality and the structure that we would desire if we limited our definition to those two parameters. But let's look at the non-favorable factor here. The non-favorable is that because it does not generate any calories compound X is not metabolized. Compound X, therefore, is not being treated as a sucrose in the system.
The other thing we would call to your attention is that compound X has chlorine atoms in its structure. If we look back into biochemical pathways and biochemical events that occur in all different cells, we will see that the ability of the cell to put chloride onto a sucrose molecule does not exist by any known biochemical mechanism, any known biochemical system. Therefore, compound X cannot be considered a metabolite in that regard. It is not a product of a metabolic event. I put enzymes very strongly into your perspective here. I want to make sure you are very clear on this. This is really what is going to determine or put real boundaries around what we would call a metabolite and what is not a metabolite.
As I said, there are substrates for enzymes that cross over products. Let me make sure you understand what I am referring to. Here we have a chemical reaction. Like all chemical reactions that occur in a cell, this reaction requires an enzyme so let's bring that into the picture. Let's say that in this chemical reaction our goal is to add this component to this structure, here. Let's call this structure glucose; let's call that phosphate. Let's say our goal is to make glucose-6-phosphate. Here is an enzyme that does that. Now, the enzyme has catalytic sites that you see here. They are designed to interact with the substrate and in the process of doing that they form what we call an enzyme-substrate complex. This is an intermediate. This can only be formed when a substrate has found a proper recognition site on the surface of the enzyme. Once that has occurred, we can now bring in the second substrate, carry out a catalytic event and produce a product.
After that has taken place, the enzyme, once it has freed the product, is now a free enzyme, capable of recycling back into the system again for another go-around. So, here we can see how enzymes typically work in the living system.
Here is a caveat that we have to bring to our attention. Enzymes in many respects are stereospecific. What do I mean by that? It simply means that if the substrate is going to engage that enzyme, it has to do so in a stereospecific manner, stereo meaning in spatial relationship to the structure of the enzyme. If you will, a key and lock type fit here tends to occur. So, there has to be proper orientation.
Stereospecificity can also be very subtle. For instance, your left hand is different from your right hand and no matter how hard you try you can't superimpose your right hand on your left hand. This is the principle of handedness that we talk about. We find that when we are dealing with molecules that have four different constituents attached to a central carbon. We call these stereoisomers, isomer implicating that they are similar in structure but, yet, have slightly different properties. For purposes of designation we call one a D-isomer and one an L-isomer. What I want to convince you of is that these are not the same molecule. You might look at them and say, oh, they are the same but let me make sure that you see what the enzyme is seeing.
Here we have a molecule. Let's make this a duplicate image, and that would look like this. So, you would agree with me that these are now identical molecules. But suppose I put a mirror up here and I brought this other molecule so it now looks in the mirror and it sees itself. The question that we would ask is, is this molecule and the one in the mirror the same? Let's see if we can answer that question by putting the molecule into the mirror. You can see that the yellow matches up with the blue; the blue matches up with the yellow--it doesn't fit. So, the mirror image is not the same molecule.
How does this apply a little more realistically to what we can consider? Let's talk about D- and L-glucose. Here is D-glucose. Here is a mirror. Here is L-glucose, the mirror image. If you look closely, it does look like this molecule is looking at itself in the mirror. All right, if we look at some of the parameters, chemical parameters, we would see that they both have the same chemical formula. They both have the same chemical properties. They both have the same molecular weight. Yet, the molecule on the left is a nutrient and the molecule on the right is not a nutrient. On that basis we can call this molecule a metabolite and this molecule, even though it has all the same parameters, except for this handedness here, we would have to say that it is not a metabolite.
Here is our take-home for that, stereospecificity must be observed for those metabolites that engage enzymes that have the capacity to select specific stereoisomers. The bottom line here is you just can't put your left shoe on your right foot.
Dr. Moore alluded to the point that metabolism can be an event where as a molecule it is away from its parent compound loses its visibility. It loses its identity to its parent structure. This is a very true principle in biochemical. The further down the metabolic pathway you go, the more your molecule becomes mixed in with the cell, and mixed in with metabolites from many other different sources. So, I would give you a little rubric here to remember and just say that metabolism is an explosive process. When we see a metabolic event taking place, we see the molecule scattering every which way. You might say, well, in that context does that actually occur? The answer is yes.
Let's look at a fatty acid for instance. This is a fatty acid that is about to undergo metabolism. You can see it has eight carbons to its chain. Well, during the course of metabolism, that gorgeous, beautiful looking, well-structured, long chain fatty acid is going to be clipped in three different positions. The explosion comes when the molecule completely dissipates, disappears from sight and becomes four acetyl-CoA units. These acetyl-CoA units could be derived from amino acids. These acetyl-CoA units could be derived from carbohydrates. They don't have to be fats. So, we can see that these molecules would just blend into the general maven of the cell, what I would call a mixed pool.
Let's examine this idea that there may be structural similarity which is the real criteria that we need to observe at the top of our list. I am going to show you a molecule of cholesterol. You can see this is a very complicated molecule. It contains a series of fused rings of different sizes. This molecule contains 27 carbons. It is assembled from those acetyl-CoA units that you saw previously in a series of well-defined--and I can assure you cholesterol metabolism is very well understood in biochemical--well-defined metabolic steps.
If we wanted to put the molecule of cholesterol together, let's go ahead and build it. What we find, as we build the molecule is that we build it on five carbon units. As we continue to clip these units together, keeping a linearity in all cases, you can see that we start to bring into perspective the cholesterol structure. As you see here, we have six isoprene units that brought that molecule about. This open change isoprene type molecule we call squalene, which is a precursor of cholesterol.
If we took the next metabolic steps, that would be to close these rings. So let's close them. When we close them we no longer have squalene. We now have the molecule that is getting closer to cholesterol. We are pretty much there, almost at the point where we want to be.
Let me call your attention to an article that appeared in this March issue of Journal of Nutrition. It concerns a compound which we call lycopene. Lycopene, I think you know, is the red coloring matter of the tomato. Here you can see a paper that is pointing out that lycopene product, tomato product enhances lycopene concentrations and it activates the capacity of an oxidant. So, it reacts against reactive oxygen species, a very important role for these type of molecules with that structure.
So, here is the question I raise to you, is lycopene a metabolite? Let's take a look at the structure. What do we see when we look at the structure of lycopene? It basically has the same 5-carbon unit we saw previously that we used to build the cholesterol molecule but, of course, this is not cholesterol. It is very close in its resemblance to squalene, with the exception that lycopene contains ten more carbons but it bears a very strong resemblance to the open chain cholesterol precursor squalene.
Let's look at this a little more closely. We see that lycopene in the tomato is actually converted into beta carotene, which we all know is vitamin A for us. That brings us to my point which is very important here. We can take beta carotene. We can break the molecule in half and derive two molecules of vitamin A, otherwise known as retinol. But we are unable to take lycopene and converted it into carotene. Wouldn't it be nice if we could do that, but we can't. Therefore, we cannot metabolize lycopene.
So, I would raise the point with you that lycopene should not be considered a metabolite. Even though it has this very interesting structural similarity, it should not be considered because no human enzyme is capable of changing it structurally to be a useful product.
By the way, in preparing these comments I went into a database to look for anything that talked about the metabolism of lycopene and I could not find it. This is a compound that we need to study more carefully to know just exactly how a living system handles this. But, as far as I know, right now there is no metabolic following compound from this through our pathways.
Let me point out some other things regarding metabolites. I think I have given you a very good idea of what it is not. I want to give you a little more insight into the properties of metabolites. I think this is something else that you have to take into your considerations here, particularly when we bring up an issue of safety, though that is not the issue I want to address with you today.
I want to address this issue in the context of how metabolites work. Here we have a pathway of A going to B going to C, in that direction. The reason this pathway goes in this direction--there are a number of reasons why it is favoring going from left to right. First of all, we have to think of energy considerations. If we go from A to C are we releasing free energy? That literally means that this is the spontaneous reaction or one favorable. Well, what happens if we suddenly raise the level of C? We have the potential now of driving the pathway in the reverse direction.
So, this indicates that a metabolite is generally kept in what we call a steady-state concentration. Don't mistake that for equilibrium; it is not. Steady-state simply means that it is being formed and being used at the same rate so there is no build-up of that compound. If it is formed faster than it is being used there will be a build-up and, of course, if it is used faster than it is formed there will be a diminution. So, steady-state just simply implies that we reach a point where we keep that level so we can drive that steady-state reaction back the other way by increasing concentration of C.
Here is a principle I am going to leave you with, because metabolites tend to exist at a constant concentration within a pathway, changes in the steady-state levels are able to control carbon flux. This could determine whether a movement towards synthesis or degradation take precedence.
The other principle I want to leave you with, and this is what I alluded to and didn't expand on but I think you can certainly see the implications here, metabolites have free existence. Metabolites turnover. In the parlance in a metabolic sense, they are constantly being replaced. They are constantly being renewed. So, this is a very important fact because this keeps a cell from building up any one particular metabolite.
Some of you might say, wait a minute, what about storage fat? I don't like that word but that is how it is called, like we have a closet where we stick fats away and forget about it. Actually, if we look at it we would see that what we call storage fat is really steady-state fat. Fat is being stored and it is being broken down. I should say it is being synthesized and it is being broken down. When we shove the direction in the form of lowered synthesis and breakdown, then we start to accumulate fat. If we have just the opposite, like in conditions that favor breakdown, then we have less fat being stored or, favorable to us as we start seeing ourselves lose weight. The principle I would leave you with is this one, that nothing is permanent in a living system.
Let me just summarize some of the points that I have made. I have provided some statements here that will perhaps help you as you consider what we mean. A liberal definition of a metabolite can include practically any substance that occurs in a cell. That seems to be fair game but you can see that there are some restrictions we have to place on that.
There are six principles that can serve as a guide here. I have tried to list them in order of priority. I am kind of imposing on you here and telling you what I think should be the most important consideration but, certainly, you have to decide that.
I would say it is important that we recognize that a metabolite has to be something created by itself. It has to be something that is a product of a biological event. A substance that is an intermediate in a pathway, this comes to play when we start talking about it is not alien to a pathway; it is actually being recognized and being changed within a pathway.
What is bringing about those changes--item number three--it has to be a recognized and acted upon by an enzyme. Item number four, it has to have a limited biological time of existence. As I told you, nothing is permanent. It has to not violate stereospecificity principles. This will not apply to all metabolites but, certainly--certainly, it will apply to many molecules that would appear to be structural like D- and L-glucose, but that have that subtle difference that would make them alien in the system. Finally, it has to have a useful purpose. I would say that that is something that tells us metabolites are serving a cell, keeping the cell alive.
This is the closing thought that I want to leave you with, and it expands on something that I told you a little bit earlier: In a living system, chemical changes never occur suddenly. One of the things that we do in our class is we ask the students how do you know that this piece of bread has this many calories in it? The student says, well, you put it in a bomb calorimeter. He is correct, absolutely. You take that piece of bread, put it in a bomb calorimeter, turn on the electrical current, put some oxygen in there and, wham, we get carbon dioxide and water and we can measure the heat. Imagine if that happened inside a cell. You take a piece of bread and, wham, you get carbon dioxide and water.
The point is that in a living system chemical changes occur gradually and at each step we are either moving forward, making a more complex molecule or making a molecule less complex and deriving energy from that transformation. So, metabolism is a series of well-engineered steps that gradually sap the energy or remodel the features of the molecule and take it in from the environment as nourishment. It is characteristic of a living system to make chemical changes gradually and to generate intermediates that meet other biochemical priorities. It is the residues of these chemical changes--that is what we call metabolites.
Questions and Discussion on Dr. Harris'
DR. DWYER: Thank you, Dr. Harris. That was great. Before we take a break, we will take some questions. Dr. Brass?
DR. BRASS: I have a number of questions so tell me to shut up whenever you want. I would just like to probe some of the underlying thinking behind some of your proposals and principles.
First of all, I listened to the phrase precursor metabolite and that sounds to me like an oxymoron. To me, X is a metabolite of Y. Just because X is a metabolite of Y, why would it not be a metabolite of other molecules? It might be but it might not be. And, why incorporate the precursor implicitly as a metabolite?
DR. HARRIS: I didn't bring that up in my talk but in the paper I did, I would consider a metabolite as something that is about to undergo chemical change. I would say that a precursor that is a metabolite would be like blood glucose. You cannot change glucose in blood. You can only change it once penetrates a cell. There are certain metabolites that you cannot change unless they can enter the organelle or compartment in a cell. For instance, you can't oxidize a fat until it enters the mitochondria and then it becomes a metabolite. But you could, in that same context, say it is a metabolite in the cytosol.
DR. BRASS: I agree with your concept that we are thinking in the context of human metabolites. I think that is a very important point. I agree to that discussion. For example, is an essential amino acid a human metabolite?
DR. HARRIS: If I would see an amino acid in the blood, and there are plenty of course, I don't think I would call those metabolites.
DR. BRASS: In the cell?
DR. HARRIS: In the cell, I would.
DR. BRASS: I think that generates a confusion because I think if it is a metabolite it has to be generated, not only be a precursor. But I understand now.
DR. HARRIS: I am just one step ahead of you, only from the standpoint that what we call a metabolite seems to be a derived substance. Now, if the answer to your question is did the cell make that amino acid, yes. But did that amino acid come from the diet and now is a digested product of a dietary protein, I would say that is not a metabolite.
DR. BRASS: I think that is why we focus on human metabolites--
DR. HARRIS: Yes.
DR. BRASS: --and if we don't we get in a morass quickly. I think if we don't make that distinction to be able to be generated by a human reaction, we open up a large morass. That is why with a precursor I am uncomfortable because of a variety of substances.
The second question is you explicitly, in your definition, exclude end products of metabolic reactions as being metabolites.
DR. HARRIS: Yes.
DR. BRASS: That is not clear to me either. Why, if it is generated clearly in a sequence of enzyme reactions that dead ends and is excreted in the urine without further change, should it not be considered a metabolite of the precursors that generate it?
DR. HARRIS: Again, I am looking at the subsequent step. If something has now been passed into the blood the subsequent step is excretion or some other byproduct. No, I would not say that those are metabolites but I am beginning to see what your point is. As you can see, in my definition I am always looking at the next step. I am telling you that a metabolite has to be recognized by an enzyme and if that substance in the blood, indeed, came from an enzymatic reaction that assembled that component, that is a metabolite, yes. I agree with that. But if it came from the diet--
DR. BRASS: I agree; I agree. Again, this is semantics but the whole discussion is semantics. Yyou said inside of a cell.
DR. HARRIS: Yes.
DR. BRASS: If the enzyme is located on the outside of the extracellular membrane, is that inside the cell?
DR. HARRIS: Now, that is a grey area. I would say that we could taper that down with a definition of recognized in the immediate environment of the cell or in the act of penetrating the cell. But a lot of times going through a cell is nothing more than a transport process.
DR. BRASS: I agree. I agree that it has to be enzymatic transformation but where that enzyme is located appears less critical. I also have a problem with the concept of proximity in a metabolic pathway for the definition of whether or not it is or isn't a metabolite. Let's imagine we have a 26-step pathway where A goes to Z. Y is clearly far away from A but unquestionably A goes to B and B is a metabolite. B goes to C, C is clearly a metabolite. I could extend that logic until I get all the way to Y and clearly Y is a metabolite. Even though it is really far from A, it has to be a metabolite simply by deductive reasoning. So, I think we have to think about proximity for the number of steps as being absolutely intrinsic to this definition.
DR. HARRIS: No, I disagree in some respects because ultimately everything goes into carbon dioxide and water. So, if something becomes carbon dioxide in water, then it is a metabolite, or is it a metabolite of a substance we are looking at?
DR. BRASS: Well, to me, this transforms into utility and application but I think, yes, carbon dioxide is a metabolite of glucose.
