a quote from "medscape"
"Creatine
Creatine is an amino acid compound produced naturally in the body.[15,48,91,105,106] Enzymes that synthesize creatine can be found in the liver, kidney, and pancreas. Creatine is synthesized from the essential amino acids arginine and methionine and the nonessential amino acid glycine, and it is also a product of muscle metabolism. In addition, creatine is obtained in the diet, largely from meat and fish. The vast majority of creatine in the body is stored in skeletal muscle where it is partially converted to creatine phosphate. There are several mechanisms whereby muscle cells produce adenosine triphosphate (ATP), which provides the energy for muscle contraction. In one biochemical pathway, the phosphorylation of adenosine diphosphate (ADP) to ATP by creatine phosphate provides initial, quick energy needs as demonstrated by the following equation[56]:
ADP + Creatine Phosphate Creatine Kinase ATP + Creatine
The amount of ATP generated via this pathway is dependent on the initial concentration of creatine phosphate. Hence, the underlying concept of using creatine as a performance-enhancing supplement is to increase the muscle stores of creatine and creatine phosphate for producing and replenishing ATP. However, the above biochemical pathway only provides the initial energy for the first few seconds of muscle contraction.
Creatine, taken to improve athletic performance, has become one of the most popular dietary supplements (excluding vitamin and mineral supplements) among both amateur and professional athletes.[26,47,107] It has also become popular with middle school and high school athletes for a variety of sports, including cheerleading.[108] Promoters of creatine supplementation claim that it increases the production of energy, promotes the growth and strength of skeletal muscle, reduces recovery time, increases stamina, and buffers lactic acid build up. Although creatine is not considered an anabolic agent, some of these claims -- directly or indirectly -- have been supported by controlled studies in specific and limited exercises. Creatine supplementation has most consistently been shown to be an aid in brief (< 30-60 seconds), intermittent, high-burst, anaerobic exercise, which would benefit sprinters in running, cycling, swimming, and rowing, and perhaps power events such as weight lifting and throwing.[15,48,109,110] Subjects with low creatine stores (e.g., vegetarians or low-protein diets) may gain the most benefit from creatine supplementation.[111] This may be one explanation as to why some subjects have shown improvement in performance while others have not. Furthermore, trained and conditioned athletes, who already may have maximized their creatine stores and work output, may not gain significant additional benefit from supplementation, but this has yet to be elucidated.
The ergogenic effect of creatine on endurance or aerobic exercises (e.g., long distance running, swimming, cycling, rowing) is less clear and inconsistent.[48,105,112] These findings can be partially attributed to the understanding that other biochemical pathways are involved in generating energy, while creatine phosphate becomes a minor source of ATP production after 10-20 seconds of maximal anaerobic work.[15,48] With respect to promoting the growth and strength of skeletal muscle, creatine may indirectly have this effect by increasing the productivity of weight or resistance training. Because of the interest in and popularity of creatine, additional controlled research studies would be beneficial to help explain its mechanisms of action and refine the role of creatine as an ergogenic aid.
The most common adverse effects reported to be associated with creatine include fluid retention and weight gain. These effects may actually be detrimental in some sporting events. Additionally, creatine supplementation has been shown to result in an increase in anterior compartment pressure in the lower leg, with some study subjects experiencing lower extremity pain or tightness in the lower leg during exercise.[113] Other anecdotal adverse effects that have been reported include dehydration, emesis, diarrhea, fatigue, muscle cramps, myopathy, polymyositis, fatigue, migraine, renal impairment, rash, and dyspnea.[48,91] There is also the possibility that with supplementation, the body will downregulate the endogenous production of creatine.
Currently, creatine is not banned by the NCAA, WADA, or other sports-governing bodies; however, many of these organizations urge caution because of the unknown long-term effects of creatine use. Despite the cautions, a substantial number of sports teams in the NFL, MLB, the National Basketball Association (NBA), and the National Hockey League (NHL) have reportedly supplied their players with creatine.[114]"