It is synthesized in the liver and kidney and its final breakdown product is creatinine, a substance that is entirely excreted by the kidney. Creatinine is unique because it is neither reabsorbed or secreted by the kidney and because of this property its excretion rate is used to measure kidney function. Energy production by all cells, including muscle, originates in a process called oxidative phosphorylation. The breakdown of ATP produces energy which is then used to drive reactions needed for cell survival. ATP is also used as the energy source of muscle contraction.
Creatine accepts a high energy phosphate from oxidative phosphorylation and becomes phosphocreatine.
This phosphagen acts as a storage form of high energy phosphate. On the other hand, when ATP is plentiful the phosphocreatine concentration can build up to act as a store of high-energy phosphate. Creatine is converted to phosphocreatine by the enzyme phosphocreatine kinase. It has been postulated that by increasing the amount of free creatine in the diet one could increase the amount of phosphocreatine in muscle which would then provide better availability of high energy phosphate for energy production during muscle contraction. It is therefore more likely that if creatine supplementation has an effect it would only be seen during a brief, anaerobic exercise.