Creatine (Cr) is one of the basic muscle energy stores, particularly in fast twitch glycolytic fibers. During exercise, phosphorylated creatine (PCr) is thought to be an immediate source for high energy phosphate groups with which to replenish ATP. There is some evidence to suggest that creatine only contributes significantly to ATP replenishment for the first few seconds of intense activity. Since PCr is generated from Cr by ATP, an ATP shortage will inhibit PCr synthesis.
Creatine has a normal turnover rate of about 2g/day, meaning that approximately 2 grams of creatine are irreversibly degraded to creatinine, and 2 grams synthesized to replace that. There appears to be a balance between intracellular creatine and creatine circulating in blood.
This balance can be altered slightly by creatine ingestion. Within a few days, intracellular Cr levels reach a new equilibrium level. A much smaller fraction of this additional creatine appears to be stored in the high-energy phosphocreatine form, however. Finally: the body appears to have a target level for circulating creatine, and ingestion or supplementation reduces synthesis by the body.
Creatine can be found in many forms. Muscle contains approximately 0.5% creatine by weight, although some of this will be degraded by cooking. Commercial supplements are also available. It has been suggested that human muscle has a maximum capacity of roughly 150 mmol creatine/kg muscle, making supplementation in excess of 20g/day pointless.
A number of studies have examined the effect of creatine supplementation on performance. The consensus appears to be that, while not increasing peak force production, creatine can increase the amount of work done (8%) in the first few short duration, maximal effort trials. The mechanism of this enhancement is not yet clearly documented, but is most likely by increasing the available PCr pool.