Diet manipulation, altered fat and carbohydrate metabolism and exercise performance in trained humans
The aim of the first two experiments was to determine whether alterations in dietary carbohydrate (CHO) intake would affect performance of high intensity exercise in well-trained individuals. This was achieved by comparing the effects of a 70 and 40% CHO diet on the performance of high intensity exercise lasting approximately 10 min (Experiment 1) or 30 min (Experiment 2). At both exercise intensities, no difference in exercise performance was found. These results demonstrated that moderate changes in diet composition during normal training do not significantly affect performance of high intensity exercise in well-trained individuals. These findings would suggest that total muscle glycogen concentration was not limiting high intensity exercise performance. The higher than normal daily energy intake of the subjects' diet may have adequately compensated for the reduced percentage of CHO on the two low CHO diets. An increased fat oxidation on the low CHO trial may also have contributed to these results. These results do not exclude the possibility that the glycogen content of individual muscle fibres was limiting high intensity exercise performance. The aim of the third experiment was to determine the effects of an exercise and diet regime, which was intended to alter initial muscle glycogen concentration, on the capacity of well-trained individuals to perform prolonged strenuous exercise to exhaustion in the heat and the cold. Exercise capacity in the heat was reduced compared with exercise in the cold, irrespective of diet. The exercise and diet intervention, when aimed at increasing muscle CHO stores, improved exercise capacity both in the heat and in the cold compared with when the intervention was aimed to reducing CHO stores.