The author of NHE - Rob Faigin - has this response when asked about the carbohydrate content of glyerol (the product used in most low-carb bars to keep the bar moist). I thought it was pretty good (and funny), so I thought I'd share it.
Low-carbohydrate meal replacement bars are the fastest growing segment of the multi-billion dollar dietary supplement industry. This fact coupled with the presence of glycerol, which enhances taste and texture, in virtually every brand makes your question a matter of widespread public concern - as evidenced by the letters I've received. You and others are reasonably suspicious of a substance that constitutes calories, but is listed neither as a protein, nor a fat, nor a carbohydrate. And, as you note in your letter, you are frustrated by the fact that available sources of information (or people willing to talk forthrightly) about the metabolic implications glycerol are scarce. I'm prepared to break the ice.
First, there is nothing unnatural or inherently hazardous about glycerol (sometimes appearing as "glycerin" or "glycerine" on package labels). Glycerol is a component of trigyceride (fat molecule). Specifically, a triglyceride consists of three fatty acids linked by an "ester bond" to a molecule of gycerol, which serves as the "backbone" of this fork-like structure (it looks like this ). During lipolysis (a term you should be familiar with from NHE), the fatty acids are cleaved from glycerol. This chemical reaction, which is the first step in the process of releasing energy from fat, is catalyzed by the enzyme lipase (which is activated by hormones) and requires three molecules of water.
lipase
Triglyceride + 3H2O --> Glycerol + 3 fatty acids
The fatty acids then travel through the bloodstream to muscle, where they are burned for energy. What happens to glycerol? That is, quite literally, the million-dollar question.
There are three potential metabolic fates for glycerol in the body. First, it can be "reesterified" back into triglyceride. Humpty dumpty (a fitting name for a fat molecule) had a great fall when it was taken apart by lipase enzymes, but in this case Humpty Dumpty can be put back together again. Another possibility is that glycerol can be converted to pyruvate and burned for energy. The third possibility is that glycerol, by donating its carbon atoms, can be converted to glucose (sugar). Technically, this makes glycerol a "gluconeogenic precursor." So one thing we know for sure is that, although not listed as a carbohydrate, glycerol can be used by the body to make carbohydrate. I'm sure this is not what consumers of low-carb bars want to hear. But let's look at this issue in greater depth.
First, it should be noted that glycerol has virtually no effect on insulin release when consumed. This is undoubtedly a favorable attribute. However, recall from NHE that this is not the whole story. (Fructose evokes the smallest postprandial insulin response of any carbohydrate; but it has a comparatively adverse impact on fat burning for other reasons, see NHE p. 145.) The more important issue is to what extent glycerol converts to sugar and inhibits fat burning, a subtler question requiring a more sensitive analysis.
Because glycerol is so versatile - it can be used to make either triglyceride or glucose, or it can immediately be burned for energy - its fate is dictated by the prevailing metabolic milieu. For example, if someone who is on a high-carbohydrate diet (i.e., in a sugar-burning state) eats a high-glycerol, low-carb bar, he/she might as well be eating a higher-carbohydrate bar because the body is going to utilize the glycerol to fulfill its ravenous sugar needs. By contrast, if a fat-burner eats a high-glycerol, low-carb bar, the conversion of glycerol to glucose will be much less. Accordingly, I recommend refraining from consuming glycerol during NHE's metabolic-shift period. After that, I do not prohibit these bars because I deem their marginal gluconeogenic effect offset by their exceptional convenience. Having said that, some measure of compensation in terms of carbohydrate consumption is appropriate where glycerol is consumed in large quantity. For example, if you are eating more than one low-carb bar per day you should aim for the mid-to-lower part of the NHE Eating Plan's downcycle range. (If you are curious to know how much glycerol a food contains, multiply carb grams by 4, add this to protein grams times 4, add this to fat grams times 9 - then subtract this sum from calories. This will tell you how many calories worth of glycerol your low-carb bar contains. Then, if you divide this amount by 4.32 you'll get grams of glycerol.)
