Answer: creatine

AnswerCrossword Clue
CREATINEAmino acid supplement popular with bodybuilders
CREATINEPopular performance-enhancing supplement for athletes
creatineamino acid, C4H9N3O2, that is a constituent of the muscles of vertebrates and is phosphorylated to store energy used for muscular contraction
creatinea chemical found within vertebrate muscle, also CREATIN, KREATINE