Creatine

Creatine is a nitrogen-containing organic compound involved in rapid cellular energy transfer. The body obtains creatine through both endogenous synthesis and the diet. It is synthesized from amino-acid-derived precursors, principally arginine and glycine, with a methyl group supplied through methionine metabolism. Creatine is distributed to tissues with substantial energy demands, particularly skeletal muscle and the brain .^1,2

Inside cells, creatine participates in the creatine kinase system. The enzyme creatine kinase reversibly transfers a phosphate group from ATP to creatine, producing phosphocreatine. When energy demand rises, phosphocreatine can rapidly transfer this phosphate to ADP to regenerate ATP. The creatine-phosphocreatine system therefore acts as a short-term energy buffer and helps transfer high-energy phosphate between sites of ATP production and ATP use .^1,2

Creatine exists in cells as both free creatine and phosphocreatine. Together, these forms make up the cellular creatine pool. A small proportion of creatine and phosphocreatine spontaneously converts each day to creatinine, which is eliminated from the body mainly in urine. This continuous loss is replaced by newly synthesized and dietary creatine .²

Creatine and creatinine are therefore distinct compounds. Creatine is an active participant in cellular energy metabolism, whereas creatinine is a breakdown product formed spontaneously from creatine and phosphocreatine. Creatine should also be distinguished from phosphocreatine, which is specifically its phosphorylated, energy-buffering form .^1,2