Quantification of the metabolic response aids in ascertaining the nature and extent of the energy requirements imposed by exercise. During high intensity exercise, virtually all of the energy is supplied by the net oxidation of glycogen while fat oxidation plays a more prominent role during lower intensity exercise. Therefore, the lower limit of carbohydrate required above resting needs is equal to the portion of the total energy cost derived from carbohydrate sources. There is no upper limit of additional carbohydrate intake that could be eaten to satisfy the extra caloric requirement since carbohydrate intake will restore any endogenous energy stores that were used during exercise, regardless of the intensity of exercise. The recommendation of a high carbohydrate intake to provide caloric balance in exercising individuals is supported by the observation that exercise performance at high intensity is improved by a high carbohydrate diet, and exercise performance at low intensity is relatively insensitive to the source of the caloric intake. Limited dietary studies are consistent with predictions based on the metabolic response. At exercise intensities below 65% VO2 max, the percent fat and carbohydrate in the diet makes little difference on exercise performance, provided adequate time is allowed to adapt to a high-fat diet. On the other hand, exercise ability during high-intensity exercise is significantly limited by a high-fat diet. A consideration of importance beyond the aspect of energy balance is the anabolic effect of insulin on muscle protein synthesis after exercise. Provision of carbohydrate after exercise is likely to stimulate muscle protein synthesis to a greater extent than a corresponding amount of fat. Dietary fats may offer practical advantages to the athlete but if fats are consumed at the expense of carbohydrate intake, many established benefits of high carbohydrate intake in terms of performance may be sacrified.