Eat More Carbohydrate!

The ability to exercise for long periods is largely dependent upon the availability of energy sources, typically carbohydrates and fats. Fat is the primary fuel for low intensity exercise and is stored in large amounts within the body, while carbohydrate fuels higher intensity exercise and is stored only in limited sites as glycogen. It is well documented that fatigue during endurance sports is closely related to depletion of carbohydrate within muscle and that increasing carbohydrate intake increases endurance. Much, although not all, research also suggests that high fat diets are associated with cardiovascular disease, some cancers, and depressed immune function. These facts, taken together, have led exercise professionals to recommend high carbohydrate diets to seriously training athletes.

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No, Eat More Fat!

There is a line of evidence, however, which suggests that seriously training athletes need not be so restrictive of their fat intake. Many of the previously mentioned studies have used sedentary or moderately trained research subjects. Well-trained endurance athletes demonstrate enhanced ability to use fats for energy at rest [Calles-Escandon] and during exercise [Karlsson].

Muscle glycogen levels are directly related to exhaustive exercise. It is apparent that one of the main effects of training is to spare glycogen by enhancing utilization of fats. Consistent with this idea is the significant depletion of intramuscular fats during strenuous exercise and the increase in enzymes which control fat oxidation in trained subjects [Karlsson].

Endurance ability has not been adversely affected by high fat diets in some studies [Phinney], and in fact high carbohydrate diets may lead to greater utilization of carbs during exercise [Jacobs], thus obliterating the "sparing" of carbohydrates that training generally produces. The possibility that increasing fat in the diet might enhance fat utilization during exercise therefore is intriguing. Work done at the University of Buffalo attempted to determine whether sedentary individuals differed from well trained subjects in their ability to use fats during high intensity exercise. Indeed, well trained subjects were better able to utilize fats, thus maintaining work output despite glycogen depletion [Pendergast]. Furthermore, trained runners given a moderate to high fat diet [35-55% of total calories] for 2 months improved running performance, presumably by increasing fat stored in the muscle and by lower lactate production [Pendergast].

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A Happy Medium...

So what do we make of this data? Do we now suggest that serious endurance athletes switch to high fat diets? Let common sense prevail. There has undoubtedly been an over emphasis by athletes on fat restriction. In doing so many have minimized a great variety of foods, including vegetables, meats and dairy, replacing them with breads and pasta, sports drinks and bars, nonfat and nondairy desserts, etc. The resulting diet is low in protein, vitamins, minerals as well as fat. It is unwise, however, to ignore the vast body of literature which relates high fat diets to heart disease and other diseases. In terms of exercise, it is important to remember that glycogen depletion is still related to fatigue and the diet should be constructed to maintain adequate muscle glycogen levels. Serious endurance athletes would best be served by focusing on whole grains, lean meats and dairy, generous portions of vegetables and fruits. This typically results in a nutrient-rich diet, approximately 30% fat, 15% protein and 55% carbohydrate. As long as the athlete is weight stable, that is, eating enough calories to avoid depletion and muscle wasting, good performance results and good health will be optimized.

For more info on this topic, check out our Sports Nutrition Corner.

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References

Exercise increases fat oxidation at rest unrelated to changes in energy balance or lipolysis.

Calles-Escandon J, Goran MI, O’Connell M, Nair KS, Danforth E.

Am J Physiol, 1996, 270:6[1];e1009-14

Dietary effects on lipoprotein lipase activity in skeletal muscle in man.

Jacobs I, Lithell H, Karlsson J.

Acta Physiol Scand 115:85-90, 1982

Body fat and exercise endurance in trained rats adapted to a high -fat and/or high-carbohydrate diet.

Lapachet RAB, Miller WC, Arnall DA.

J Appl Physiol 80[4]:1173-1179, 1996.

Effect of dietary fat on metabolic adjustments to maximal VO2 and endurance in runners.

Muio DM, Leddy JJ, Horvath PJ, Awad AB, Pendergast DR.

Med Sci Sports Exerc. 26[1]:81-88, 1994

The human metabolic response to chronic ketosis without caloric restriction: preservation of submaximal exercise capability with reduced carbohydrate oxidation.

Phinney SD, Bistrian BR, Evans WJ, Gervino E, Blackburn Gl.

Metabolism 32:769-776, 1983