DR. HARRIS: Or a metabolite of a hundred--
DR. BRASS: That is correct. This is about defining a metabolite in an absolute sense, not whether or not a specific molecule is a specific metabolite of another specific substance.
DR. HARRIS: Yes, let me back up one second. You are touching on a very important point, I realize that. If compound B or compound C that is formed maintains much of the parent structure so that we could say that those compounds actually came from the substance that we looked at as a reference compound, then we are on good grounds. But if that has gone all the way down to a point where now, as I showed you, it is an explosive process where we cannot identify its origin--well, technically it is a metabolite--
DR. BRASS: I think, for reasons you said, similarity is a trap because there are many molecules that are very subtly different that are biologically worlds apart and other structures that look very different and have very similar functions. To me, the important point is whether or not increasing the precursor results in net synthesis or net increase in flux--
DR. HARRIS: That is a kinetic argument.
DR. BRASS: That is why, for example, glucose can't be considered a metabolite of palmitate even though we label palmitate or bicarbon in glucose. That seems to me a more compelling argument than proximity or similarity.
DR. DWYER: Before you answer, would you repeat what you pointed out about the precursor?
DR. BRASS: Yes, it seems to me that if something has a precursor-metabolite relationship, if I increase the amount of the precursor I have to increase the amount of metabolite and that has to be a net result. The counter example is if I put radioactivity in palmitate I can follow that radioactivity into all kinds of molecules in metabolism that are really not net metabolism. To me, that is the difference between a mish-mash of metabolism and a precursor metabolite reaction that I can actually change.
DR. HARRIS: I agree with you. I didn't want to approach this from an experimental perspective but that is exactly the type of experiment so people could identify potential metabolites. As you pointed out, there are a great many traps here, mine fields that you can fall into. The bottom line is you cannot make glucose from palmitate in a net way but you can still find the carbons there.
DR. DWYER: Ed?
DR. BLONZ: My name is Ed Blonz, and I want to thank you for your presentation. As a biochemist, I also would like to make some comments because here we are talking about dietary supplements and with dietary supplements we have botanical products and botanical products are not necessarily essential to the human body. That was one of your points, for being a metabolite it has to be essential to the human body. As a biochemist dealing with nutritive substances, obviously this would be an exception to that and I wanted your comments on that. Then I have one other question.
DR. HARRIS: I don't know why you would say they are not essential. Obviously beta carotene is a botanical. You can't make that.
DR. BLONZ: There may be some botanicals but would you say that kava-kava is essential to the human body?
DR. HARRIS: I am not sure what kava-kava is.
DR. BLONZ: There will be many dietary supplements that are botanicals that at the present time we could not define as essential.
DR. HARRIS: I suppose that is going to have to come down to what you mean by essential. You are a biochemist so you know the nutritional definition. If the body can't make it to maintain the pace of metabolism, then it becomes essential. If the body can make it, it still can be a metabolite but not necessarily, in that context, essential. Essential means that it can only be supplied by the body.
DR. BLONZ: Well, I think the definition of essential would actually delve into the realm of philosophy with some botanicals. But I personally would tend to think that that might be an exception for the need for a metabolite to be essential in terms of nutritional substance of the human body.
DR. HARRIS: Yes, I almost look at the word essential as indispensable. Yes, I am trying to see if we are talking about two different things in that regard.
DR. BLONZ: The other thing had to do with recognition by an enzyme.
DR. HARRIS: Yes.
DR. BLONZ: There might be many botanicals which will not be recognized by an enzyme and they might leave through the kidney or through some other route--
DR. HARRIS: Those are what I would rule as not being metabolites. I would exclude them. Lycopene was the example that I brought to your attention.
DR. DWYER: Any other questions? Yes, Dr. Schiff? DR. SCHIFF: I think it gets very difficult when you start to compare a vitamin or a single product with a botanical which, in essence, is a mixture of hundreds and hundreds of different compounds. Maybe the structure of all those compounds are known or maybe they are not known. Maybe something about their typical kinetic metabolic fates is known and maybe they are unknown. That is a whole leap in a whole other direction. The variables are incredible.
So, you know, I would thank you for placing perspectives on it but I want to emphasize single products. I also tend to agree with Dr. Brass and a lot of his comments as far as some of the concerns he has. Thank you for asking those. Those were questions that I had. I wouldn't want us to refer to a botanical in the same sense as we refer to a single amino acid or a single vitamin because of the complexity.
DR. BRASS: If I could just follow-up on that. It seems to me, to answer both those questions if it is not further metabolized in the body is not relevant. The metabolite generated from a botanical is a constituent of a substrate or a metabolic enzyme.
DR. SCHIFF: My issue would be is that if there would be a product that would be one step away from an existing botanical, that would be metabolized outside of the human milieu but might be marketed as a supplement under the metabolite definition. If this would be a substance that would not be recognized by an enzyme inside a biochemical system, would it then not be subject to that inclusion?
DR. BRASS: That is why I emphasized the human metabolite because clearly one could find bacteria or other cellular species that can generate all kinds of metabolites that are formed in human metabolism. That is not in any of the regulatory documents but it seems to me that we have to be talking about human generated metabolites in this conversation.
DR. HARRIS: Let me comment on that too. I think that is a very important point. When you talk about a substance being recognized by an enzyme that is not necessarily a natural substrate within that cell, let's say a xenobiotic comes in, we have many instances where that happens, like cytochrome p450. This enzyme is designed to rid the body of substances that it is not naturally producing in its metabolic ways but simply taken in by accident, benzothyrines and things like that from cigarette smoke. These are enzymes that will act on the substance and they will recognize it and cause some kind of a change. That is why I emphasized that the next step, after you make that change, is important. Let's say an enzyme changes a compound to another, if that compound now stops at endpoint, that first compound is not a metabolite.
DR. BLONZ: But the second--
DR. HARRIS: But the second compound would be.
DR. BLONZ: Yes.
DR. DWYER: Dr. Shannon?
DR. SHANNON: I am struggling with the definition that the substance be recognized and acted upon--
DR. HARRIS: Can you say that again?
DR. SHANNON: If you have a definition that a substance must be recognized and acted upon by an enzyme, and it seems to me if my recollection isn't in error, at least some reactions that occur in the body don't require an enzyme and they produce a product. I remember the term non-enzymatic hydrolysis for example. So, I wonder why an enzyme has to be absolute in order to talk about a product as a metabolite of something else.
DR. HARRIS: I think I could probably name the number of non-enzymatic events. I am talking now about chemical change. I am not talking about changing the state of a molecule or maintaining its structure pretty much intact. That would be something like folding a protein. Those could be spontaneous events. But even those, we find, require certain outside agents that affect their properties.
The only non-enzymatic event that I am aware of is cross-linking collagen. Once you form those residues that allow cross-linking to occur, then it becomes a spontaneous event. I am not aware of too many biochemical processes that do not require enzymes. The ones I could name I could put on my first two or three fingers.
DR. SHANNON: But there are some.
DR. HARRIS: I am waiting to hear them.
DR. SHANNON: No, no, that is what you just said. Right?
DR. HARRIS: I am illustrating the points where spontaneity occurs mainly from the standpoint of finished molecules, not molecules that are going to be changed. All right, let me be a bit more specific. When I take the elements of collagen I have two groups that I am going to use to form the Schiff base to link them together. Those groups are there. All I need to do is put those two together and the chemical reaction occurs. But the enzymes prepared me for that step, and the proximity of the molecules lying next to one another rolled the reaction in that direction. So, I have to say that is a non-enzymatic reaction at that last step, but to say collagen cross-linked is non-enzymatic is wrong. Enzymes are involved in all the steps to that point where they actually performed the condensation product.
DR. SHANNON: Explain to me again why non-enzymatic hydrolysis wouldn't be included.
DR. HARRIS: I am trying to think of illustrations or instances where there is non-enzymatic hydrolysis.
DR. DWYER: Glycosylation--
DR. HARRIS: Johanna, I am sorry?
DR. DWYER: Glycosylation of hemoglobin in the blood.
DR. HARRIS: That I really can't comment on. I am not sure. I would have to project that there are glycosyl transferases that put those carbohydrate units on that hemoglobin. Wait a minute, if the hemoglobin is unmodified and you are forming glycosidic bonds, I am sorry, that is not a spontaneous reaction; that is energy dependent. Is that wrong? Okay, I will step aside on that then. Some of you may be much more aware of that particular reaction.
DR. DWYER: Dr. Mehendale?
DR. MEHENDALE: Substances are biotransformed in the gut by bacteria. This is a well-known phenomenon, as you are aware. I didn't hear anything in your presentation that addressed the issues where botanicals might be metabolized by bacterial flora in the gut.
DR. HARRIS: We are talking about microflora here.
DR. MEHENDALE: I don't think there are many individuals in this room who do not carry bacteria in their stomachs.
DR. DWYER: You need to use the mike.
DR. HARRIS: I have to stay at the podium, I am afraid. I understand your question about microflora and definitely they are in the human gut, yes. So, should we consider anything that is metabolized by microflora, which are really bacteria? Should we feel that is fair game? I am going to have to let you decide that.
DR. MEHENDALE: I can also think of endogenous or exogenous compounds that get into what is known as enterohepatic cycle, the enterohepatic recirculation.
DR. HARRIS: Like cholesterol.
DR. MEHENDALE: Like cholesterol.
DR. HARRIS: Bile salts.
DR. MEHENDALE: Therefore, they are very much part of the definition because they will go out of bacterial metabolism in the gut and then in and out of the human cell metabolism.
DR. HARRIS: Yes, but there again those reactions are well defined. What are the bacteria providing other than enzymes to carry out those reactions?
DR. MEHENDALE: One further question I have is, you know, the requirement of enzymatic action--there are compounds that, to me, undergo metabolism and interact with the substance and, therefore, evoke a response. Where would we put this, this kind of substance? Generally we consider protein as enzymes but the substrates may not undergo a change and, therefore, I suspect--
DR. BRASS: If that would be a metabolite of catecholamines, I would say no.
DR. BLONZ: What about final estrogen that would bind to an estrogen receptor?
DR. BRASS: What is the question?
DR. BLONZ: Well, it is not acted upon--
DR. BRASS: But it is not a metabolite. Its chemical structure isn't changed. The example that was posed is, does binding a receptor lead to a chemical reaction where a molecule is changed, whether or not you have a product precursor relationship, that was implicit in my thinking.
DR. DWYER: Dr. Dickinson?
DR. DICKINSON: I would like to go back to your example about lycopene and vitamin A. As I understood it, you said that the reason it wouldn't be a metabolite is that although it may be broken down to beta carotene, it doesn't eventually goes to vitamin A and, therefore, it isn't a metabolite. But there are hundreds of carotenoids that are broken down into various other compounds in the body, and I am troubled by the conclusion that simply because it doesn't go to vitamin A, all those metabolites wouldn't necessarily count as metabolites although they might have functions.
DR. HARRIS: What I was trying to point out when I brought the lycopene up is simply the first step in the metabolism of lycopene is something that we can't do. But a tomato can convert it into beta carotene. I said we could take beta carotene which we would then consider a metabolite because we have enzymes to conduct further metabolism with that compound. What I tried to indicate is that we have no enzymes for lycopene.
DR. DICKINSON: In the case of other carotenoids that might break down into component parts, would those component parts then be metabolites of those carotenoids?
DR. HARRIS: If they themselves could undergo further chemical change through enzyme-catalyzed reactions, yes.
DR. BRASS: But what if they are end products? What if they go through a couple of steps and you get an end product? I think has to be considered a metabolite, the original dietary ingredient. Just like I think that something that is a botanical and is metabolized into p450 and gets hydroxylated, that hydroxylated product is a metabolite of what you gave. I think linking function to the metabolite is fallacious. I mean, the precursor-metabolite relationship is a biological chemistry one, not a functional one. In fact, the metabolite could be a detoxification. Just like your concept in the written document that says it must not represent a threat to homeostatic mechanisms. That is a concentration argument. So, I think trying to evoke all these non-chemical, non-biochemical definitions into what a metabolite is cloud the water rather than clarify it for what is a much more straightforward precursor-product relationship that occurs in the body.
DR. HARRIS: My strongest point that I want to make to you is that we are talking about defined biochemical pathways and if we are talking about intermediates in those pathways, we would be perfectly legitimate.
DR. BRASS: Okay, p450, a non-metabolic pathway. It hydroxylates a botanical precursor to a hydroxylated product that has no function in man.
DR. HARRIS: That is why I emphasized the next step as being important.
DR. BRASS: That is why I emphasized not to count end products because end products are generated by metabolism. Just because B doesn't go any further doesn't mean it didn't come from A.
DR. HARRIS: You are saying that a chemical reaction is a pathway.
DR. BRASS: An enzymatic pathway can occur in one step, a natural product exposed to p450 gets hydroxylated and excreted in the urine. Because a hydroxylated compound was generated by the body from a precursor I don't know how to avoid concluding it is a metabolite. I don't know any theological reason--
--that says it has to be greater than one step. Obviously I am biased because as a pharmacologist this is what we do all the time. We give a foreign substance to the body, most of which are derived from natural products, metabolized by reactions to the body to metabolites. Some of those are active and some of those are inactive but they are clearly metabolites. Again, the implication of that definition, that is somebody else's problem but it seems to me that trying to put arbitrary boundaries on what is or isn't just makes it more confusing.
DR. HARRIS: Well, I can see your argument and I can also see the danger in the argument because if I can take any compound, stick it inside p450 and hit it, it is a metabolite, I don't like that.
DR. BRASS: Well, many of us have problems with parts of the law but that is not our charge. I mean, it is to come up with a working definition of what a metabolite is. Under the law, if that precursor turns out to meet any of the precursor definitions that are in the law and a metabolite may be p450 I feel trapped. I don't like being trapped. I don't understand the negative potential consequences of it but I feel trapped in trying to come up with a definition of a metabolite trying to avoid these traps just leads me into semantic traps.
DR. DWYER: I think it is time for coffee. I just wanted to mention that Dr. Yetley came in while we were in the midst of this. Why don't we take a break and give Dr. Harris a rest and be back here in 15 minutes?
DR. DWYER: Could everybody sit down, please? We are going to start again. There are a couple of issues that emerged as we were metabolizing coffee. The first is, I hate to keep getting you up to the mike but I wondered if you could go back and just give us a little bit perspective on the kava-kava question that came up, the whole issue of dietary ingredients and where metabolite fits in that whole thing?
DR. MOORE: Do you want to wait for a couple of your AWOL committee members? Do you want me to wait until they all get back?
DR. DWYER: Well, while you are waiting could you please tell us where you come from in your terms of your background? What are you? A lawyer?
DR. MOORE: Oh, no.
DR. DWYER: A doctor.
DR. BRASS: That wasn't very nice; what did he do to you!?
DR. MOORE: Before I say no to that, I should ask the lawyers to raise their hands so I will know how to answer that! No, my undergraduate and doctoral training was in nutritional sciences. Then I spent six years and did my postdoc. at Grand Forks, USDA lab, and then spent six years in the Army's Occupational Medicine Research Program. I came here in '94 and had the misfortune or good fortune, depending on your perspective--I was hired right when DSHEA was happening. I have been there ever since.
DR. DWYER: Thanks. Are you a metabolite of DSHEA?
DR. MOORE: The question that Dr. Dwyer asked, to sort of walk through again, is the construct for in the statutory definition for dietary supplement. It is important to recognize that the dietary ingredient clauses is but one of many, many requirements that a product would have to meet before it could be lawfully marketed. The issue of dietary ingredients is really the gatekeeper clause. You don't have to worry about whether it is safe, or is approved as a drug, or it is intended to supplement the diet or anything else because if it isn't statutorily "a dietary ingredient" it can't be a supplement and all those other requirements go away.
