Creatine
Creatine, substance in the bloodstream of vertebrate animals that is used for supplementary energy transport in certain body systems. (Arganine serves a similar role in invertebrate animals.) Adenosine triphosphate , or ATP, is the main immediate source of usable energy in the body, but brain , heart , and muscle cells require enormous amounts of energy that creatine also helps to supply. Produced in the kidneys and liver , the chemical is absorbed into cells and picks up the high-energy phosphate group from excess ATP at cellular bodies called mitochondria, then shuttles back and forth to other cells to supply the energy.