Creatine

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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.

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