When muscle glycogen runs low, a greater proportion of fat is used for energy. During a diet and exercise program, an athlete may gain muscle while losing fat. So the athlete may not lose weight. Since muscle is more dense than fat, the athlete's clothing should fit more loosely. Body fat percentages below these values impair athletic performance, as well as endanger normal body physiology.
Athletes can train their muscles to store more glycogen and to use their limited supply sparingly.
A balanced diet rich in complex carbohydrates that allows their the glycogen supplies to be replenished completely, along with both anaerobic and aerobic training are important.