There are three major bioenergetic systems that regenerate ATP: phosphagen, glycolytic and aerobic system. Proteins, fats and carbohydrates are possible substrates for the formation of ATP spent in the muscle contraction. All of the above substrates can be used during exercise; however, the intensity and duration of activity determine the relative rates of substrate utilization. A basic aspect of training and performance optimization through nutrition is consumption of enough calories to compensate for increased energy expenditure. As a primary source of energy, muscles use carbohydrates stored in the form of glycogen. Adequate carbohydrate intake through the diet is important for glycogen recovery. Protein is used during exercise to repair and build muscle protein and, to a lesser extent, serves as a source of energy. Athletes involved in intense training can benefit from ingesting approximately twice as much RDA protein in their diet to maintain protein balance. Maintaining energy balance, replenishing intramuscular supplies of triacylglycerides, and adequate consumption of essential fatty acids are important for athletes, all of which is obtained through dietary fat intake in no more than the usual amount of fat as recommended for general population.