Review articleScientific basis and practical aspects of creatine supplementation for athletes
Introduction
Creatine supplementation has become one of the most popular ergogenic aids among athletes and one of the most profitable for the supplement industry. A relatively large scientific body of literature has been generated over the past decade documenting the physiologic and performance effects of creatine supplementation in diverse populations. The large number of studies on creatine and widespread interest among scientists have prompted many review papers covering broad topics1, 2, 3, 4 and those with a specific focus in relation to acute performance,5, 6 training adaptations,7, 8, 9 factors affecting muscle creatine uptake,10 pharmacokinetics,11, 12 therapeutic potential in neuromuscular and other metabolic diseases,13 use in adolescents14 and the elderly,15 reasons for inconsistent results,16 and potential adverse effects.17, 18
Although there are some inconsistencies, most studies on the effects of creatine supplementation on exercise performance have demonstrated an ergogenic effect.9 These discrepancies may be due to the large variability associated with muscle creatine uptake after creatine supplementation, but this is difficult to assess because many studies do not assess muscle creatine. Small samples and inadequate statistical power to detect the small benefits afforded by creatine supplementation are other possible explanations for the discordant findings between studies that demonstrate an ergogenic effect of creatine and those that do not.16 Perhaps the most consistent and important finding for athletes is that creatine supplementation in conjunction with resistance training leads to greater gains in lean body mass, maximal strength, and weight-lifting performance.8 In this review we briefly discuss the studies showing a beneficial effect of creatine on resistance-training adaptations, with an emphasis on cellular effects and mechanisms of action. Because significantly increasing muscle creatine levels appear to be necessary to achieve the full benefits of creatine supplementation, we also overview the most important factors contributing to muscle creatine uptake. We conclude with more practical aspects related to the effects of creatine supplementation on sports performance and issues related to dosing and maximizing creatine delivery to muscle.
Section snippets
Effects of creatine supplementation on chronic reponses to training
There is evidence to indicate that athletes are ingesting creatine for several months and often in conjunction with progressive resistance-training programs.19, 20 As an example, Juhn et al.19 reported that collegiate baseball and football players ingest creatine for 3 and 5 mo, respectively, in the off-season to increase lean body mass and strength. Athletes hope that increased muscle creatine and phosphocreatine stores before exercise and accelerated phosphocreatine resynthesis after exercise
Mechanisms of action
The physiologic mechanisms underlying increased muscle strength and improved weight-lifting performance after creatine supplementation in conjunction with resistance training are not known with certainty, but several theories exist and have been investigated.
Factors affecting skeletal muscle creatine uptake
After creatine supplementation, some individuals experience a marked increase in muscle creatine concentrations (>30%), whereas others experience little or no change.55 Several factors can explain this large intersubject variability, but initial muscle creatine content may be the most important determinant of muscle creatine uptake after supplementation.55 For instance, subjects with lower muscle creatine concentrations have the largest increase in muscle creatine after supplementation, whereas
Maximizing creatine delivery
Blood creatine levels are sometimes used by manufacturers to indicate that one creatine product has greater bioavailability than another. However, the ergogenic effect of creatine supplementation is derived from increased muscle phosphocreatine after supplementation, so plasma creatine values without accompanying muscle creatine measurements may be misleading. For instance, Rawson et al.63 reported similar plasma creatine pharmacokinetics between old and young subjects ingesting creatine, but a
Summary
A large number of studies has been published on creatine supplementation. A substantial number of these studies has observed an ergogenic effect on performance when muscle creatine levels are significantly elevated. It has become apparent that accumulation of creatine in skeletal muscle or other tissues affects a variety of cellular processes that could account for its ergogenic and therapeutic potential. Recent work has started to provide a better picture of the physiologic mechanisms by which
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