Creatine is a naturally occurring amino acid (protein building block) that's found in meat and fish, and also made by the human body in the liver, kidneys, and pancreas. It is converted into creatine phosphate or phosphocreatine and stored in the muscles, where it is used for energy. During high-intensity, short-duration exercise, such as lifting weights or sprinting, phosphocreatine is converted into ATP, a major source of energy within the human body.
Creatine supplements are popular among body builders and competitive athletes. It is estimated that Americans spend roughly $14 million per year on creatine supplements. The attraction of creatine is that it may increase lean muscle mass and enhance athletic performance, particularly during high-intensity, short-duration sports (like high jumping and weight lifting).