Creatine, is a naturally occurring non-protein compound. It belongs to the class of organic compounds known as alpha amino acids and derivatives. These are amino acids in which the amino group is attached to the carbon atom immediately adjacent to the carboxylate group (alpha carbon), or a derivative thereof. Creatine is found in all vertebrates where it facilitates recycling of adenosine triphosphate (ATP). Its primary metabolic role is to combine with a phosphoryl group, via the enzyme creatine kinase, to generate phosphocreatine, which is used to regenerate ATP. Most of the human body's total creatine and phosphocreatine stores are found in skeletal muscle (95%), while the remainder is distributed in the blood, brain, testes, and other tissues. Creatine is not an essential nutrient as it is naturally produced in the human body from the amino acids glycine and arginine, with an additional requirement for methionine to catalyze the transformation of guanidinoacetate to creatine. In the first step of its biosynthesis glycine and arginine are combined by the enzyme arginine:glycine amidinotransferase (AGAT) to form guanidinoacetate, which is then methylated by guanidinoacetate N-methyltransferase (GAMT), using S-adenosyl methionine as the methyl donor. Creatine can also be obtained through the diet at a rate of about 1 gram per day from an omnivorous diet. A cyclic form of creatine, called creatinine, exists in equilibrium with its tautomer and with creatine. Clinically, there are three distinct disorders of creatine metabolism. Deficiencies in the two synthesis enzymes (AGAT and GAMT) can cause L-arginine:glycine amidinotransferase deficiency (caused by variants in AGAT) and guanidinoacetate methyltransferase deficiency (caused by variants in GAMT). Both disorders are inherited in an autosomal recessive manner. A third defect, creatine transporter defect, is caused by mutations in SLC6A8 and inherited in a X-linked manner. Creatine is widely used as a supplement by athletes. Its use can increase maximum power and performance in high-intensity anaerobic repetitive work (periods of work and rest) by 5 to 15% (PMID: 24688272). Creatine has no significant effect on aerobic endurance, although it will increase power during short sessions of high-intensity aerobic exercise (PMID: 9662683).