Although fats and protein contribute to energy demands of exercise, carbohydrate, principally glycogen, is the preferred fuel for muscular activity. Because of its limited storage, depletion of muscle glycogen has been shown to be one factor responsible for fatigue and exhaustion during prolonged exercise. Thus, dietary carbohydrate plays a key role in exercise performance and training. When the athlete's diet is low in carbohydrate, little glycogen is resynthesized between training sessions, leaving the individuals with low muscle glycogen and a state of chronic fatigue. The most sensitive period for glycogen resynthesis is within the first few hours after exercise. Optimal recovery from an exhaustive exercise bout depends on a reasonably rich carbohydrate diet soon after the exercise. Such feedings serve to replenish carbohydrate stores in both liver and muscles. Exertional hypoglycemia can occur when liver glucose output falls below the rate of muscle glucose uptake. Though this seldom occurs in well-fed and highly trained individuals, sugar feedings during long-term exercise has been shown to enhance performance. Thus, the important role of dietary carbohydrate before, during and after endurance activities is well established, whereas our understanding of the nutritional needs for protein and fat remain unclear.