Once it becomes a dietary ingredient, then you superimpose on that all of the other requirements, does it meet the safety standards? Is it excluded? Is it thrown out of the box by one or more of these exclusion clauses? If it is a dietary ingredient, is it then a new one that would require a notification or is it eligible for exemption for notification? So the issue of the dietary ingredient definitional construct is simply that it possibly gets you into being a lawful ingredient but it is not the end-all. What that section says is that dietary ingredients are vitamins, minerals, amino acids, herbs or other botanicals, the sort of dietary substance for use by man. In a broad sense, things that are articles of the usual food or drink of man is the way to look at that.
Then, the last one is that it is a constituent, a metabolite, a concentrate, an extract or a combination of any of those others. For example, to use kava which came up earlier, it gets into the dietary ingredient box because it is an herb or botanical. The things in it may get into the box because they are a metabolite of it, a constituent in kava, a concentrate or an extract of it. So, metabolite isn't the only way a particular substance can get into the dietary ingredient box. It is simply one of a myriad possibilities, and something that is a metabolite might also be a constituent where it might fit into some other category but this is simply one of many.
DR. DWYER: Just to follow-up on that, could you tell us, lycopene could also be in the box by other ways, couldn't it?
DR. MOORE: Right. The most straightforward way is it is simply a constituent. You know, herb or other botanical means plants, if you will. So, tomatoes are clearly plants, bioengineered or not. So, lycopene clearly is a constituent of a plant material and that is the simple, straightforward way to get in.
DR. BRASS: Again, as I read your background summary and your list of vitamin, mineral, amino acid and herb, in my construct of precursor-metabolite relationships I view that upper list as the precursor which might be a substrate to generate a metabolite in the final clause.
DR. MOORE: Clearly, that is what the statute envisioned because that is what it says, a metabolite of something aforementioned.
DR. BRASS: Right.
DR. MOORE: So, it can be a metabolite of some ephemeral substance. If that ephemeral substance isn't listed above, than that metabolite isn't a dietary ingredient.
DR. BRASS: Right, but trying to keep my syntax consistent, those would be the precursors and I wouldn't have to argue whether they were metabolites or not. They are already the precursors and not metabolites.
DR. DWYER: So, those are amino acids, vitamins, minerals, herbs, botanicals and what is the last?
DR. MOORE: Dietary substance for use by man to increase the dietary intake. That, like metabolite, is also not explicitly defined or discussed in the statement of agreement. How we have applied that or interpreted it is simply that those dietary substances are those substances that are in the usual food and drink of man. So, that is a fairly broad category.
DR. DWYER: Shall we go around the table? Any questions? Annette?
DR. DICKINSON: No, I thought your clarification earlier that many of these substances that we have been talking about, like lycopene, would be dietary ingredients in and of themselves for other reasons, regardless of the metabolite clauses, is a point that I am glad you made.
DR. MEHENDALE: I have a question.
DR. DWYER: Identify yourself.
DR. MEHENDALE: This is Hari Mehendale. I have a question concerning whether we have some historical perspective. When this law was introduced somebody framed this metabolite. Surely, somebody had input into this and I wonder if there is any information on people who might have been involved at that time who introduced this term metabolite. That might be helpful, if somebody has that information, what was thought at that time when whoever did this introduced this term metabolite.
DR. DWYER: That is a very good question and I will answer what I know and then the experts can answer from what they know. There is no legislative history for this. It was the first thing I asked when I was told the purpose of this business. So, that means whatever was discussed was expunged from the record. What that leaves you with is the analog of a hundred people feeling different parts of the elephant and explaining--what is it?--a hundred blind people trying to explain what the elephant was. So, there doesn't seem to be very much common agreement on what the dialogue was that led to this particular thing. Perhaps there are others who would like comment on it.
DR. MOORE: No, I think you are correct from a purely legal perspective for interpreting and applying the law. While I am sure one could mine the Congressional Record and find what was the Congressional Record, for legal purposes it is struck and can't inform whatever decision we make or consider.
DR. DWYER: Anyone else over here have any comments on that?
DR. BLONZ: This is Ed Blonz. I don't know the exact pathway but let us say we are dealing with a substance such as garlic. Garlic, when it is in its full state has a set of compounds, but when you crush garlic you activate an enzyme system which then breaks down one compound into another compound that would be a metabolite of the naturally occurring compounds in garlic. Now, if we were to take those metabolites of garlic and use those as a supplement, see, we have an enzyme system outside the human body acting on a naturally occurring compound. How would that fit into our definition of a dietary supplement?
DR. MOORE: Probably simply as constituents again because the limited statement of agreement says that physical acts, if you will, on things that are dietary ingredients don't change anything. So, to the extent that garlic, at least in some normal way of use is squashed, that is still the plant or botanical and the substances that are in that are constituents of it. Regardless of how they got there, they are still constituents.
DR. DWYER: Dr. Schiff?
DR. SCHIFF: Paul Schiff. I think the point is well taken but it goes back to the big picture versus many small pictures. So, if a plant has its own set of metabolites that it has produced, you grind it up, the constituents mix, there is some water, there are enzymes present. A glycoside in one cell becomes an A-glycan and a human being ingests ultimately both the glycoside and the A-glycan--they are all metabolites I think.
DR. BRASS: Except I think it is important to think about the separation. In the context of this they are constituents and not metabolites because, to me, if you open the metabolite to non-human fates--
DR. SCHIFF: No, no, pardon me, they are phytometabolites, I agree with you.
DR. DWYER: We will get back to that definition again. There are a couple of other questions, Dr. Moore--you are not off yet. One was Dr. Mehendale's question but maybe you can phrase it better. It is this whole business of the human rumen. Again, it is sort of like the crushed garlic question, but the issue of some of the isoflavonoids, I can't remember which one it is that is converted in the gut by the bacteria and the question is whether the bacteria in the gut are in or out. What is the FDA's official view of that?
DR. MOORE: I don't think we have an official view.
DR. DWYER: This is a first, isn't it?
DR. MOORE: That is one of those areas of ambiguity that aren't readily clear from the plain language of the statute or the statement of agreement. I think that discussion as to where those types of metabolites or transformed substances fit may be part of your discussion that will inform our consideration of that, but we have not outlined a policy about that.
DR. DWYER: Probionics?
DR. MOORE: As to where they fit in the dietary ingredient mode? Probably, again, to the extent that some microorganisms are typical parts of fermented foods, some of their constituents, some of them have a long history of use simply as food maybe not in the U.S. but in other cultures. So, arguably, depending on the bug, they would be either dietary substances for use by man or constituents of other foods.
DR. DWYER: One other thing, are metabolites organic?
DR. MOORE: The statute is silent on that. I don't think it limits it to inorganic or organic or any other.
DR. DWYER: I guess it is time to get Dr. Harris back up.
DR. MOORE: Okay.
DR. DWYER: We are giving him training as an expert witness for a murder trial. You make big money when you do murder trials; here it is nothing!
Could you tell me if you think metabolites are organics?
DR. HARRIS: No, I would not limit it to that, no.
DR. DWYER: What is an example of one that is?
DR. HARRIS: Well, up until probably the last fifty years or so a lot of attention has been placed on inorganic elements in living systems. I don't have to tell you Tufts is very famous for work on calcium and iron and my work on copper. We now know that these elements are clearly metabolized, not in the usual sense that they are being broken down to other things; obviously not, but they are able to engage in compounds that assist in their transport around the cell and ultimately determine their positioning into enzyme structures, and so forth. So, these are well-defined pathways that are now coming into more common understanding and, hopefully, more in the literature so you won't have to ask that question again.
DR. DWYER: Thank you. Let's start back with some questions for you.
DR. HARRIS: Can I raise one question?
DR. DWYER: Of course.
DR. HARRIS: Regarding the comment about microflora again and returning to that, by a physiologist, quite some time ago, I was told that anything in the gut is outside the body, waiting until until we enter the system. The case in point here could easily be seen, you don't produce antibodies to bacteria even though the bacteria are in the gut. They don't form any type--it is more of a symbiotic relationship. So, I raise that question with you. When you talk about the gut are you talking about in the body or out of the body?
DR. BRASS: I was with you until you got into the immunity part. There is mucosal native immunity. I think this is a grey area, but I think to incorporate both Dr. Shannon's comment and the comment about the gut flora, if you take that definition, then for example the stomach acid is outside the body and, clearly, the stomach acid may be a mediator of a chemical modification of a dietary substance. So, it seems to me that one is trapped into some kind of precursor-metabolite relationship following oral ingestion, or some other kind of exposure to the body that incorporates all the body functions and that includes natural flora to the degree that that is a natural constituent. It seems to me that, again, there is a little bit of a trap here, that you are talking about oral ingestion and the fate of the precursors after oral ingestion and another molecule that is generated is a metabolite.
DR. DWYER: Thank you. Dr. Childs, you had a very interesting thing that we were just sort of chatting about in the break in terms of your thinking about toxicity and the whole issue of where these things all fit.
DR. CHILDS: I think we have already covered that with Dr. Moore. I had been concerned about going back to the language in the original statute in the sense that there were multiple pathways to be considered a dietary supplement or dietary ingredient. I believe, Johanna, that is what we had talked about.
DR. DWYER: Did Dr. Moore's explanation clarify what you were thinking?
DR. CHILDS: Yes, it did, although I am still in a grey area over the bacteria issue. I am not as certain how that will shake out. Then, I am also just wondering, just trying to leap ahead for how this language might be applied in the future, thinking about GMO organisms and thinking about other methods of ingestion that might not be dietary, particularly if you think of aroma and if we see that emerging as a channel for dietary supplements. I am just trying to be very far out there in what ways this might move forward.
DR. DWYER: I think there are already B12 inhalers.
DR. DICKINSON: Right, but the Act defines dietary supplement as something that is ingested.
DR. BLONZ: You have to swallow.
DR. DICKINSON: Yes.
DR. CHILDS: All right, so that takes care of that.
DR. MEHENDALE: I know the examples to illustrate a metabolite and metabolism--the examples that you used generally dealt with intermediate metabolism and production of energy, important areas and you did a very, very good job. But if you think about benefits, obviously we can have many, many benefits outside of those areas as well, not just production of energy. You could also have interventional benefits in people who might have other metabolic disorders or may not have what we would like to prevent, disorders, and there are many other areas of benefit even coming from intervention in toxicities, pharmacological interventions; there are many of those.
So, I don't know if benefit with regard to definition of metabolite can be extended in those areas as well. You may not have limited that but your presentation was primarily using those examples, and I wanted your comments on that.
DR. HARRIS: As I understand it, you want me to justify why the word benefit was put into my perspective of what it should be.
DR. MEHENDALE: Right.
DR. HARRIS: I suppose looking at how cells function, obviously cells have all kinds of different mechanisms that are really geared toward cell survival and basically that is to sustain life. So, I am looking at certain events here, and they could be protective measures but actually let's look at it from the standpoint of keeping alive, stepping aside from anything that might harm us. That is where I put metabolism, in that perspective.
I see metabolism as basically two processes. Typically biochemistry talks about anabolism, which is building new structures from smaller molecules, or catabolism, which is breaking down for the purpose of either yielding energy or other purposes. I see all of these as well-designed steps that keep a cell alive. I also see where mutations in any enzyme systems can lead to diseases and these diseases ultimately feed back into some malfunctioning pathway. So, I am looking at maybe more of a divine nature of a living system rather than a practical nature.
DR. DWYER: Your definition implies that there is sort of--
DR. HARRIS: Ultimate purpose.
DR. DWYER: --a purpose.
DR. HARRIS: Yes.
DR. DWYER: One doesn't have to assume if one doesn't want to.
DR. HARRIS: That is correct.
DR. BRASS: I agree with everything you said but I don't think it is relevant to the definition of what a metabolite is or isn't. A metabolite relationship, again, is a chemical, biochemical, precursor-product relationship. Even for the natural intermediates there are concentration considerations so let's take a molecule like propionate. Propionate is a natural product of odd-chain fatty acid breakdown, branched amino acid production. It is important for generating Krebs cycle intermediates. But in high concentrations it is clearly toxic to cells, unambiguously toxic to cells. Carbon monoxide is a metabolite of heme breakdown in man. Carbon monoxide is pretty toxic but it is generated in endogenous metabolism and it is clearly a metabolite, in my mind.
So, I think trying to prejudge or demand some kind of benevolence or malevolence in the definition of what a metabolite is really clouding the issue. I think for relevant dietary supplements those issues are taken care of, to the degree they are, in other aspects of the law.
DR. HARRIS: Eric, pardon me for saying this but I think some of those comments are a cloudy issue. Water. Water is a metabolism product. Obviously, water is a dangerous substance. I mean, if you want to sit here and drink a barrel of water, be my guest but you will not survive. You will upset all your homeostatic mechanisms.
I guess what I am saying really is looking in the context of normal--if you want to call it normal--
DR. BRASS: Again, we are talking about exposure to compounds that are not endogenous to man. We are talking about xenobiotics. We are talking about other molecules that are not endogenous when we talk about plants that contain a number of molecules that are not endogenous to man. Therefore, their metabolism and generated metabolites don't mimic human metabolism, and trying to force the square peg into the round hole doesn't work if I am trying to simply define whether or not an individual molecule is a metabolite of another molecule.
DR. DWYER: Let me make sure, your point is that dietary supplements include xenobiotics in some cases.
DR. HARRIS: Let's make sure we understand the meaning of xenobiotic.
DR. DWYER: Sorry?
DR. HARRIS: I said let's make sure we understand the meaning of xenobiotic.
DR. DWYER: Fair enough. Do you want to comment on that?
DR. HARRIS: I think Eric brought it up.
DR. BRASS: Well, I included it in molecules that are contained in other organic matter that are not endogenous to human metabolism. Fair?
DR. HARRIS: Yes.
DR. BRASS: Without being judgmental about them.
DR. DICKINSON: But by that definition a vitamin would be a xenobiotic.
DR. BRASS: That is a good point.
DR. DICKINSON: One thinks of a xenobiotic as things other than naturally occurring materials. One thinks of xenobiotics as being a synthesized--
DR. BRASS: No.
DR. DICKINSON: Well, in nutrition we do.
DR. BRASS: But, again, I think a substance that is present in a plant that is not used in normal metabolism but can be metabolized by p450 is going to be a xenobiotic.
DR. DICKINSON: But it is not a xenobiotic to the natural biological system--
DR. BRASS: When you ingest it.
DR. DICKINSON: --to a biological system generally though.
DR. HARRIS: There is an important point here that has to be made and that is, is it a xenobiotic before the fact or after the fact? If you are talking about after the fact, then a vitamin is not a xenobiotic because it is acted upon by a series of enzymes that converts it into a profactor so it cannot be foreign to the body.
DR. BRASS: I don't want to get distracted into an argument about what xenobiotics are. Everyone will grant me there are molecules in organic matter that you can put in your mouth that are not part of endogenous human metabolism. Those molecules can get metabolized and generate metabolites. That is my point, and without deciding whether those metabolites are benevolent or malevolent.
DR. CHILDS: I was just wondering if phytoestrogens are an example.
DR. HARRIS: I think Ed brought up the point--I think it was your point that phytoestrogens will interact with estrogen receptors.
DR. BLONZ: They will, but they are also naturally occurring constituents of a number of--
DR. BRASS: And undergo metabolism in man.
DR. HARRIS: Yes.
DR. BRASS: And those metabolic products are metabolites of phytoestrogens.
DR. HARRIS: Yes.
DR. BLONZ: Clearly, one of the things that we are dealing with here is once we get past many of the ingredients that are entailed or are included in the definition of a dietary supplement, the one category which is problematic is the botanicals because we are not dealing with essential nutrients per se, although there can be essential nutrients in botanicals. So, this is where we introduce a lot of the compounds which might be foreign to the human body but they are still within our definition and we need to understand how the definition of metabolites applies to them.
DR. HARRIS: I think I would argue with you that mere interception with a receptor type mechanism is not sufficient to say that chemical changes have taken place.
