Creatine, a substance naturally present in lean muscle tissue, is a popular food supplement some student-athletes to use to increase their muscle mass. Some of the most famous pro players in sports use it. Mark McGwire, Sammy Sosa, and quarterback Troy Aikman are among the big-name athletes who take creatine to enhance their athletic performance. The body manufactures creatine as its main fuel source for muscle activity. Creatine is most often found in skeletal muscle in the form of creatine phosphate, which helps to regenerate a molecule called adenosinetriphosphate, also known as (ATP). ATP provides energy for the body by releasing one of its phosphatemolecules, but in order for activity to be maintained, ATP must be replenished with another phosphate molecule. Creatine supplies the missing phosphate. "ATP and creatine phosphate are the body's immediate sources of energy," said Bob Girandola, a professor in exercise science. Other sources, like fats and carbohydrates, have to be metabolized first before they can be used as energy, he said. However, the body has a limited supply of ATP and creatine phosphate, and when the reserve is close to running out, another energy system called glycolysis takes over. Glycolysis produces a lactic acid as a by-product, creating a burning sensation that often keeps people from continuing a workout. Some studies said that you could experience an increased capacity for strength, power, and speed, but not for endurance. Because creatine replenishes the body's main energy source, the more one has, the more activity one can maintain. Those who take creatine supplements are able to work out longer with less fatigue. It's not the creatine itself that builds more muscle mass; it is its secondary effect. One increases muscle mass because one can do more work. One student athlete, John, used to use creatine as part of his training for the men's crew team.