‘Forbidden’ Fruit

We are constantly told that fruit is good for us, and that it should comprise a healthy share of our daily diet. Yet, research indicates that there may be some drawbacks to consuming large amounts of fruit’s natural sweetener, fructose.

According to the USDA, the per capita consumption of fructose equates to 10% of the total energy intake in the typical American diet.

Gram for gram, fructose is 20 times sweeter than normal table sugar. It is a major ingredient in candy, soft drinks, and as a major component of table sugar. Promoted as the “healthier” sugar, health conscious Americans have continued to increase their consumption of fructose without understanding the consequences. The problem with overconsumption of fructose is in its unique metabolism by the liver. Because of its molecular structure, fructose bypasses a key regulatory step of glycolysis resulting in several rapid changes in fat and carbohydrate metabolism. By allowing glycolysis in the liver to progress in an uncontrolled manner, fructose causes an immediate increase in pyruvic acid and lactic acid and an activation of an enzyme, pyruvate dehydrogenase. This, in turn, produces a shift from fatty acid oxidation for energy to fatty acid esterification in preparation for release from the liver in the form of VLDLs (very low density lipoproteins) which are commonly stored in fat cells or are responsible for artery clogging plaque.

Chronic overconsumption of fructose will also cause enzymatic adaptations promoting fat formation and VLDL production and secretion rather than glycogen formation. This results in higher cholesterol/triglyceride levels (triglyceridemia), poor glucose tolerance, and oversecretions of insulin (hyperinsulinemia). So does this mean that fruit lovers are doomed to high cholesterol levels, increased body fat, and lower energy levels? Probably not. Research shows that the body’s capacity for efficiently absorbing fructose from the intestines to be something less than 50 g (200 kcal) at any one time.

Chances are small amounts of fruit each day are not going to cause severe or adverse affects. However, it’s probably a good idea to avoid combinations of a great deal of table sugar, soft drinks, and large quantities of fruit on a regular basis. It should also be noted that exercise has been shown to reduce the efficiency by which fructose has been absorbed just as with any other simple sugar, so that piece of fruit or high fructose carb drink right before or during your workout might not be such a good idea. Don’t get the wrong idea. Fruits are obviously healthy.

But, as with all good things, should be taken in moderation. The fact that fructose rates low on the glycemic index is an indicator that it is not broken down very efficiently. The question would then be… Is this a bad thing? The answer is no.

The wrong thing to do, however, would be to frequently ingest large quantities of any food that is absorbed slowly. This contributes to poor digestion.

 Fujisawa, T., et al. The effect of exercise on fructose absorption. Amer J Clin Nutr. July ’93 p75(5).

Lamb, DR. Physiology of Exercise: Responses & Adaptations. 2nd edition. MacMillan Publishing Company, New York. 1984

Mayes, PA. Intermediary metabolism of fructose. Amer J Clin Nutr. Nov. ’93 p754S(12). Metabolic effects of fructose. Nutrition Research Newsletter. v11. April ’92 p51(2).

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These resources are for the purpose of personal trainer growth and development through Continuing Education which advances the knowledge of fitness professionals. This article is written for NFPT Certified Personal Trainers to receive Continuing Education Credit (CEC). Please contact NFPT at 800.729.6378 or [email protected] with questions or for more information.