DR. BRASS: I agree with that.
DR. DWYER: Could everybody turn to these questions because I don't know how long you get to stay, Dr. Harris--
DR. HARRIS: I will be here all day.
DR. DWYER: You will be here all day? But I would still like to focus on those and we do want to go back to these out of body experiences.
Miss Hardy just asked whether we want two public comments before the questions. What I want first though is to get Dr. Harris right now, while it is fresh in our minds, to make sure there are no questions we want to ask him right now.
So, question one, is it possible to identify particular scientific criteria, principles, or conventions that enable a determination to be made about when a substance is or is not a metabolite of another dietary ingredient?
First of all, do you have any additional comments you want to make having listened to our questions so far? You will have other chances but we want to focus on the issue because these are the questions the agency wants us to answer.
DR. HARRIS: One of the comments I would make, first of all, is I really enjoy the intercourse that is taking place between us. When I bring you a strictly biochemical perspective, I realize there are exceptions. For example, the exception of non-enzyme-catalyzed reactions, and other exceptions could be brought to the fold.
I found it rather difficult trying to give you numbers. So, if I said a metabolite has to be within a certain distance number-wise, reaction-wise from a reference compound, I found that difficult. I think you don't want to do that but, at the same time, we want to observe the principle.
I think for everything we are going to be saying there is going to be an exception and what I am really concerned about is that we don't do a carte blanche here. Let me return to the p450. We have been talking about that enzyme quite a bit. That enzyme is called a mixed function oxidase. That means we haven't really nailed it down as to what molecules it actually acts upon because there is such a wide spectrum of molecules. I think Eric would agree with me on that.
Just because that enzyme is acting on anything we throw at it, is that enough to say that we have seen metabolism taking place and that we have generated a metabolite? So, that is why I really wanted to bring up the idea that, in a metabolic sense, we are dealing with both a substrate and a product. In other words, we are going to make something but that something we made has to be converted into something by an enzyme reaction. To me, I would be relaxed when I think of that being a metabolic event.
DR. DWYER: Thank you. Are there any other questions on question one now?
DR. MEHENDALE: I just wanted to comment. Before we realized that the mixed function oxygenases consist of individual members of a family, I think the mixed function probably applied to the time when we realized this group of enzymes carries out many, many functions. After the advent of the individual isozymes being separated, now cloned and so on and so forth, I think we are using those terminologies less and less even though we realize that individual members have some overlapping activities. I just wanted to clarify that.
DR. DWYER: Very good. For question two, are there any things that immediately come to mind?
DR. BLONZ: Let me ask you a procedural question. I have looked through the materials and I have comments to make on all of them, but I also would like to make sure that I have listened to all the public comment before I come to any conclusions. So, how do we handle the conduct of the meeting as regards that?
DR. DWYER: We are going right to the public comments as soon as you ask any questions of Dr. Harris you want to ask.
DR. BLONZ: So, this is only as regards addressing Dr. Harris?
DR. DWYER: That is correct. We are just trying to make sure that any immediate type of questions are raised but public comments are coming right along. Then we will go back to the issues again. Anything on question two? Anything on question three that you want to ask now that you have jotted down? If not, I think we are ready. The public comment can proceed. I don't know if the people are here. If they are, we would love to hear their views. Thank you, Dr. Harris.
MS. HARDY: We have two people that are on the agenda that are going to speak today, the first one being Dr. Philip Harvey. Each commentor will be permitted to talk for ten minutes. The timer is on the podium while you are up there and then I will be watching it too and I will give you a signal when you have two minutes left. So, Dr. Harvey?
DR. DWYER: Dr. Harvey has written a thoughtful letter.
Open Public Comment
DR. HARVEY: The letter was written by Scott Bass and Emily Marden. You have the letter. I mean, I was just going to read through that. I don't know if it is redundant or not.
MS. HARDY: Many of the people in the audience do not have the letter.
DR. HARVEY: Okay, I am just going to go ahead and read the written comments that were submitted by Sidley and Austin. There are some introductory points but I will just get to the specifics.
I am chief science officer for the National Nutritional Foods Association, or NNFA. NNFA appreciates FDA's solicitation of comments on the definition of metabolite. NNFA believes that the term has a clear scientific definition and was explicitly included in the DSHEA definition of dietary supplement on those terms. NNFA, therefore, takes the position that there is no need for FDA to redefine or narrow the category.
One, the definition of a dietary supplement in DSHEA includes metabolites. I think this was discussed extensively in terms of the DSHEA definition, defining a metabolite or a vitamin, mineral, herb or other botanical, amino acid or dietary substance. Specifically, the term dietary supplement means a product, other than tobacco, intended to supplement the diet that bears or contains one or more of the following dietary ingredients: (a) a vitamin; (b) a mineral; (c) an herb or other botanical; (d) an amino acid; (e) a dietary substance for use by man to supplement the diet by increasing the total dietary intake; or, (f) a concentrate, metabolite, constituent, extract, or combination of any ingredient described in clause (a), (b), (c), (d) or (e).
Two, the definition of a metabolite is not contested. Metabolite is uniformly defined in scientific and general use dictionaries as any substance produced by the body's metabolism or by one of the body's metabolic processes. A metabolite thus includes any substance that is created by the body as it builds up, breaks down, or converts the nutrient.
This inclusion of the term metabolite in the DSHEA definition of dietary supplement indicates that the drafters of DSHEA contemplated allowing any compound that results from metabolism of one of the other defined dietary ingredients to be marketed in dietary supplements.
The range of metabolites is potentially broad, including what are called intermediary metabolites, molecules that are not identical to the original nutrient/supplement but are created to get to another step along a chain of alterations that leads to the molecule the body is trying to make or excrete.
Three, a wide array of dietary supplements are already on the market as metabolites. A wide range of dietary supplements already on the market as metabolites of dietary ingredients, and here are a few examples. One would be MSM, a metabolite of DMSO, a garlic-oyster smelling substance. Glucosamine sulfate, a metabolite of chondroitin sulfate; pantetheine, a metabolite of pantothenic acid; methionine, a metabolite of SAMI, or a metabolite of homocysteine. Those are just some examples.
Four, like other dietary supplements, metabolites must meet the statutory safety standard. The definition of metabolite is, therefore, very clear and there is no reason FDA should move to redefine or narrow this category.
At the same time, all dietary supplements, including metabolites, are subject to the statutory safety standard, which NNFA urges FDA to continue diligently enforce. Under this standard, dietary supplements must not present a significant or unreasonable risk of illness or injury under conditions of use recommended or suggested in labeling, or under ordinary conditions of use. Some products on the market clearly fail that standard regardless of their definitional status under Section 21 of the U.S.C.
Five, new dietary ingredient provisions also apply. The new dietary ingredient, or NDI, provisions of DSHEA apply to all dietary supplement ingredients including metabolites. NDI is defined as a dietary ingredient that was not marketed in the United States before October 15, 1994. DSHEA requires that manufacturers of dietary supplements containing an NDI file a notification establishing the safety of the ingredient 75 days before it is placed on the market.
The safety of a dietary supplement containing an NDI is established if there is a history of use or other evidence of safety establishing that the dietary ingredient when used under the condition recommended or suggested in the labeling of the dietary supplement will reasonably be expected to be safe.
Thus, manufacturers of dietary supplements containing NDIs are subject to both the general safety standard noted above, as well as the NDI standard. NNFA believes that these safety standards, together with FDA enforcement, are adequate to ensure that metabolites are adequately regulated.
Based on these comments, we urge FDA not to modify its interpretation of the term metabolite. Thank you.
DR. DWYER: Could we take questions now or would you be willing to answer some questions?
DR. HARVEY: Sure.
DR. DWYER: Yes?
DR. BRASS: Do you agree that human metabolite is implicit in the phrase metabolite?
DR. HARVEY: Yes.
DR. SHANNON: Do you really think the definition is not contested?
DR. HARVEY: You know, that is our position.
DR. DICKINSON: Could we ask a question of Dr. Moore related to that? Could we ask whether there has been a warning letter or other action related to whether something is or isn't a metabolite?
DR. MOORE: I don't know that there is a warning letter. I know that there are untitled letters where we have asserted that certain analogs of things that would fit into one of the other categories are not metabolites. For example--I remember the colloquial name but not the chemical identity--but it is an analog of coenzyme-Q. That was argued to be a metabolite of and we asserted that it is not a metabolite; it is a chemically prepared substance derived from that, that fell outside the scope. We haven't really defined or carved out much more than that for metabolites.
DR. BLONZ: Dr. Harvey, in your definition of metabolite, in the first paragraph you say that a metabolite, thus, includes any substance that is created by the body as it builds upon, breaks down or converts the nutrient. Now, not all of the dietary ingredients are nutrients.
DR. HARVEY: Correct.
DR. BLONZ: Then you also put in the third paragraph to get to another step along a chain of alterations that leads to the molecule the body is trying to make or excrete. Could you expand on what you mean by that?
DR. HARVEY: You know, listening to the prior comments, I mean these compounds are precursor substrates and are making one intermediate constituent or are converted to another. Some of those are excreted based on metabolism and, you know, it is just that process specifically.
DR. BLONZ: It was my thought that in many cases they might be trying to make or excrete something in response to the presence of that very ingredient. So, it might not be a physiological process that happens outside the presence of that ingredient; it could be just a reaction to its presence. Again, coming back to our friend cytochrome p450, there are many reactions to putting foreign substances in the body.
DR. DWYER: One issue that is troubling here is the purposefulness again.
DR. BLONZ: Yes.
DR. BRASS: Yes, I don't think you should try to decipher what the intent of the body is.
DR. DWYER: Could I ask just probably a very stupid question? In your section four, it says all dietary supplements are subject to the statutory safety standard. Of course, NNFA is urging diligent enforcement. The question is there could be a standard after the fact. Right? If you leave that very broad and you don't know something is harmful and then it turns out to be harmful, what is the protection to the consumer? Is this a license to cause trouble?
DR. HARVEY: I am not going to speak specifically for FDA but, you know, there is a possibility--you know, these things are kind of living--that new evidence comes out based on a better understanding of these substances that could possibly change that. But generally, as we see that as constituents that the body could use or eliminate, and there are things for which we still don't one hundred percent know the exact mechanism or what is going on but unless there is some physiological change or condition of abnormality, then there is a possibility that that could be addressed. In terms of FDA enforcement, you know, they would be looking for some type of an illness or injury, some event and a sequence of that and, you know, they may consider enforcing on that.
DR. DWYER: Other questions?
DR. BLONZ: Let me just ask one more thing, by this definition you would be arguing to say that testosterone or estrogen would be a metabolite of a fatty acid and could be included as a dietary supplement, as a metabolite of that substance.
DR. HARVEY: Correct.
DR. BLONZ: Thank you.
DR. DWYER: Dr. Dickinson?
DR. DICKINSON: It might under the definition by a metabolite but whether it could legally be included would depend on other provisions of the law, such as not including ingredients that are drugs or that are controlled substances that are otherwise excluded.
DR. BRASS: This is where I think the concept of net product and not simply tracing carbons through pathways makes a big difference. Again, you can ingest any carbon and find it in any other molecule in the body under a wide variety of circumstances, but if it doesn't result in net structural contribution then I don't see how it can be viewed as a metabolite.
DR. DWYER: I am troubled again. The word is used in the standard, as I understand it, as the last in a paragraph that lists specific things, and it seems to me a metabolite is a derivative--
DR. BRASS: No, it is an alternative way. My interpretation is the things above it are the precursors--
DR. DWYER: Yes, right.
DR. BRASS: --and metabolites of those things.
DR. DWYER: Of those things.
DR. BRASS: So, again, because glucose is a dietary constituent and just because you can feed somebody glucose and find glucose and carbons in every molecule in the body doesn't mean all the other molecules immediately become metabolites of it. There has to be net contribution to synthesis.
DR. DWYER: Back to our colleague at the mike, then do you consider substances, other than things where precursors were amino acids, vitamins, minerals, herbals and botanicals or dietary substances for use by man, to be metabolites?
DR. HARVEY: They could be, yes.
DR. DWYER: Would you give an example? Is an example estrogens, metabolites of fat?
DR. HARVEY: Estrogens metabolites of fats? I mean, carbon from cholesterol is a steroid. You know, it just depends on how far downstream.
DR. BRASS: No, but if you eat more lipid, you eat more palmitate you don't get net incorporation of palmitate into estrogen. You might get carbons traced through infinite pathways that end up there, but you don't get net production so it can't be a metabolite.
DR. HARVEY: I am thinking of another example of arachidonic acid into cell membranes. I mean, this does occur over a period of time, lipid especially. You know, you can change, especially with fatty acids, over time net incorporation of those dietary substances from either foods or dietary supplements into cell membranes.
DR. BRASS: The specific phospholipids that are metabolized might be argued as metabolites because, again, there is a product-precursor relationship. But you can't say because it is incorporated into the cell the cell is a metabolite of the lipid.
DR. DWYER: Any other comments or questions? Are those all clear? Does anyone at the table over here have any? I was puzzled by this precursor-product relationship. Is that your understanding?
DR. TAYLOR: I think we are here to hear the discussion you folks would have about that, and we certainly don't have anything in our record that would be relevant to it.
DR. DWYER: Thank you. Thank you, Dr. Harvey.
DR. HARVEY: Thank you.
MS. HARDY: Dr. Dentali?
DR. DENTALI: Thanks very much. I am Steven Dentali. I am the Vice President for Scientific and Technical Affairs at the American Herbal Products Association, a trade group that represents manufacturers of botanical products. I don't have a written statement. I didn't come prepared with one but I would like to respond to the interesting discussion from this morning.
I have a doctorate in pharmaceutical sciences, a minor in pharmacology and specialization in botanical and natural products chemistry.
From the natural products chemistry, from botanicals, I just want to say for the record that flavonoids, terpinoids, alkaloids but not altoids but certainly the menthol in the peppermint oil that goes in the altoids, all these others fall into a class of secondary metabolites. That doesn't really have relevance here for the discussion because we are talking about metabolites of compounds, and botanicals is already included, and constituents of botanicals are already included. So, if you are taking menthol, that is already included but it is well within the definition of the metabolite standard within natural products chemistry. I know Dr. Schiff can back me up on that.
In the same way--I am sorry, I have forgotten your last name--Brass, thank you--I took a course in pharmacology called drug disposition and metabolism. So, I was uncomfortable with the limitation of the excellent discussion on metabolism, of intermediate metabolism and human cellular metabolism to be limited to just that. It is clear the course I took was talking about xenobiotic metabolites. So, the design of drugs and metabolism of drugs--this is clearly within the definition of what a metabolite is and shouldn't be arbitrarily excluded.
I think we see some of the limitations and try to limit it to within the body and to limit it within the realms of intermediate metabolism when, clearly, there are plasma esterases that occur; there are reactions that take place outside cells. You know, we seem to move the discussion from cells to the body but clearly in plasma metabolites are formed. There are non-enzymatic reactions that produce metabolites. Neurotransmitter metabolism, just as an example.
That is really the point I wanted to make. I think the point of secondary metabolites is something to recognize. There are metabolites that plants make. In fact, these are pretty much the compounds of interest for botanical products. They don't come into play here because of the way the statute is written.
I do think that the metabolism as known in pharmacology--certainly there is no reason to exclude that from the definition. I don't see that we have any charge to do that. That is pretty much what I have to say.
DR. DWYER: Come back to the secondary metabolism. What is primary metabolism? How do you define that?
DR. DENTALI: Well, it would be the compounds that we know are necessary for the structure of the plant, basically proteins, carbohydrates, fats. We know exactly what they are for. We didn't really know exactly why--I am not sure we still know why coffee beans contain caffeine. I don't think they are doing it for our benefit. But alkaloids are considered a secondary metabolite of a plant metabolic process. They may be doing it for their benefit since we eat the coffee beans and coffee is still around and is grown quite a bit; surviving well; there is a lot of coffee around because it contains caffeine.
