Dietary creatine supplementation and exercise performance: Why inconsistent results?

Over the past few years there has been considerable interest in both the use of creatine (Cr) supplementation by athletes and the documentation of its effects by scientists. Some believe that this nitrogen-containing compound found in meat and fish has a performance-enhancing capability as important for brief intense exercise efforts as dietary carbohydrate is for activities where glycogen supplies limit performance. The mechanisms thought to be responsible for any ergogenic effect of acute (few d) Cr supplementation include: increased stores of muscle phosphocreatine (PCr), faster regeneration of PCr during exercise recovery, enhanced adenosine triphosphate (ATP) production from glycolysis secondary to increased hydrogen ion buffering, and/or possible shortened post contraction muscle relaxation time. With chronic (wk & mo) supplementation when combined with strength training, Cr may alter muscle protein metabolism directly (via decreasing protein breakdown or increasing synthesis) and/or indirectly as a result of a greater training load made possible by its acute ergogenic effects on strength and power Cr supplementation is not banned by the International Olympic Committee and, with the exception of a small increase in body mass (similar to1 kg) over the initial 3-6 d, does not appear to have any adverse side effects, at least with short-term use. Few scientific data are available for more prolonged use (mo or y) but considering the large numbers of athletes using Cr over the past 6(+) y and the absence of reported problems, it may be that the often discussed somewhat nebulous long term adverse effects are presently being overestimated. Intakes of 285-300 mg Cr/kg body mass (.) d(-1) over 3-6 d or 30-50 mg/kg body mass (.) d(-1) over similar to4 wk are sufficient to produce benefits (muscle mass and high intensity power gains); however not all study results are consistent. The focus of this review is to outline some possible explanations for the inconsistent observations reported in the literature. Clearly, if proven to be consistent the benefits of Cr supplementation could extend far beyond the athletic arena to include individuals who experience muscle weakness for a variety of other reasons (e.g., age/disease, muscle disease, exposure to microgravity, etc).