In muscle, creatine exists in two forms, free creatine and phosphocreatine. Phosphocreatine is important for energy metabolism during muscle contraction, primarily by maintaining levels of ATP. (ATP is the "energy currency" that allows muscle contraction to take place.) Therefore, the main theory behind the use of creatine as a performance enhancing aid is that increasing the amount of phosphocreatine will help in this maintenance of ATP supply and, as a result, the athlete is able to develop more muscle power during exercise. Higher muscle creatine also would be of benefit when repeated bouts of exercise are performed by allowing a more rapid restoration of ATP concentrations. It also is proposed that creatine supplementation results in an increase in muscle mass, although evidence of this is shaky at best. In humans, numerous studies have shown that as little as four to five days of creatine supplementation results in a 20% to 50% increase in muscle creatine content, with more modest increases in phosphocreatine. Although this increase in muscle creatine appears to have minimal effect on single bout sprint exercise performance, there is evidence that weight lifters and other athletes performing repeated exercise tasks benefit from creatine supplementation. As for the equine athlete, research has shown that creatine is poorly absorbed by the horse. When creatine is fed to horses at doses similar to or even higher than those proven effective in humans, there is no change in muscle creatine concentration. Thus, there is no evidence that creatine supplementation will help the performance horse; without absorption of the ingested creatine and an increase in muscle creatine concentration, there can be no ergogenic (exercise enhancing) effect.
CREATINE Among human athletes, particularly those involved in strength- or sprint-oriented events, use of creatine has become widespread, writes Dr. Ray Geor in the August edition of The Horse. Similarly, there has been a great deal of interest regarding the use of creatine as a nutritional supplement to improve performance in horses. Creatine is a natural substance synthesized in the liver and kidney, then transported to, and stored in, skeletal muscle. Also, in meat- and fish-eating species, a significant amount of creatine is provided in the diet. Importantly, however, the creatine content of plant material is low, and therefore, it is not a component of a horse's natural diet.