DR. DWYER: So a secondary metabolite is defined primarily by human knowledge?
DR. DENTALI: Human knowledge--could you repeat that, please?
DR. DWYER: If we went back to 150 years ago, everything would have been a secondary metabolite because we didn't know about anything--
DR. DENTALI: Yes, we knew a little bit about alkaloids but not a lot. Yes, it is based on our knowledge of plant chemistry; our knowledge of human metabolites back about the same time.
DR. DWYER: How about some questions for Dr. Dentali?
DR. DICKINSON: You made the point that, as an intellectual matter, metabolites of xenobiotics should still be considered metabolites. But would you argue that those are necessarily dietary supplement ingredients?
DR. DENTALI: Not necessarily, no. However, for flavonoids in a botanical it would be a dietary supplement ingredient. A drug metabolite, if it is already sold as a drug, does not make it a dietary supplement ingredient.
DR. DICKINSON: And a secondary metabolite in botanicals or the botanicals themselves would be dietary ingredients for reasons other than that they are metabolites.
DR. DENTALI: They are already constituents.
DR. DICKINSON: Right.
DR. DENTALI: Yes, the garlic was a very good example, made after the fact.
DR. SCHIFF: I think when Steven was talking about secondary metabolites, I would want to emphasize that he means secondary phytometabolites in phytometabolism. He referred to caffeine and, if I remember correctly, glycine carbon dioxide and ammonia, assembled correctly, gives you caffeine in the plant. To me, something primary in a plant would be an amino acid.
DR. DWYER: Fair enough. Any other questions? Anybody over here have anything? No? Thank you. It was very enlightening.
DR. DENTALI: Thank you.
DR. DWYER: Let's just go around and make sure there are no other questions that have come to mind for Dr. Harris or any of the folks who have been kind enough to share their views with us, or any questions of clarification that are needed from the scientists representing the agency. Yes, Dr. Dickinson?
DR. DICKINSON: At the risk of complicating this unnecessarily, I have been sitting here, fretting about one thing that Dr. Harris said and what it means in the context of other ingredients that we know are used in dietary supplements. That is, he put a great deal of emphasis on the stereoisomer that is produced naturally in the metabolic system, and is suggesting that that be one of our criteria for what is an appropriate metabolite of one of these other ingredients as we talk about it.
I just want to separate that concept from the established recognition that there are many essential nutrients where there are synthetic forms of the naturally occurring nutrient that are well accepted as equivalent forms of the nutrient, and that that is a separate issue from this question of whether a metabolite is the appropriate stereoisomer. I wondered if somebody from FDA would comment on that.
DR. BRASS: But you are not quibbling that stereoisomers have to be considered distinct molecules--
DR. DICKINSON: Yes.
DR. BRASS: --in whatever category one is thinking about?
DR. DICKINSON: Yes.
DR. DWYER: Shall we take an hour for lunch or do you want to take less? Let's take sixty minutes which will bring us to about 12:35. We will be back here at 12:35. So, we are a little ahead of the schedule, which is fine with me.
[Whereupon, at 11:35 a.m., the proceedings were recessed for lunch, to be resumed at 1:40 p.m.]
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A F T E R O O N P R O C E E D I N G S
Discussion of Charge and Questions by Subcommittee
DR. DWYER: Does everybody have these questions? What is your pleasure? Shall we just go through the questions one by one? All right?
DR. BRASS: It might be useful--I have kind of a conceptualized definition that was useful for me in trying to integrate various aspects of this. I don't pretend it is perfect. It is not worth-smithed but it is conceptually how I thought about this. That might be helpful?
DR. DWYER: Yes.
DR. BRASS: It is a functional definition. What it says, in my mind, is X is a metabolite of Y if ingestion of Y by humans results in net production or increased flux of X incorporating structural elements of Y.
So, there are two parts to it. First of all, it involves ingestion by man. Two, it involves a true precursor-product relationship that Y is giving net production of X and it is a direct structural interaction. In other words, the fact that catecholamine increases cyclic ANP wouldn't fit the definition because cyclic ANP incorporates no structural elements of catecholamines.
So, it is in two parts. It is ingestion by man which, as we heard, was the key for supplements; yields net production of and is structurally a precursor of. Those seem to me to be the elements which I thought were critical in incorporating the issues we talked about with Dr. Harris' presentation, an attempt to relate those in a way that made functional sense to me.
DR. SHANNON: I think that is useful. We would probably need to work on what structural similarity means--
DR. BRASS: Incorporating structural elements of.
DR. SHANNON: Right. What I mean is if that can be quantitated in any way because otherwise, again, if it has two carbons--
DR. BRASS: Well, I was trying to find exceptions where, if there was a direct net production and incorporation of the elements--I was just trying to differentiate and gave the cyclic ANP example that was so well brought up earlier where these other indirect hormonal effects are some downstream effects when you produce a compound. Again, it is not word-smithing but that was the concept I was trying to get across.
DR. DWYER: I have one addition to that. It has concerned me all along. It is getting all of this to something that is useful from the standpoint of the regulators. If you will bear with me while I draw some circles--
DR. BRASS: I didn't intend it to be this formal.
DR. DWYER: You talk about metabolites and we are talking about other dietary ingredients, and it seems to me that what they are talking about here that needs to be defined is this, that intersection in the van Dyke diagram. Does everybody agree with that on the committee?
DR. BRASS: So, this was a clever ploy to get me away from the microphone! I think that I do disagree.
DR. DWYER: All right.
DR. BRASS: Because I think that from conceptualization, the other dietary constituents or ingredients are the precursors of metabolites. Now, a metabolite might also turn out, by coincidence--
DR. DWYER: That is what this is supposed to be.
DR. BRASS: No, I view that as the same thing.
DR. DWYER: How would you draw this diagram?
DR. BRASS: Well, I personally would draw them as two non-overlapping circles, acknowledging that it may turn out, by coincidence--see, this again is an interesting definition--by coincidence it might turn out that some of the metabolites also are constituents of dietary supplements, but that is not intrinsic to their definition.
DR. DICKINSON: These are dietary ingredients. These are metabolites and some of those metabolites are dietary ingredients.
DR. SHANNON: What makes them dietary ingredients? What makes those fractions dietary ingredients?
DR. DICKINSON: You are right, I almost need two circles over here, don't I? These are legitimate dietary ingredients as defined elsewhere. These are their metabolites. Actually, all of those metabolites that meet that definition are dietary ingredients also. But also--
DR. BRASS: No, they are not.
DR. DICKINSON: --there are many other metabolites.
DR. BRASS: The way you initially drew it is dietary ingredients generate metabolites. It may turn out, purely by coincidence and not by definition--again, I am trying to focus on the definition which is our charge--it may turn out that some of those metabolites turn out to be dietary ingredients by coincidence. For example--
DR. DICKINSON: No, no, I am saying--
DR. BRASS: --glucose is a metabolite of sucrose. Right?
DR. DICKINSON: Yes, it may also by itself be a dietary ingredient.
DR. BRASS: And by coincidence glucose turns out to be a dietary ingredient but not intrinsic to the definition.
DR. DICKINSON: No, I meant to show just that it is only metabolites of ingredients that are already dietary ingredients that are going to be ingredients because they are metabolites.
DR. BRASS: What you are doing is defining X.
DR. DICKINSON: Y.
DR. BRASS: Y, I am sorry. You are defining Y by saying Y has to be a dietary ingredient or one of those precursors that are listed above.
DR. DICKINSON: Yes.
DR. BRASS: And I agree with that. Again, I was trying to focus on what a metabolite was.
DR. DWYER: Come and play with us over here with the circles.
DR. BRASS: I like my own game! You don't share!
DR. DICKINSON: All right, what fraction of the second circle is relevant to us?
DR. BRASS: A hundred percent.
DR. DICKINSON: All of it.
DR. BRASS: Because that is the definition of a metabolite, and that is what our charge is.
DR. DICKINSON: But all of those may not be legitimate dietary ingredients.
DR. BRASS: Well, there may be other parts of the statute but in terms of defining what a metabolite is, that is the definition.
DR. DWYER: Then there is a whole bunch of other substances that are metabolites but not under the law.
DR. CHILDS: Doesn't that get you back to your original drawing?
DR. DWYER: Yes, I think so.
DR. CHILDS: Which I thought was a clear portrayal.
DR. BRASS: Again, defining whether it is a metabolite per se doesn't define whether or not it is an appropriate thing to be marketed under DSHEA.
DR. DWYER: Right.
DR. BRASS: And that is what I am trying to keep absolutely separate as we define what a metabolite is, or at least for me when I think about what a metabolite. That is, I have to keep that separate or I get into all kinds of caveats that I can't incorporate into a definition.
DR. DWYER: Isn't something you said a lot earlier, that the upper list, the amino acids, vitamins, minerals, herbals and botanicals or dietary substances for use by man, are the precursors?
DR. BRASS: Again, in my definition that is the list of things that are in Y. Again, I was trying to write a generic definition because I don't want to be trying to interpret--we are having enough problems interpreting one word and I am trying to avoid getting us distracted and trying to interpret a bunch of other words. So, this seemed to me to be a generic way of saying that, but I would agree with you and my intent is Y would be that list of precursors.
DR. DWYER: Could you read the definition again, Eric, to make sure I understand it?
DR. BRASS: X is a metabolite of Y if, one, ingestion by humans of Y results in net production or increased flux of X. The phrase increased flux is to recognize that there may be a metabolic intermediate which doesn't accumulate, as Dr. Harris was referring to, but simply increases the flux through it in a way that affects biologic function. But the other part is two, that X has to incorporate structural elements of Y. That is, there is a direct precursor-metabolite structural link. Again, that is to get rid of the hormonal thing. If Y was a hormone, there are a variety of Xs that change in response to hormones. Those would not be metabolites because they don't incorporate structural elements of the hormone. Again using Dr. Harris' example, if a molecule was an allosteric regulator of a pathway it would affect the concentration or flux of a number of substances, but because it didn't represent a structural precursor those would not be true metabolites of the precursor. So, those are the scientific criteria I was trying to incorporate into the syntax.
DR. DWYER: Dr. Schiff?
DR. SCHIFF: Eric, if I could ask you a question because I think this is very good conceptually and I just want to make sure I am on the right page with you here, let's go back to a tougher example. We will take a botanical. We will pretend it has 200 ingredients in it. Let's isolate ingredient 144--I just picked that, being ingested by a human through some process, defined or undefined; it hydroxylates to hydroxy-144; hydroxy-144 you would consider a metabolite.?
DR. BRASS: It would be a metabolite of Y.
DR. SCHIFF: That is correct.
DR. BRASS: Absolutely correct. That was my intent.
DR. DWYER: Any other questions or comments? Dr. Harris, do you have comments you wish to make on this? Not allowed?
DR. DICKINSON: Can I ask for clarification? We have asked questions before at advisory panel meetings of experts who were in the audience. No?
MS. HARDY: Well, you know, we had time allotted for questions earlier in the day. Now it is supposed to be just you deciding answers to the questions, having already heard answers to the questions earlier. Technically, he is invited by the FDA so, you know, it is FDA's perspective.
DR. BLONZ: I had prepared answering each of the individual questions, hoping that discussion of each of those points would lead us up to formulating a definition. We very well may come back to this but with some different understanding with more commentary. Are we going to go thorough these individual charge questions?
DR. DWYER: Yes, we are in just a moment. First I need to get what is wrong with this diagram in terms of circles, and then we will go to the individual questions. Basically, I am still having trouble with why this first diagram I drew is not in line with your thinking.
DR. BRASS: Well, it very well may be.
DR. DWYER: I think it is.
DR. BRASS: Okay.
DR. DWYER: What I am saying is that a dietary ingredient, other than amino acids, vitamins, minerals, herbals and botanicals or dietary substances for use by man, a metabolite of those substances is what we are talking about. We are only talking about metabolites in that defined way, that a substrate or a precursor, as I guess you called it, has to be one of those things.
DR. DICKINSON: That is what that shows.
DR. BRASS: Yes, I think that is what that shows. I agree. I agree with Annette. I think that shows the relationship of causality and product precursor, and not the overlap.
DR. DWYER: Does it imply Y always has to be before X?
DR. BRASS: Because you have the arrow.
DR. DWYER: There is directionality and there is temporality?
DR. BLONZ: That is why Y came first.
DR. CHILDS: I think what is happening is that we think of this as your starting diagram and your diagram as the concluding diagram.
DR. BRASS: It will turn out that there is an overlap between but that area of overlap is not intrinsic to the definition.
DR. BLONZ: One of the issues that I have with the definition is what would happen if you were to have an amino group as part of a reaction? Would the donation of that amino group then make that amino acid a precursor to the product?
DR. BRASS: It would depend on the specific. The answer could be yes. Is an amino acid a precursor of urea?
DR. BLONZ: Well, urea is a pretty uncomplicated substance.
DR. BRASS: But that is the example.
DR. BLONZ: One of the examples we have is an amino group from glutamine which then makes it glutamic that shovels that compound down the road. Are you saying it is the metabolite of both?
DR. BRASS: Yes, conceivably that is true. I don't know for a fact--I apologize--whether adding glutamine increases the net flux through a set of reactions. I don't know which way it is limiting. So, again, both clauses have to be true under the functional definition I proposed but, yes, one metabolite might have more than one precursor, i.e., might be a metabolite of more than one molecule.
DR. DWYER: One other question that might be asked is are there other ways, such as a structural definition rather than a functional definition, that might be thought of? It seems like there are some because they are in the paper. It has to be something that is an identifiable organic compound.
DR. BRASS: No.
DR. DICKINSON: It doesn't have to be organic.
DR. DWYER: It doesn't have to be organic. It has to be an identifiable compound.
DR. BRASS: You have to know what X is. X is a specific molecule so, yes, there has to be a specific molecule.
DR. DWYER: Does it have to have any similarity to an amino acid, vitamin, mineral, herbal, botanical?
DR. BRASS: In my opinion, the answer is no because we would argue all day whether molecule X and Y were similar, and we could go through 300,000 individual examples and develop 300,000 individual answers from the people on this committee but we wouldn't get insight into an extrapolatable definition.
DR. CHILDS: But isn't that incorporated in your definition?
DR. BRASS: Well, that is why I put structural element because that is different than similarity. Again, is an L-amino acid similar to a D-amino acid? It depends on whether or not you transfer RNA or not; one you recognize and one you don't. Again, I was trying to avoid that kind of discussion which I think would require individualization of every kind.
DR. DWYER: Are there any other criteria? Do you have to know exactly?
DR. BRASS: In my opinion, you do not. So, again, it terms of scientific principles, I do not think you need to know the pathway. I don't think you need to know the number of steps between. I don't think you need to know whether it is a good or bad molecule. I don't think any of those things are intrinsic to the definition of what a metabolite is or isn't. So, I tried to be reductionist. Again, I apologize that it is not clear and needs word-smithing but conceptually I have tried to incorporate what I think are the core concepts of a definition of metabolite that would apply to the maximum number of situations I can think of.
DR. MEHENDALE: Would humans include also microflora in the gut?
DR. BRASS: Well, I avoided that very carefully by saying ingestion by humans, and I think a reasonable interpretation of that would be ingestion by humans who have bugs in their gut, and acid in their stomach, and water in their blood and all the other non-enzymatic mammalian processes.
DR. MEHENDALE: So, it is not intended to mean only by human cells, human tissues.
DR. BRASS: I was impressed by the discussion both by you and Dr. Shannon this morning about other non-enzymatic pathways so I modified what I had originally scribbled on the airplane.
DR. DWYER: Could I just ask one other clarifying thing? You called this a functional definition. I guess last night I was thinking about relationships and I guess it comes under function but, to me, they are a little different. One thing you might want to think about in the definition is what does it have to do with safety. Anything?
