There are only two studies which provide substantial evidence for this view7 8. Costill and colleagues reported that the ingestion of a concentrated glucose solution 45 min before the start of submaximal treadmill running caused a greater rate of muscle glycogen utilization than when exercise was performed after drinking water. This increased rate of glycogenolysis would, it was proposed, lead to an early onset of fatigue during prolonged exercise. Foster and colleagues8 confirmed this hypothesis in a study which showed that cycling time to exhaustion was reduced by 19% when their subjects ingested a 25% glucose solution 30 min before exercise. Muscle glycogen concentrations were not measured but the study did show an elevation in serum insulin and a reduction in serum fatty acid concentrations8. The cause of the early onset of fatigue was, according to the authors, a decrease in the availability of serum fatty acids for muscle metabolism which, in turn, increased the rate of muscle glycogen depletion and caused the early onset of fatigue9.Therefore, the explanation offered by Foster and colleagues8 for the paradoxical onset of fatigue following the pre-exercise ingestion of a concentrated glucose solution is, in theory, sound. But closer examination of the results of their study leaves several questions unanswered. For example, the rates of carbohydrate oxidation were the same during the glucose and water trials. If there had been an increased rate of glycogenolysis then this latter result is difficult to explain. Furthermore, there were no differences in perceived rates of exertion recorded during the two trials and yet their cyclists exercised for 10 min less on the glucose trial.More recently, however, Fielding et al.10 showed that the pre-exercise ingestion of fructose and glucose solutions had no greater influence on the rate of muscle glycogen utilization during exercise than the pre-exercise ingestion of water. McMurray et al. 1 provided some evidence to show that the pre-exercise ingestion of either glucose or fructose solutions by female runners, before exercise, has no detrimental influence on running performance. The exercise duration in this latter study was only approximately 60 min, probably because of the high exercise intensity (80% VO2max) which in turn resulted in blood lactate concentrations of about 5mmol)1-1 at Br