Carbohydrate Supplementation and Heat Fatigue

Consuming carbohydrates during exercise, under conditions of high heat, improves performance by lengthening the time before fatigue occurs, according to a new study in Pflugers Archives—European Journal of Physiology (2003;446:211–9).

Prolonged exercise in cool to moderate temperatures depletes carbohydrate stores in the liver and muscles. Because carbohydrates are an important source of fuel during exercise, carbohydrate depletion is associated with the experience of fatigue. Carbohydrate supplementation during exercise delays the depletion of carbohydrate stores and has been shown in several studies to increase exercise capacity in moderate temperatures by lengthening the time of exercise before fatigue.

In higher temperatures fatigue is experienced before carbohydrate stores are depleted. It has been suggested that some biochemical change other than carbohydrate depletion, brought on by prolonged exercise in the heat, is responsible for the experience of fatigue and the inability to continue exercising. A number of studies have found that carbohydrate supplementation increased exercise capacity in people exercising in the heat, despite the evidence that carbohydrate depletion is not the cause of fatigue under these conditions. Other studies have found no change.

In the current study, eight athletic men were observed during prolonged periods of moderate and high intensity exercise in a chamber kept at 35? C (about 90? F). Each participant received a drink with maltodextrin (6.4%), a source of carbohydrate calories, during one session of moderate and one session of high intensity exercise. A placebo drink, containing no maltodextrin but otherwise identical to the carbohydrate drink, was provided during another session of each type of exercise. Participants exercised until fatigue occurred, or until reaching a core temperature of 40? C (about 104? F) at each session.

Exercise performance was better with the carbohydrate drink in both the moderate and high intensity sessions. Time to fatigue during moderate intensity exercise was almost 15% longer with the carbohydrate drink than with placebo, averaging 145 minutes with carbohydrate and only 123 minutes with placebo. Time to fatigue during high intensity exercise was 61 minutes with carbohydrate and 51 minutes with placebo, a 14% increase.

The results of this study show that carbohydrate supplementation during prolonged exercise in hot conditions can improve exercise performance by lengthening time to fatigue. The physiologic mechanism for the onset of fatigue in the heat is not known.

Maureen Williams, ND, received her bachelor’s degree from the University of Pennsylvania and her Doctorate of Naturopathic Medicine from Bastyr University in Seattle, WA. She has a private practice in Quechee, Vermont, and does extensive work with traditional herbal medicine in Guatemala and Honduras. Dr. Williams is a regular contributor to Healthnotes Newswire.

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