Objectives. - In the last few years many athletes and persons engaged in recreational sports activities have begun using creatine supplementation. Creatine feeding is possible by oral administration of creatine monohydrate. The objectives of this paper are to recall the mechanisms by which creatine might improve performance, to discuss the known effects of creatine supplementation on exercise performance, and to examine its side effects. Topics. - The rate of turnover of creatine for a 70 kg male has been estimated around 2 g/d. Creatine is partly supplied by the diet that provides I g/d through meat and fish. Recent studies have shown that ingestion of about 20 g of creatine monohydrate per day is able to modulate total muscle creatine, free creatine and phosphocreatine. The aim of this article is to provide an overview of recent knowledge on the effects of creatine supplementation on exercise performance. Many studies demonstrate that creatine supplementation has beneficial effects on performance of short-duration exercises, during repeated isokinetic or isometric contractions of the quadriceps muscle, jumping or high-intensity cycling exercises. The beneficial effects of creatine supplementation on performance capacity are strongly related to the efficacy of the treatment for enhancing muscle creatine pool. If is thus clear that phosphocreatine stores play a key role for ATP resynthesis during muscle contraction and recovery. The improvement in performance following creatine supplementation is dependent on the characteristics of the exercise. It has been suggested that human skeletal muscles have an upper limit for total creatine concentration. In contrast with sedentary subjects, in athletes and well-trained subjects who have high initial total creatine concentrations in skeletal muscle, only a slight improvement in exercise performances is expected. Taken together, the results of most studies published to date suggest that only performances of repetitive high-intensity exercise bouts are positively affected by creatine supplementation. During this type of exercise, the expected increase in total creatine contributes to the fast resynthesis of phosphocreatine during recovery. Until recently, it was well accepted that except for a slight increase in body weight, no adverse effects have been associated with creatine supplementation. However, a recent report described a clinical case of renal dysfunction that was associated with oral creatine supplementation. Future prospects. - If creatine supplementation is without any risk for health, creatine ingestion could delay the onset of muscular fatigue during repeated bouts of high-intensity exercise. Future studies should verify the lack of side effects of repeated creatine supplementations. (C) 1998 Elsevier, Paris.
