Dietary creatine from a typical mixed diet is equivalent to around 1 gram per day with a further 1 gram per day synthesised mainly in the liver and kidney. Creatine is synthesised from the amino acids glycine, arginine and methionine. The rate limiting step in the formation of creatine, the enzyme arginine:glycine amidino-transferase (AGAT) prevents synthesis rates exceeding 1 gram per day. Once produced, creatine negatively inhibits AGAT to slow further creatine formation, and synthesis of creatine is also regulated by thyroid hormone, growth hormone, testosterone, ornithine, vitamin E deficiency and fasting. Diet and endogenous synthesis equals the phosphocreatine degradation rate of around 2 grams per day. Any intake over 1 gram per day leads to accumulation of creatine, which is distributed 95 % in skeletal muscle and 5 % in the brain. Total body stores of creatine for a 70 kg male are equivalent to 120 grams. Creatine is excreted through the kidney and dietary creatine increases excretion and blood creatine levels.
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