There is scientific evidence that short term creatine use can increase maximum power and performance in high-intensity anaerobic repetitive work (periods of work and rest) by 5 to 15%. This is mainly bouts of running/cycling sprints and multiple sets of low RM weightlifting. Single effort work shows an increase of 1 to 5%. This refers mainly to single sprints and single lifting of 1-2RM weights. However, some studies show no ergogenic effect at all. Studies in endurance athletes have been less than promising, most likely because these activities are sustained at a given intensity and thus do not allow for significant intra-exercise synthesis of additional creatine phosphate molecules. Ingesting creatine can increase the level of phosphocreatine in the muscles up to 20%. It must be noted creatine has no significant effect on aerobic endurance, though it will increase power during short sessions of high-intensity aerobic exercise.
Since body mass gains of about 1 kg can occur in a week's time, many studies suggest that the gain is simply due to greater water retention inside the muscle cells. Other studies, however, have shown that creatine increases the activity of satellite cells, which make muscle hypertrophy possible. Creatine supplementation appears to increase the number of myonuclei that satellite cells will 'donate' to damaged muscle fibers, which increases the potential for growth of those fibers. This increase in myonuclei probably stems from creatine's ability to increase levels of the myogenic transcription factor MRF4.
In another study, researchers concluded that changes in substrate oxidation may influence the inhibition of fat mass loss associated with creatine after weight training when they discovered that fat mass did not change significantly with creatine but decreased after the placebo trial in a 12-week study on ten active men. The study also showed that 1-RM bench press and total body mass increased after creatine, but not after placebo. The underlying effect of creatine on body composition has yet to be determined, as another study with a similar timeframe suggests no effect on body composition, but had less overall emphasis on metabolic effects.
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