During exercise, the body uses both glucose and fat to supply the demands of energy. The proportion of fat to glucose changes with the intensity of the exercise as fat is used as the primary energy source. Most athletes do not need a special diet as long as they eat a balanced diet that includes a variety of nutrients from all basic food groups. The difference between a sensible diet for a normal person versus an athlete is the total caloric intake number required during physical activity, especially if it's prolonged.