DR. BRASS: Nothing. Other criteria apply. I mean, X may be safe, may be unsafe or may be safe at low doses or certain doses and not others. Those are all unrelated. They are not intrinsic to the definition of what a metabolite is.
DR. DWYER: Essential function?
DR. BRASS: Irrelevant.
DR. DWYER: Bioaccumulation?
DR. BRASS: Irrelevant.
DR. DWYER: Stereoisomers; regulatory function?
DR. BRASS: Irrelevant.
DR. DICKINSON: Except that X may only be one isomer.
DR. SHANNON: But that is not intrinsic to the definition.
DR. BRASS: Yes, X is a molecule. That is why I clarified earlier that we acknowledge that stereoisomers are different molecules.
DR. DWYER: Proximity?
DR. BRASS: Irrelevant.
DR. SCHIFF: Proximity--you do have incorporating structural elements. There is certainly an element of proximity there.
DR. DWYER: An element of proximity.
DR. BRASS: Yes, but it is not numeric--
DR. SCHIFF: No.
DR. BRASS: It is functional proximity.
DR. DWYER: And then directionality, you said yes.
DR. BRASS: Again, that is because X is a metabolite of Y.
DR. SCHIFF: Eric, let me ask you, is not, to coin a term, the beauty of your definition that it more or less removes all of the mechanistic functionality involved, and it simply says we have this and somehow it is getting to this?
DR. BRASS: Well, that was my intent. That was my intent because every time I tried to word-smith syntax I was arguing with myself the way I argued with Dr. Harris, and any time I tried to get specificity I could think of examples and I could think of no reason to pose those limitations on my functional definition in the context of what we are trying to do.
DR. DWYER: Dr. Schiff, would you repeat what you said?
DR. SCHIFF: I will try. I think I said that perhaps the beauty of the definition was that it tended to stress independence of mechanism, or as many of the other considerations dealt with a lot of functionality and a lot of mechanism.
DR. DWYER: Are we about ready to go to questions?
DR. BLONZ: Yes, let's go to questions.
DR. DWYER: Question one, can we go around the table and try to answer this one? Maybe your best answer would be ditto if one person says it. Is there someone who would like to start?
Is it possible to identify particular scientific criteria, principles, or conventions that enable a determination to be made about when a substance is or is not a metabolite of another substance?
DR. DICKINSON: Yes, and Dr. Brass has outlined that for us very nicely I think.
DR. SHANNON: Ditto.
DR. SCHIFF: Ditto.
MS. HARDY: Identify yourself.
DR. SHANNON: Michael Shannon, ditto.
DR. SCHIFF: Paul Schiff, ditto.
DR. BLONZ: Ed Blonz. We were given a number of definitions from various sources, both biochemical and from dictionaries, and they were really all over the place. It is my opinion that we need to come up with something as it applies to DSHEA, not necessarily one that would be an English definition but one that would apply to a dietary supplement legislation.
I also think that in the definition we have to have some means of stating that this reaction should be independent of any exogenous influences. So, the chemical reaction would take place only when there was a certain chemical agent in the body. That should be independent of the fact that this would happen when there were no outside influences. So, when an individual is under the influence, or they have liver problems and so forth and certain metabolites might be created, would those be metabolites? So, perhaps we could have under ideal conditions or in normal situations, some sort of caveat for that.
DR. DWYER: Let me understand, you said in many ways you agree but it must apply to DSHEA. Then the second point was less clear to me. It must be independent of what?
DR. BLONZ: Of any exogenous affecters.
DR. BRASS: Otherwise unadulterated human.
DR. BLONZ: Exactly.
DR. BRASS: I agree with you about the exogenous thing. I am less sanguine about liver disease--
DR. DWYER: Let's go around the table. Hold your comments until then.
DR. BLONZ: Well, liver disease might not have been the most ideal circumstance.
DR. DWYER: Does everyone understand what is meant by exogenous affecters? Give one example.
DR. BLONZ: Alcohol.
DR. DWYER: Thank you. We will go around. Dr. Mehendale?
DR. MEHENDALE: I have similar comments. Susceptible populations is another example. Aged populations is an example. Exposure to alcohol, smoke is another example. But I don't think we need to worry about that in this definition because those are safety issues and, hopefully, the safety issues will cover those aspects.
So, I am a little ambivalent but I did want to bring my thought process into play at the table. My thinking is that those observations will be covered under safety regulations. I don't know that we need to worry about those in this definition.
DR. DWYER: But the safety is determined after the fact rather than before. In one case you have a license to put something in and then you determine the safety by body counts, or whatever. In the other case you don't allow the thing in. I mean, isn't that the difference?
DR. DICKINSON: It is a difference but it isn't a carte blanche. There are procedures to go through before you put a new ingredient on the market.
DR. CHILDS: I am comfortable with the definition, and I agree with the fact that there are safety and other factors built into the larger Act.
DR. DWYER: Eric?
DR. BRASS: In response to question one, I would say yes, as I think we tried to articulate in our discussion.
In response to the comments that have been made, I think intrinsic to the definition--what I proposed is DSHEA specific in that it incorporates human ingestion and that is the relationship that is unique to DSHEA. This would not be a definition that would apply in any other context, other than DSHEA, because a metabolite need not occur after human ingestion in other contexts. If I give a drug IV it gets metabolized. A plant itself, as we have heard, metabolizes itself. Animals and bacteria have unique metabolic pathways. So, the DSHEA-ness of my definition, if you will, is the ingestion by humans because I agree with you, it is a very important differentiation.
With respect to whether or not the definition has to incorporate special classes of endogenous conditions--age, liver disease, renal disease, etc.--I think that is really hard to try to do and would carry over, I think, to other safety concerns. If it requires an exogenous enzyme so you have to take Y and an enzyme that mix in your stomach, then I think the exogenous affecter clause might come into play.
DR. DWYER: I think we are getting some place. My sticking points are, first, I don't think ingestion by humans is specific enough and, if you want it to be a definition that is meaningful under DSHEA it has to say specifically amino acids, vitamins, minerals, herbals, botanicals, or dietary substances produced by man.
DR. BRASS: Well, that is what Y is.
DR. DWYER: That is fine; let's say it in the definition.
DR. DICKINSON: Or let's add a second sentence that says for purposes of this definition X is--
DR. BRASS: Y is, yes.
DR. DICKINSON: Right.
DR. DWYER: Then the second thing is again about the safety thing. I am not sure that I am satisfied about that.
DR. BRASS: Well, believe me, I would love to put in a clause that says it has to be a safe molecule, but I don't think anybody would allow me to do that because to the degree safety is an issue, it is addressed in DSHEA itself, not in the definition of metabolite, unfortunately from some people's perspective.
DR. SCHIFF: I agree.
DR. DWYER: Okay, I still am not satisfied.
DR. BRASS: Is there a way to deal with it?
DR. CHILDS: Johanna, is that any different than the fact that herb or botanical is listed, and we know there are toxic herbs and botanicals that are automatically excluded by the prior language in the Act? So, how would this be different than that kind of generic reference to herbs and botanicals?
DR. DWYER: I don't know.
DR. SCHIFF: Johanna, the definition we are looking at, we seem to be reasonably close to agreeing on. When it comes to the nomenclature that is involved here, Eric's definition really refers to not a botanical but the individual constituents of the botanical, which makes it very different from a vitamin identifiable, a mineral identifiable, an amino acid identifiable, but for the an herb or other botanical really by direct implication he means the constituents, the individual constituents present in the herb or other botanical.
DR. DWYER: Okay.
DR. SCHIFF: I know we can't modify the law, but for the record, I think that is the intent of the definition.
DR. DWYER: Well, for the record, we are defining metabolite and we don't intend to expunge the legislative history.
DR. BRASS: But, again, he was trying to address your concern about what Y is.
DR. DWYER: Okay, but I would like to see a definition of Y, and the definition of Y then also includes constituents of herbals and botanicals.
DR. CHILDS: To support Johanna, it is in that herb and botanical area where we have had the issues in the past with DSHEA.
DR. SHANNON: Excuse me, I didn't understand what you just said about constituents.
DR. DWYER: That it isn't herbals and botanicals, as Dr. Schiff said, it is specific compounds in herbals and botanicals.
DR. SCHIFF: This definition could not apply to a whole plant. It would apply to the individual chemical compounds that are present in the plant.
DR. SHANNON: I see.
DR. SCHIFF: As it could apply to a single vitamin or a single mineral. That is my point.
DR. BRASS: And I appreciate that clarification--
DR. SHANNON: I am not trying to cloud that.
DR. BRASS: No, it is critical. It is part of my trying to avoid defining what an herb or other botanical encompasses.
DR. SHANNON: That is fine.
DR. BRASS: And if herb or other botanical means all the constituents of that herb or botanical, then I agree. If there is some other interpretation of what those words mean that limit what would be the definition of Y, then I would accept those limits. But my understanding is what your understanding is.
DR. DWYER: Yes, Dr. Dickinson?
DR. DICKINSON: Herb or other botanical means whole herb or any of its parts, including the roots, the flowers, the seeds or whatever. But it isn't logical that there would be metabolites of the whole fruit. Their constituents are also covered in part d), e) or whatever it is that refers to the constituents or extracts or so forth. But for metabolites we are still talking about metabolites of single compounds.
DR. BRASS: Again, I apologize because what it says is a concentrate, metabolite, constituent, extract or combination of any ingredient named above. Constituent is not above. Constituent is in the same clause. What is above is herb or other botanical. So, again, I have interpreted that to mean the herb, botanical, all its parts down to the molecular level. If there is some other interpretation of that that takes precedent, then that would be critical to know because that is what metabolite applies to. That is the universal Y.
DR. DICKINSON: Well, an herb or botanical for purposes of normal products is some large piece of it or some extract of it, not a single compound of it. But we could use the word constituent or component as a foregoing word that applies to--
DR. BRASS: Again, it doesn't say metabolite of a constituent because constituent is not above. It says metabolite of an herb or other botanical. So, again, I was trying to avoid that in the definition of metabolite by saying Y is whatever Y is. If Y is an herb or botanical, I imply Paul's implication but if that violates some other interpretation I don't want to impose it.
DR. DICKINSON: But we could add a sentence that says, you know, logically or biochemically speaking, a metabolite of an herb or botanical will be a metabolite of a particular constituent.
DR. BRASS: I think that is very reasonable and common sense.
DR. DWYER: Say that again.
DR. DICKINSON: For purposes of an herb or other botanical, a metabolite is understood to be a metabolite of a particular constituent.
DR. SCHIFF: Individual constituent.
DR. DICKINSON: Individual constituent.
DR. DWYER: Does everybody agree with that?
DR. BRASS: Yes, as long as it doesn't violate any legal or regulatory interpretation that has already been made. Again, there may be some history that excludes constituents from that discussion that would make this re-interpreting the regulation. I just don't know.
DR. DICKINSON: Would Dr. Moore be able to comment?
DR. DWYER: Dr. Moore, are you able to comment on that? And would you also comment on the safety issue?
DR. MOORE: I will do that one because it is straightforward relatively. The safety issue is separate because there are separate provisions of the Act to deal with safety. So, from a definitional point of view, it wouldn't need to be there.
We haven't specifically addressed the issue of things that fit into dietary substances by man, herb or botanical, which may be an entity with lots of other smaller entities within it. Arguably, once something is used--and, again, this isn't something in case law or policy, but arguably, once something is used, like an herb or botanical, it becomes a dietary substance because now it is part of the usual food or drink of man. We have interpreted that dietary substances include those things in it as dietary substances if those substances are the reason, in part or in whole, that someone would use that thing.
So, I think while we don't have nice language that is cleared by our attorneys, the concept that has been articulated, while we haven't tried it in policy, in a practical sense is consistent with how we view it.
DR. DWYER: Good.
DR. BRASS: Paul, let me try another spin on your concern because I think it is very important. Another way to think about it is, okay, let's say you can't do it. Let's say you give Y as a complex plant part and you generate X. X may or may not be a metabolite of that herb because in order to meet the second clause, the structural relationship, you are, in fact, going to have to practically identify the molecular constituent in the herb to show the product-precursor relationship. So, whether you know it, when you define Y as an herb or not to show that, in fact, there is a molecular element that is a precursor for X you are going to have to identify the constituent and show the product-precursor relationship.
DR. SCHIFF: I just think inherent in this is the big difference, when you say herb Y is a mixture, whereas when you say amino acid Y is a single--
DR. BRASS: Absolutely correct; absolutely correct.
DR. DWYER: Have we settled on that one? We will go on and then at the end maybe we can play a little bit with this and try some compounds, if we can think of some compounds.
DR. SHANNON: Can I just ask what kind of notes you took in terms of resolving the issue of the role of exogenous substances being required to produce the metabolite? I am not sure what kind of resolution we got on that issue.
DR. DICKINSON: You thought that might be irrelevant.
DR. BRASS: Well, I could think of no example but, again, Dr. Blonz' point was a fair one, that implicit is that this is metabolism that is occurring by an otherwise unadulterated body. So, if one put an enzyme mix--I can't think of an example.
DR. SHANNON: But you could ingest a substance as well as another substance that is a foreign enzyme and now you have created a metabolite. The question is would we be satisfied with that product or not?
DR. BRASS: No, because that is not a human metabolite and, again, that is the DSHEA-ness of the definition, in my opinion.
DR. DWYER: Do you have some language there that would be helpful, Dr. Shannon?
DR. SHANNON: Well, I think Dr. Brass kind of said it. We are talking about the ability of humans to create that metabolite rather than an exogenous substance that is creating that metabolite. I think we have accepted that we would include endogenous normal microflora but we would not include anything else that might be considered exogenous.
The other issue I am not sure what kind of resolution we had about was humans and whether or not this definition would refer to a reference human or does it even have to be a healthy human. If so, is it age specific, gender specific, pediatric patients in terms of kind of breadth to give that term, humans, for the purposes of defining metabolite?
DR. DWYER: Does anyone have an opinion around the table?
DR. BRASS: I think it is a really valid question. Every time I tried to get around it I came up with 16 pages worth of, you know, verbiage that ended up creating more questions than it answered. So, I think it is a valid question. I just don't know how to address it, other than to say human. Again, I could imagine a supplement being differentially metabolized in different people, for better or for worse--
DR. SHANNON: Based on age, gender, disease.
DR. BRASS: Yes, sure.
DR. SHANNON: Perhaps we could say in normal, healthy adults.
DR. BRASS: But that would exclude a unique metabolic pathway that existed in somebody who is sick who might generally benefit from the supplement.
DR. SHANNON: Or a pediatric patient.
DR. BRASS: Right, etc. As I say, I thought it hard to get more specific. From my perspective, if there were a group of people with a disease, healthy, male, female, you know, the phytoestrogens are going to be metabolized differently on a gender specific basis; children--I think that would be okay with me but I see no way around it being okay.
DR. DWYER: Weren't we saying that would all come under the safety? Dr. Moore, do you have an opinion on this? No?
DR. BRASS: Again, if a metabolite were uniquely generated in women, should it be given to adolescent boys? That again becomes a safety issue, not a definition of metabolite issue, it seems to me.
DR. DWYER: Is that, in fact, how the agency looks at that? Is that a safety issue? Is it an issue inherent in the definition? Or, are we asking you something you don't want to answer?
DR. WALKER: This is Dr. Walker. Just inherently, I think we are interested really in the broader definition of metabolite. The safety issues generally are taken care of under a different rubric. So, I think if we can confine it really to the definition of metabolite it is a very helpful discussion for us.
DR. DWYER: Let's go on to question two, which is very long. There is a shorter version up there. Consider and discuss the scientific strengths and weaknesses of the following concepts with respect to their usefulness in identifying whether a substance is or is not a metabolite of another dietary ingredient.
