American Journal of Clinical Nutrition, Vol 25, 85-90, Copyright © 1972 by The American Society for Clinical Nutrition, Inc.

Dietary carbohydrate and work capacity

¹ From the Bioenergetics Division, United States Army Medical Research and Nutrition Laboratory, Fitzsimons General Hospital, Denver, Colorado 80240

Although the requirements for carbohydrate have not been established, there is sufficient evidence that the adult body requires at least 100 g/day for essential functions of the neurological system and other tissues. If carbohydrate is not provided in the diet, it is then synthesized in the body mainly through the deamination and conversion of amino acids.

Carbohydrate appears to be the preferred energy source for the working muscles, although it can be replaced by fat during starvation periods or during carbohydrate-deficient intakes. The ratio of the carbohydrate to fat utilized in the production of energy for the working muscles rises with an increase in work intensity or an increase in dietary carbohydrate. This has been demonstrated repeatedly by the studies of RQ changes during work, and by variations in nutrients in the arterial and venous blood that supplies the working muscles.

Muscle glycogen stores can be altered by changes in the diet and an increase in glycogen stores increases endurance during strenuous physical activity. Muscle biopsies have shown that glycogen stores were depleted during exhausting work and that when these levels were critically low, the individual was unable to continue work. Free fatty acids and triglyceride levels in the muscle also decreased but did not fall to critical levels, which are associated with exhaustion.

Carbohydrates have been found to be 4 to 5% more efficient than fat as an energy source for the working muscles. This difference may appear to be minimal, but it could often mean the difference between winning or losing for an athlete.

An increased protein intake for athletes has been the general popular concept of trainers and coaches, but scientific evidence indicates that protein is not consumed by the working muscles. An athlete in training will consume a greater quantity of food to remain in body weight equilibrium, which will result in an increase in protein intake for muscle synthesis during this period. An increase in protein may be required during periods of combined body weight reduction, physical conditioning, and growth.