You suggested that this might be a case of "clever marketing" on the part of the supplement companies. It is unquestionably clever, but whether it is unethical is not so clear. At first blush, it seems reprehensible that these companies know that the amount of carbohydrate reflected on the label is, though technically accurate, a practical understatement - and allow this situation to persist. However, as is often the case with ethical matters, a more searching examination muddies the waters of moral certainty and makes more difficult a categorical declaration of right or wrong.
To a large extent, whether or not the supplement companies' policy regarding glycerol is ethical entails an inquiry into whether they have a duty to be honest or candid.
Here's my conceptualization of these two words.
Honesty: Saying that which is factually true.
Candor: In addition to being honest, preemptively resolving a false inference likely to be drawn from a factually true statement.
Examples
Honesty: I've never lost a fight.
Candor: I've never lost a fight, nor have I ever been involved in one.
Honesty: I got straight A's in high school.
Candor: I got straight A's in high school until I dropped-out during the first semester of my sophomore year.
Honesty: During a six-month period in which I took "Fat Away" pills I lost 20 pounds.
Candor: During a six-month period in which I took Fat Away pills and began exercising I lost 20 pounds.
Honesty: I looked both ways before turning.
Candor: I looked both ways before turning south onto a one-way street running north.
Honesty: I once went out to dinner with Julia Roberts.
Candor: I once went out to dinner with Julia Roberts, my grandmother's 96-year-old friend.
Honesty: I didn't hear the phone ring.
Candor: I didn't hear the phone ring when I was five miles away from the phone at the saloon drinking with my friends.
Honesty: I look back on my Harvard days with great fondness.
Candor: I look back on my Harvard days with great fondness, because I enjoyed working as a janitor.
Honesty: I have not yet completed all the tasks that you assigned to me.
Candor: I have not yet completed all the tasks that you assigned to me, nor have I completed any of them.
Aside from the honesty/candor issue, it is, for the reasons explained above, impossible to quantify how much carbohydrate a given amount of glycerol supplies. Therefore, even if the supplement companies were to conceive their duty to the public in broader terms than literal truthfulness and were inclined candidly to disclose information that would help the public but not necessarily increase profits, how would they do so in this case? They would have to say something like: "glycerol, though not a carbohydrate, can be converted to carbohydrate at an indeterminate and variable rate." This same statement applies to gluconeogenic amino acids, of which there are many. Because many substances can be converted to other substances by the body, trying to represent precursor potentials can become unwieldy. For this reason, and bound by my oath to be fair and impartial in my judgments, I grant the supplement companies a pass on this one.
References
Trimmer JK, et al. Autoregulation of Glucose Production in Men with a Glycerol Load during Rest and Exercise. Am J Physiol Endocrinol Metab 2001;280:E657.
Jahoor F, Peters EJ, Wolfe RR. The Relationship between Gluconeogenic Substrate Supply and Glucose Production in Humans. Am J Physiol 1990;258:E288.
Landau BR, et al. Glycerol Production and Utilization in Humans: Sites and Quantitation. Am J Physiol 1996;271:E1110.
Maughan RJ, Gleeson M. Influence of a 36 h Fast Followed by Refeeding with Glucose, Glycerol or Placebo on Metabolism and Performance during Prolonged Exercise in Man. Eur J Appl Physiol Occup Physiol 1988;57:570.
Nurjhan N, et al. Insulin Dose-Response Characteristics for Suppression of Glycerol Release and Conversion to Glucose in Humans. Diabetes 1986;35:1326.
Gleeson M, Maughan RJ, Greenhaff PL. Comparison of the Effects of Pre-Exercise Feeding of Glucose, Glycerol and Placebo on Endurance and Fuel Homeostasis in Man. Eur J Appl Physiol Occup Physiol 1986;55:645.
Miller JM, et al. Effect of Glycerol Feeding on Endurance and Metabolism during Prolonged Exercise in Man. Med Sci Sports Exerc 1983;15:237.
Baba H, Zhang XJ, Wolfe RR. Glycerol Gluconeogenesis in Fasting Humans. Nutrition 1995;11:149.
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