So, we are talking about strengths and weaknesses and the first thing they ask you to talk about is direct or indirect participation in catabolic and/or anabolic sequences. Why don't we start here for a change and go around this way? Dr. Mehendale, do you have a comment on this one, participation in catabolic or anabolic sequences?
DR. MEHENDALE: I have to think about it a little bit.
DR. DWYER: Nancy, do you have any comments?
DR. CHILDS: No comment.
DR. DWYER: Eric?
DR. BRASS: Irrelevant. Again, what specific kind of pathway is or isn't involved is not relevant to the definition.
DR. DWYER: Annette?
DR. DICKINSON: It seems to me that we have already been through these points under question two and have considered them and incorporated them into the shorter definition, have either considered them irrelevant to this definition or have included consideration of them, for example in specifying that structural elements must be incorporated.
DR. DWYER: So, what is your answer?
DR. DICKINSON: I that think a), b), c) and d) are all--
DR. DWYER: Irrelevant?
DR. DICKINSON: Well, not necessarily irrelevant, but if they are relevant they are covered by our short definition.
DR. DWYER: How about you, Dr. Shannon?
DR. SHANNON: I agree, not irrelevant but I think they are encompassed by the short definition. I will say that in terms of a), I was a little confused by both the term participation and pathway. When I first read it I was assuming it to say that if the substance appears anywhere or occurs anywhere along the metabolic pathway, but it seems that is not really what it is saying. At any rate, I do think that the issues here are encompassed by this definition we are heading towards.
DR. DWYER: So, you would put proximity, which is b), also in the same category?
DR. SHANNON: Yes.
DR. DWYER: Encompassed by the definition?
DR. SHANNON: Yes.
DR. DWYER: How about you, Dr. Schiff?
DR. SCHIFF: I completely agree with Annette. I think what we have up there and what we have talked about literally covers most of these variables, although I have a hunch that probably we are dealing with "metabolites" as more than being catabolic but we don't need to go into that.
DR. DWYER: Could you expand upon that because I found it difficult in just thinking about how I have always used the word in my own work.
DR. SCHIFF: Break down, not build up.
DR. DWYER: Correct, I have always thought of it as being catabolic rather than anabolic. I guess for the output of an anabolic process I have always used a different word than metabolite. I have used words like product for example. Maybe I am the only one but I am a little uncomfortable.
DR. SCHIFF: I don't think it is relevant to the definition as we are looking at the definition.
DR. BRASS: The definition of metabolite encompasses what you have described as products.
DR. DWYER: Dr. Blonz?
DR. BLONZ: The strengths and weaknesses of these four, I have both a) and b) as yes. They definitely play a role in the definition or metabolite and they are very well covered by our definition. For c), I do not consider a precursor to be a metabolite. So the preceding portion of c) is not there. Then, d), similarities to other ingredients, I don't think that analogous substances are metabolites as well.
I had hoped that somewhere we might talk about biofunctionality but to get into that might be delving into more specifics than the simple definition that we are hoping to come out with. I also had some desire that we talk about metabolism vis-a-vis critical control points. If you would have a metabolite that would be past a critical control point and would be treated differently, it would under this definition be a metabolite but it would be treated differently in the body.
DR. DICKINSON: I didn't understand that.
DR. DWYER: I don't think I understand it.
DR. BLONZ: There are a number of mechanisms whereby--
DR. DWYER: Well, wait, is it relevant to question two? We can discuss more at the end but really I would like to get question two nailed down because that is what we are supposed to be doing.
DR. BLONZ: Well, let's postpone it until afterwards.
DR. DWYER: Fine, bring it up at the end.
DR. BLONZ: Yes.
DR. DWYER: And your answer for d) was what?
DR. BLONZ: I would place that as a weakness because I do not feel that analogous substances can be considered metabolites.
DR. DWYER: And c)? Where did you put c)?
DR. BLONZ: Precursors, a weakness because precursors are not metabolites unless, of course, there were some sort of loop as in a Cori cycle type arrangement where a precursor could become a metabolite and then it cycles around. So, if there was a cycle a precursor could be a metabolite but that would be that circumstance. Biochemistry is very complicated.
DR. DWYER: I am still not sure we have answered this question. Am I right that the group feels that direct or indirect participation in catabolic and/or anabolic sequences or pathways is not useful in identifying whether a substance is or is not a metabolite? Is that correct?
DR. DICKINSON: No.
DR. BRASS: No. I do not find it useful, but I want to emphasize that the indirect clause is outright misleading and dangerous, and that goes to the allosteric regulation part or the hormone part, that a molecule can indirectly influence a pathway but have no structural particular precursor relationship. So, I think the indirect is absolutely inappropriate and the direct is unnecessary.
DR. DICKINSON: But the direct is incorporated.
DR. DWYER: Okay, that clarifies it a little bit. Anything else we want to suggest to the agency about direct or indirect participation? Back to the proximity, what are the strengths of proximity in terms of number of steps away?
DR. DICKINSON: The number of steps I think we have said here are not relevant, but participation in the pathway is relevant because this definition requires that it result in an increase in the end product or in the X.
DR. BRASS: Of the metabolite. It has to be a product-precursor relationship.
DR. DWYER: So, the strength is that it is product-precursor relationship?
DR. BRASS: Just to go back to some of the things Dr. Harris talked about, apparently when A goes to B goes to C goes to D, all those are metabolites of A, no matter how downstream they are, as long as they continue to be directly affected by the amount of A present.
DR. DWYER: But is that true within this context now? We are talking about trying to provide some guidance to this agency that has to administer a law on dietary supplements. It is not to define metabolite for students in pharmacology.
DR. BRASS: No, it is absolutely critical. Take a compound like a phytoestrogen, it may be sequentially metabolized to five different molecules. All of them are metabolites of the original phytoestrogen.
DR. DWYER: And what is the weakness of proximity?
DR. BRASS: Its vagueness. You can't define a number of reactions that are both necessary and sufficient.
DR. MEHENDALE: I am trying to understand why the proximity was put here. So, in trying to understand the intent I wonder if proximity was thought of because certain metabolites can interfere with other pathways, and the proximity becomes an issue where in that pathway the interference might occur. To give an example, with porphyria you can have interference very early or sometimes later in the synthesis of heme, and the product preceding the step that is interfered is excessively accumulated, excessively excreted. In that sequence then, the later the interference, the lower the adverse effects. I don't know if I am making that clear. Therefore, if there are ten steps, if you are interfering with the first step the impact is enormous. If you are interfering with step number nine the impact may not be as enormous. And, I don't know if that proximity is because of such considerations.
DR. BRASS: I think that is fair. My guess is that what the proximity was trying to differentiate is, again, net influence versus carbon tracing or atom tracing. Again the example I spoke of earlier, if you put a radiolabel in palmitate you will get a radiolabel in glucose but there is no net synthesis of glucose from palmitate. It represents just mixing in metabolic pools. So, in my mind, proximity means close enough to have a net influence on the "metabolite" not simply contributing an atom to it some place down the line. As Dr. Harris nicely illustrated, there are certain parts of metabolism where pathways pool. Acetyl CoA is a good example. So, anything that generates acetyl CoA will put carbons into a billion things even though it doesn't contribute to their net synthesis.
DR. DWYER: Is the agency able to tell us why they asked the question? Just as a person who doesn't work with some of these pathways every day, I would think that it just would make sense to the common person that something closer to a thing, absent all of the knowledge around this table, would be more likely to be the thing, or close to the thing. If it walks like a duck and talks like a duck, it is a duck.
DR. WALKER: I think you have done a good job of discussing issues related to proximity, and I think that those discussions are helpful for us. I don't think we need specific number of steps or that type of a discussion. On proximity, I think what might be helpful in the discussion of proximity relates a little bit to the structural elements. If you could discuss structural elements in the context of proximity, whether or not you think that the proximity is relevant to structural elements, that would be very helpful for us.
DR. BLONZ: So, we are really talking about anabolic versus catabolic processes. If you are dealing with structural elements where you can get a contribution to acetyl CoA which would then be a structural element that would lead to steroid hormones, cholesterol and so forth, you do have a substance that ends up being--can't physically be in there but it is a building block. So, do we want to say number of steps or are we talking about some sort of functionality issue as well? Can we separate catabolic from anabolic processes in this?
DR. DWYER: Well, the people who administer the law or the people who write the law aren't necessarily chemists so the simpler one keeps it, the better.
DR. BRASS: I agree with all the comments. This link between both net production and structural contribution--I mean, originally I had the word significant in front of structural contribution but didn't want to answer what significant meant. It is easier to visualize for catabolic because most of the molecule can be traced through. For anabolic I think this net influence component is really key. There are a whole lot of things you can give that contribute carbons to acetyl CoA that don't affect the rate of cholesterol synthesis; that don't affect the rate of anything synthesis. They just mix into that pool because the regulation is different.
I think your point earlier about are you proximally distal to regulatory steps that you put in also is intended to implicitly incorporate by this net concept. If you are proximal and you don't change the regulatory step you are not going to get any change in that flux.
DR. DWYER: On this business of catabolic versus anabolic, is everybody satisfied? I am still uneasy about that.
DR. SHANNON: I am too.
DR. DWYER: You are satisfied?
DR. SHANNON: I am uneasy.
DR. BRASS: But think about amino acids contributing to the synthesis of glutathione. Think of N-acetyl cysteine. Is glutathione or the acetaminophen conjugate, are those metabolites of N-acetyl cysteine?
DR. SHANNON: No, I wouldn't think of them as metabolites of N-acetyl cysteine. The analogy that comes to my mind is if you make catecholamine from an amino acid, is the catecholamine a metabolite?
DR. BRASS: Yes, I think it is.
DR. DWYER: Well, we don't.
DR. BRASS: Because I think you are thinking narrowly catabolically.
DR. DWYER: The problem is different scientific disciplines are looking at this differently, basically pharmacologists.
DR. BRASS: Well, that would make it easy because thinking as a pharmacologist it is only catabolic.
DR. DWYER: Ah!
DR. BRASS: But I don't think that is right in the context of what dietary supplements, vitamins and minerals do. They are both building blocks and broken down too. I think any interpretation in the context of what has been posed has to recognize that putting two molecules together may generate a metabolite as well as breaking one molecule into two different ones.
DR. DWYER: Okay, but the problem is that we are here as scientists so I am concerned that the shoemaker should be true to his last and, you know, that is how we use the term in our--
DR. BRASS: But this goes back to the DSHEA-ness of the definition. If you want be to put DSHEA aside and simply give a definition metabolite, I would give you something completely different. I wouldn't have written what I wrote on the board.
DR. DWYER: I see your point. I am still uneasy about anabolic. Could we get another opinion from any one of the three of you in terms of how we can provide assistance to your quandaries? I am looking at the table in the back.
DR. SCHIFF: Johanna, can I ask a question? If we accept the precepts of that definition with its provisos, is it mandatory that we open the catabolic/anabolic box? I don't mean to push it away or not do my duty, but I don't know that we need to do that. Am I the only one who thinks that from the definition, not from the duty standpoint?
DR. MEHENDALE: I also agree. I don't know that we really need to concern ourselves with whether it is anabolic or catabolic. In the example that Eric was discussing with Dr. Shannon, you know, it was also referred to as a synthetic reaction and even by that definition it is a metabolite because structural elements are in the product. So, I am not sure we need to concern ourselves with anabolic or catabolic so I am falling in that column as well.
DR. BRASS: Again, I don't want to pretend this definition solves all problems because you could take what I have put literally here and say superficially that every protein is a metabolite of lysing and, therefore, I can give any protein as a dietary supplement and that may or may not be a logical thing to say.
DR. DWYER: You know, as a nutritionist, I am very uncomfortable with that.
DR. BRASS: Again, that is why I originally had "significant" so that if you contribute one amino acid out of twenty no reasonable person could say that you are contributing to the net synthesis of.
DR. DICKINSON: But, Johanna, I don't understand what you are uncomfortable with because as a nutritionist what you deal with all the time is nutrients going to build body parts basically.
DR. SHANNON: Which she doesn't call metabolites.
DR. DWYER: I don't call them metabolites; I call them products.
DR. DICKINSON: Well, can you learn to call them metabolites?
What is it that you are afraid of? What is it that you see coming that we don't see?
DR. DWYER: As I said, you know, it makes the definition useless.
DR. BRASS: I think you mis-extrapolated what I said. What I said is there are obvious limitations to any scrolled up definition and as part of the legislative record, I was saying, clearly that just because an amino acid is a precursor of a protein, that should not be construed as a metabolite by this definition because there is no way to construe that a single amino acid would change the rate of synthesis of a given protein, i.e., the net production clause or, two, that any single amino acid contributes significantly to the structure of any given protein. So, what I am saying is that it required common sense interpretation. I think it contains enough specificity that with the legislative record and the thousands of word-smiths that exist in the FDA they can turn it into an interpretable clause. I was, in fact, arguing not to get hung up on these points as long as the concepts are clear to the agency who will turn it, if they want, into a regulatory context that will resemble nothing I wrote.
DR. DWYER: Well, that is their right I guess. What about possessing qualities or similarities to other dietary ingredients relative to speed/time, compartmentalization and fate?
Oh, we skipped semblance. Are there any other things that would be helpful? I think we have gone about as far as we can get on the structural business. Is that correct? What about semblance to another dietary ingredient? Do we have any guidance there in terms of strengths, weaknesses or other comments for function, structure or combination? Dr. Mehendale?
DR. MEHENDALE: Just to open it up, I might share a couple of thoughts. Semblance is similar but it can mean it actually participates in the pathway of another ingredient in this case. Or, sometimes it could also interfere with that pathway, antimetabolite. Metabolites and antimetabolites closely resemble each other. So, don't know if that was the intent there, semblance meaning either overload a pathway or interfere with a pathway.
DR. DWYER: So, you are saying the strength is that structure sometimes is associated with function?
DR. MEHENDALE: Yes.
DR. DWYER: And structure sometimes isn't associated with function?
DR. MEHENDALE: Yes, in that context those points could be relevant.
DR. DWYER: Anybody else?
DR. BRASS: No, other than, again, similarity is a trap. I think it is impossible to define and it can be misleading, as was just said.
DR. DWYER: Nancy?
DR. CHILDS: I have nothing to add.
DR. DWYER: Annette?
DR. DICKINSON: I just have a question. We talked a lot about enzymes. I am assuming enzymes might also be metabolites in that they are synthesized in the body from protein and other components and certainly an enzyme does not share the functional characteristics of the amino acids that went into it. So, function may, in fact, change in the metabolite as opposed to the precursor but I don't see that as a problem. I see that as just how the system works.
DR. BRASS: There are a lot of examples with small molecules where the native molecule--the ingested molecule is inactive; it gets metabolized; it is active, and the ingested molecule is active and the metabolite is inactive and the structural modifications can be very subtle, taking away double bond.
DR. DWYER: I don't see how with that definition, with the caveats we have here, an enzyme could be regarded as a metabolite.
DR. BRASS: That goes to the protein example that I spoke to earlier, that it would not be a reasonable extrapolation.
DR. DWYER: Because of the clauses down there.
DR. DICKINSON: It would not be what?
DR. BRASS: A reasonable example because if you ingested an amino acid you would have a hard time showing the change in net synthesis of any given enzyme.
DR. DWYER: Dr. Shannon?
DR. SHANNON: Nothing to add.
DR. DWYER: Dr. Schiff?
DR. SCHIFF: No, nothing to add.
DR. DWYER: Dr. Blonz?
DR. BLONZ: I had said on c) that I did think it was helpful in helping define it because I didn't feel that a precursor was the equivalent to a metabolite
DR. DWYER: Very good. Thank you very much.
DR. DWYER: Let's go to d) and then we will be done with this one; d), is possessing qualities or similarities to another dietary ingredient relative to speed/time, and they got into reaction rates and so forth; compartmentalization; and fate? Shall we start with Dr. Blonz?
