Phosphocreatine [PCr] (or creatine phosphate, CrP) is used as a source of energy during high intensity exercise such as weight training. Energy is released from the molecule when phosphate is donated to adenosine diphosphate, creating adenosine triphosphate or ATP. Muscle stores of PCr are limited and tend to be lower in vegetarians. Typical creatine intake is estimated to be 1 gram daily in meat-eating persons. The best dietary sources are animal products although creatine can also be synthesized endogenously.
The purpose of creatine supplementation is to increase either total creatine stores, or PCr stores, within muscle. Supplementation increases the rate of resynthesis of creatine phosphate following exercise (see image below). Various studies have shown increased muscle PCr levels after supplementing with 20-30 grams of creatine monohydrate daily. The amount stored is variable between individuals and is greatest in those who begin with the lowest PCr levels.
The amount stored may be further enhanced if the creatine is combined with a simple carbohydrate, presumably due to an insulin-mediated effect. The maximal storage capacity of the muscle appears to be 150-160 mmol/kg. Accordingly, studies have employed a loading method of supplementation of 20-30 g/day for 5-7 days, followed by 5 g/day of creatine to maintain muscle CrP levels.
Creatine supplementation appeals to those involved in strength training. Greater strength gains were seen during resistance training with creatine use as compared to placebo. Creatine may enhance maximal intermittent exercise capacity and result in greater gains in lean body mass. Weight gain is an almost universal observation in all studies and may represent increases in muscle water.
Generally speaking there are marginal and in some cases statistically significant improvements with creatine supplementation over placebo in strength studies employing isotonic and isokinetic exercise, less so for isometric exercise. Creatine supplementation for improvement in aerobic, endurance exercise has scant justification in the literature.
No health risks have been documented with creatine supplementation in the recommended doses of 20-30 g/day. Anecdotal reports of GI upset, muscle cramping and delayed healing following muscle strains have been made. The level of supplementation in these reports is unknown. Currently, it is purely speculative to say that supplementation in amounts higher than those reported in the literature results in the same low incidence of side effects. The long term health benefits and risks of supplementation (for greater than 1 year) have not been systematically studied.
For more info on how muscles derive energy from creatine, see our Muscle Physiology Primer.