We studied the effects of pre-exercise meal composition on metabolic and performance-related variables during endurance exercise. Eight well-trained cyclists (VO2max 65.0 to 83.5 ml.kg-1.min-1) were studied on three occasions after an overnight fast. They were given isoenergetic meals containing carbohydrate (CHO), protein (P) and fat (F) in the following amounts (g per 70 kg body weight): High -carbohydrate meal, 215CHO, 26P, 3F; high-fat meal, 50CHO, 14P, 80F. On the third occasion subjects were studied after an overnight fast. Four hours after consumption of the meal, subjects started exercise for 90 min at 70% of their VO2max, followed by a 10 km time trial. The high-carbohydrate meal compared with the high-fat resulted in significant decreases (P < 0.05) in blood glucose, plasma non -esterified fatty acids, plasma glycerol, plasma chylomicron -triacylglycerol and plasma 3-hydroxybutyrate concentrations during exercise. This was accompanied by an increase in plasma insulin (P < 0.01 versus no meal), plasma epinephrine and plasma growth hormone concentrations (each P < 0.05 versus either of the other conditions) during exercise. Despite these large differences in substrate and hormone concentrations in plasma, substrate oxidation during the 90 min exercise period was similar in the three trials, and there were no differences in performance on the time trial. These results suggest that although the availability of fatty acids and other substrates in plasma can be markedly altered by dietary means, the pattern of substrate oxidation during endurance exercise is remarkably resistant to alteration.
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