DR. BLONZ: With this one, I had said I didn't think this is helpful because I didn't think that analogous substances would be equivalent to being a metabolite.
DR. DWYER: Dr. Schiff?
DR. SCHIFF: I think our definition handles this well. I wouldn't pick anything in particular out of here.
DR. DWYER: Are there any strengths or weaknesses there?
DR. SCHIFF: No.
DR. SHANNON: I would agree with Dr. Blonz that a weakness would be the issue of analogs.
DR. DWYER: Dr. Dickinson?
DR. DICKINSON: No comment.
DR. DWYER: Dr. Mehendale?
DR. MEHENDALE: No comment.
DR. DWYER: Dr. Childs?
DR. CHILDS: No comment. I am assuming that we have reconciled the in and outside the body issue.
DR. BRASS: Nothing further.
DR. DWYER: No pros, cons, strengths, weaknesses?
DR. BRASS: I think they are all a weakness to the point that they don't help intrinsically define what a metabolite is, can be misleading and to the degree anybody would think a chemically synthesized analog would be a metabolite would be very dangerous.
DR. DWYER: And this goes for speed and time, compartmentalization and also fate? We are all agreed that those are not particularly helpful? Is there further guidance for the whole last issue? Is there further guidance that you would like with respect to this?
DR. WALKER: I think you have covered it very well. I think what you are doing so far is very helpful to us.
DR. DWYER: Any other comments? Shall we do question three first round, then take a break and come back and finish, hopefully?
DR. BRASS: It depends on how long it is going to take. If it is quick, we might just as well finish.
DR. DWYER: You never know. Okay, discuss the scientific validity and likely usefulness for identifying when a substance is or is not a metabolite of another dietary ingredient. If so, what characteristics associated with the criterion make it valid or useful? Why don't we go around and discuss a little more or at least jot down--we think we have a definition but then what characteristics are associated with the criterion that make it valid or useful? We need some characteristics.
DR. BRASS: It emphasizes the product-precursor relationship. It emphasizes that the formation of the metabolite is the result of human ingestion. It emphasizes that there has to be a product-precursor relationship in the form of net production of the product from the precursor, and that it has to incorporate structural elements, i.e., it has to follow along the same processing metablic pathway whether it is enzymatic or non-enzymatic.
DR. DWYER: Could you give them again?
DR. BRASS: Could the transcriber read that back?
[The transcriber read back the record]
DR. DWYER: Anyone have any other suggestions? Annette?
DR. DICKINSON: No, I think it is both valid from a science point of view and I think it is also very likely to be useful in terms of putting this human scope around it and limiting it in that fashion.
DR. DWYER: Dr. Shannon?
DR. SHANNON: No comment.
DR. DWYER: Dr. Schiff?
DR. SCHIFF: I like its functionality, its flexibility but, yet, with enough scientific rigor to put flesh on it.
DR. BLONZ: My concerns have to do with the biofunctionality issue. I would like to see there be some sort of way of relating the originating substance, the Y, to the X in terms of biofunctionality.
DR. DWYER: What do you mean by biofunctionality?
DR. BLONZ: Well, when you are not dealing with a substance that all of a sudden is doing something different in the body, something that was nothing like the original substance--
DR. DICKINSON: But it is; it will be.
DR. SHANNON: It doesn't have to be.
DR. BRASS: Well, it may or may not be. Again, if Y is an inactive molecule and it is activated metabolically to an active X, would you like to exclude that from a definition of a metabolite? Their functionalities are completely different. One is no function; the other is very functional.
DR. BLONZ: That is a good point where you could get activation of a substance. My concern, of course, is for the darker side of things.
DR. BRASS: Yes.
DR. BLONZ: I wouldn't want to see products on the marketplace that might start out as benign that would fit our definition of metabolite but would end up being precursors to other substances.
DR. BRASS: I agree 10,000-fold but I don't think the definition of metabolite is a way to exclude those products from the market.
DR. SCHIFF: If we consider a lot of the herbals the biofunctionality is, at best, only modestly defined if not ill-defined, and then the exact molecules responsible are rarely defined. So, that opens the door--and I agree with these gentlemen that the dark side is a concern--but that opens the door in the definition that I would rather not open.
DR. BLONZ: Well, perhaps the important thing is to get it on the record then.
DR. DWYER: Dr. Mehendale, do you have a comment?
DR. MEHENDALE: I think the definition has scientific merit. I think it is going to be highly useful, likely to be highly useful. The agency asked us to provide some boundaries and I think in the definition we have pretty good boundaries actually. So, I like it. I think it is a good product.
DR. DWYER: Dr. Childs?
DR. CHILDS: Nothing. I think it is a useful and concise definition. I still have a little bit of a concern about it applying to all populations, particularly when it goes all the way down potentially to young children and to disease states, but I am not sure the definition is the place to address that. That might be elsewhere in the statute.
DR. DWYER: Dr. Brass?
DR. BRASS: No.
DR. DWYER: If it worked, then we could go back to the marketplace examples which are in the back of Dr. Moore's presentation this morning. One was cholesterol to steroid hormones.
DR. BLONZ: In each of those the first substance would have been a precursor, if you will, and the second one would not be the metabolite.
DR. DWYER: Let's start with some vitamins. Anybody think of a precursor? I think of a provitamin beta carotene. Does the definition work for this?
DR. SHANNON: I am sorry, what are you looking for?
DR. DWYER: I am trying to see if we can go through these examples that he gave us and whether it works.
DR. BRASS: You can eat a lot of cholesterol but you will not change your testosterone level.
DR. DWYER: So, the steroid hormone would not be a metabolite of cholesterol.
DR. BRASS: That is correct. That would be my interpretation. It falters at the second step because you don't get net production.
DR. DWYER: No net production.
DR. BRASS: I am sorry, you don't increase the amount of steroid hormone, that I know of.
DR. DWYER: What about prostaglandins and arachidonic acid?
DR. BRASS: That is a complicated one. I think the answer is probably prostaglandins are a metabolite of arachidonic acid. I don't know that much about the oral fate of arachidonic acid, whether it survives in the stomach, but even if it got into the mucosal cells of the stomach if you dump arachidonic acid on a mammalian cell it will make more prostaglandins.
DR. SHANNON: Yes, it will.
DR. BRASS: Now, whether that is the result of oral ingestion or not--I have never given oral arachidonic acid and followed its fate, but in concept it very well might be, yes.
DR. DWYER: Dr. Moore, did you give me your arachidonic to prostaglandin sequence? Thanks, Dr. Moore. Does everyone see this? Let's just look at this. Arachidonic is where?
DR. BRASS: Right in the middle.
DR. DWYER: So, we are right here and we are saying that prostaglandins are where? Down here.
DR. BRASS: No, to the right; they are all over the place.
DR. DWYER: And we are saying it is--
DR. BRASS: It may or may not be.
DR. DWYER: PG2 is a metabolite.
DR. BRASS: Here are the data I know, if you take a mammalian cell or a mammalian organ and provide it additional arachidonic acid it increases its prostaglandin production. Okay? So, that leads me to think the answer is yes. However, after oral ingestion of arachidonic acid, I don't know if it reaches enough cellular cyclooxygenase to change net prostaglandin production after oral ingestion. So, my answer is I don't know. I wouldn't be surprised if the answer was yes but I don't know that the answer is yes.
DR. DWYER: Does anyone know around the table here?
DR. DICKINSON: Yes.
DR. DWYER: Yes, you do know and it will increase it?
DR. DICKINSON: I know someone on the audience who I believe understands this pathway and he says it does. He says oral ingestion will increase it.
DR. BLONZ: I think one of the connections is with heavy meat consumption too which requires arachidonic acid. It has been shown to increase, yes.
DR. DWYER: Anything more on that particular one? What about catecholamines?
DR. BRASS: Let me point out again this is an issue where the time factor and the proximity might be of interest to some people because if the mechanism is that arachidonic acid gets incorporated into cellular phospholipids and only later comes out and does it, or something like that but, again, I think it is conceivably a yes.
DR. DWYER: What about catecholamines and amino acids?
DR. BRASS: I assume you mean a specific amino acid.
DR. DWYER: Yes, I will give you a specific amino acid, glycine.
DR. BRASS: No.
DR. DWYER: What about methionine?
DR. BRASS: How about the precursor amino acid for catecholamines? Tyrosine?
DR. DWYER: What about that?
DR. BRASS: Yes.
DR. DWYER: So, the answer is like the rabbi said, yes and no?
DR. BRASS: Well, it depends. See, for tyrosine, I don't know the answer after oral tyrosine ingestion. I only know it for in vitro and organ perfusion. I don't know if oral tyrosine changes catecholamine levels or not.
DR. SHANNON: I don't think it does.
DR. BRASS: I don't know. If it fails, it fails because of the oral ingestion clause.
DR. DICKINSON: And because there are other regulatory steps.
DR. BRASS: Yes, there are other regulatory steps but in some cases you can overwhelm those by giving enough, but I don't know; I don't know.
DR. BLONZ: Let me add in here. I don't know what would happen with tyrosine as a supplement, but this is part of the rationale for having high protein breakfasts and having hefty levels of neurotransmitters during the day, it is primarily because of the tyrosine composition. I don't know what the effect would be if you were to take tyrosine on an empty stomach and whether that would affect neurotransmitters.
DR. BRASS: As I said, I don't know either.
DR. BLONZ: It would make a difference as to what we would decide as to how it fits our definition.
DR. BRASS: I think the point is the definition would work because if it didn't, it wouldn't be and if it did, it would be.
DR. DWYER: Any other comments on amino acids? Yes, Dr. Mehendale?
DR. MEHENDALE: I think there are also examples with tryptophan. Everyone knows about the cheese and wine combination, and so on. So, I guess the answer would be yes.
DR. BRASS: But also for serotonin.
DR. DWYER: How about substrates, inosine-5 monophosphate?
DR. BRASS: I just don't know what the regulation of that pathway is and whether ingesting would change it or not. I know the pathway, I don't know the regulation.
DR. DWYER: Let's get some vitamin examples. I have been trying to think of some and I can't. Well, vitamin D, what do you think, hydroxy--
DR. BRASS: 125 is a metabolite.
DR. DICKINSON: But it is also a prescription drug and so out of balance for this purpose.
DR. BRASS: Yes, so are catecholamines.
DR. DWYER: The metabolite would be what? Vitamin A, retinal?
DR. DICKINSON: We don't make retinal but you could go from retinal to some of the vision compounds, rhodopsin.
DR. DWYER: So, rhodopsin would be a metabolite? How about a mineral example just so that we work through all of them?
DR. DICKINSON: We have all of a sudden become ignorant!
DR. SHANNON: Calcium gluconate. I guess that is organic, isn't it?
DR. DICKINSON: I mean, I can think of examples where deficiencies of minerals inhibit things, but I don't know to what degree extra minerals increase, say, some of the antioxidant enzymes.
DR. BRASS: I don't know if, you know, superoxide dismutase is sensitive to copper exposure or magnesium exposure. I don't know. I can't think of any immediately. Iron and ferritin, ferritin as a metabolite of iron. There is not a lot of structural contribution there so it would be hard-pressed to do it. That is, again, where the oral ingestion in humans came in so that, for example plutonium is not a metabolite of uranium because you just can't do it.
DR. DICKINSON: But if you give extra folic acid and B12 you change red cell synthesis. I don't know what all the intermediates are there but there are some metabolites that get created.
DR. BRASS: It is not a product-precursor relationship.
DR. DWYER: Any mineral example? How about herbal, botanical examples or constituents of those?
DR. SCHIFF: Let's see, saw palmetto is loaded with long-chain fatty acids, straight-chain fatty acids and those get chopped down and metabolized. That would be an example, I suspect.
DR. DWYER: I think we are pretty much toward the end of the end-game. Shall we just go around the table once more and try to conclude? I know, Dr. Blonz, you had a question.
DR. BLONZ: Well, I think we have covered it. That had to do with the biofunctionality and also with the idea that a metabolite would end up past a critical control point and might have an undue influence on metabolism whereby the originating product would not. This is something that, hopefully, we would deal on the safety side of things, but it is still a possibility from a biochemical standpoint.
DR. BRASS: An example of that, again, if you give methenolone you can increase cholesterol synthesis, unlike if you give the acetyl CoA precursor.
DR. BLONZ: Right.
DR. DWYER: Anyone have any comments on this for Dr. Blonz, a clarification? If not, I would just like to go around the table one last time to make sure we have any other additions or insights on all of this clarified for the record. I would also like to ask the agency to email us the final disposition of this, rather than just posting it on the web, if it is okay.
DR. BRASS: You will see in the Congressional Record of the proposed rule.
DR. DWYER: I doubt that, but if it is possible we would like to see it. Are there any other issues you wish to have us answer, Dr. Moore?
DR. MOORE: No.
DR. DWYER: Fine. Do you have other--
DR. BRASS: No, I don't. I think the discussion has been good. My final thought would be just to emphasize the concepts that have been discussed, not the specifics, because I think the concepts here are what is important, not any errors in word-smithing or mis-articulation.
DR. CHILDS: I simply want to mention, as we had the agency reinforce for us earlier, that safety is outside of the context of this definition.
DR. DICKINSON: But safety needs to be addressed.
DR. DWYER: It may be outside the context of the definition but is truly very critical.
DR. CHILDS: Yes, I agree completely. I was just simply trying to say we have addressed that. We have focused on it, and what has been brought to our attention is that that is elsewhere in the definition--not in the definition, elsewhere in the language.
DR. DWYER: Dr. Mehendale?
DR. MEHENDALE: Yes, I also want to emphasize that the definition, as we have evolved it, is really in addition to other regulatory areas.
DR. DWYER: Dr. Blonz?
DR. BLONZ: I think that the definition is very silent but, again, we are talking about DSHEA and it opens the door and it really puts the onus on the agency in a post-regulatory environment to be able to police and check for those substances that may fit the definition of a metabolite but really might have a dubious place in the marketplace.
DR. DWYER: Do you have suggestions for how best to protect the public?
DR. BLONZ: Well, to my way of thinking the best way is improved surveillance. I think they are doing as good a job as their budget will allow them but, given the surveillance with MedWatch, and so forth, you really do need to keep in consult with biochemists, toxicologists and so forth, and find out what is going on and what people are taking. It is really do the best you can. That is all I can urge.
DR. DWYER: Dr. Schiff?
DR. SCHIFF: I am pleased with the definition. It is more than I thought we could do, to be honest with you. It is functional; it is broad-based; it is moderately cohesive. I think it gives the agency a good base from which to build.
DR. DWYER: Dr. Shannon?
DR. SHANNON: As others have said, I hope that today was useful to the agency in terms of finding ways to make these products safer.
DR. DICKINSON: I share Dr. Schiff's feeling. I came into this meeting thinking we might stir around for quite a while and not come up with much, but I think we have come up with something that is useful and I think it has been a very productive discussion.
DR. DWYER: Thank you for a very useful meeting. I too am concerned about the issue of surveillance being necessary and the safety issue. I think the resources that are being devoted to the Middle East rather then the East Coast, and I work on the East Coast and live on the East Coast--I am a little concerned about the resources available to the agency to do the safety mandate. Nevertheless, we have made a lot more progress than I was afraid we were going to make with a green chairman.
Can I urge you to fill out the questionnaires that the folks at the University of Maryland gave us? If you could do it today and drop it in the box, that would be great. If not, drop it in the box when you get back to your airport.
We will look forward to getting together probably shortly for yet another session on these esoteric issues that are of such importance to the country. Dr. Walker, will you end the session?
DR. WALKER: Yes, I would like to thank you very much on behalf of our office and the agency. I would like to thank Dr. Dwyer for leading a very interesting session. I think what has been said today will be very, very useful for us, and I would like to thank you all for the time and the interest that you have shown by coming here and joining us in this discussion. So, thank you all very much.
[Whereupon, at 2:25 p.m., the proceedings were adjourned.]
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