Glycerin
Chester J. Zelasko, Ph.D. | January 22, 2002

The health headlines read, "Manufacturers of energy bars caught mislabeling products!" Sensational headline, but does it have any substance? Let's begin with the nutrient at the center of the controversy--glycerin.

Glycerin is a three-carbon molecule usually found in the body as a by-product of the breakdown of triglycerides. Triglycerides contain one glycerol molecule (chemically identical to glycerin) and three long-chain fatty acids. This is the way most fat is transported in the bloodstream and the form in which it's stored. When your body needs to use fat as a fuel, the triglyceride is broken down and all parts are metabolized as fuel--including the glycerol. It usually enters metabolism in the citric acid cycle (also known as the Krebs cycle). This is the metabolic process that occurs in the mitochondria and after glycolysis.

Glycerin is also used in the manufacture of foods such as energy and protein bars. It adds moisture to the bars and is also slightly sweet to the taste (the name comes from glykeros, the Greek word for sweet). Exogenous glycerin does contain calories--about 4 per gram--so it will be used for energy. The advantage is that it will not induce a significant glycemic response.

Glycerin has other uses. Athletes have used glycerin to help maintain hydration levels during long-term exercise. Simply put, glycerine helps retain water, thereby preventing dehydration (1). Under the right conditions, such as starvation, glycerol could be used to make glucose via gluconeogenesis, but it is a minor metabolic pathway in a reasonably fed individual. Even on a weight loss program, as long as an individual were ingesting at least 1,000 calories per day, this would be an insignificant source of glucose production.

So why the controversy? Most manufacturers did not put glycerine on the label as a protein, carbohydrate, or fat because it seems to be in-between macronutrient categories. If you multiplied the number of grams of protein and carbohydrate by 4 kcals and the number of grams of fat by 9 kcals and added them up, it wouldn't result in the total calories listed on the label. Most manufacturers stated on the labels that the remaining calories were from other sources such as glycerin or maltitol (a sugar-alcohol sweetener). However, the Food and Drug Administration has ruled that glycerin is a carbohydrate and as such, it must be added to total grams of carbohydrates on the product labels. As a result, the label of your favorite protein or energy bar may suddenly seem to have jumped in grams of carbohydrate. It hasn't really changed. The manufacturers have no choice but to comply.

This is an example of trying to put a square peg in a round hole. Scientists will argue about whether glycerine is a carbohydrate or not. Because it doesn't appear to affect blood glucose levels, it is not. Rather, it appears to be a category unto itself. The controversy is not really about what glycerine is or how it is used, but rather how it's portrayed on a food label. Given that it took almost a decade for the last change to occur and that trans-fatty acids are still not listed on nutrition labels, don't look for this to be resolved any time soon.

Bottom line: If you like these types of products as part of your weight management program and they are effective for you, use them--regardless of what the label says.

References:

  1. Robergs RA, Griffin SE. Glycerol. Biochemistry, pharmacokinetics and clinical and practical applications. Sports Med 1998. 26(3):145-67.
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