Creatine is a compound, synthesized from the amino acids arginine, glycine, and methionine, which occurs in muscle. Combined with phosphoric acid, in the form of creatine phosphate (or phospho-creatine), it is an important reserve of energy for muscle contraction, which is released when creatine phosphate loses its phosphate and is converted to creatine. The creatine is then excreted in urine (at a rate of 1.2–1.5 grams per day in humans).