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Oxford Lipid Metabolism Group, Radcliffe Infirmary, Oxford OX2 6HE; University Department of Anaesthesia, University Hospitals of South Manchester, Withington Hospital, Manchester M20 2LR; and Centre for Sport and Exercise Science, Liverpool John Moores University, Liverpool L3 3AF, United Kingdom
We studied the effects of preexercise meal
composition on metabolic and performance-related variables during
endurance exercise. Eight well-trained cyclists (maximal oxygen uptake
65.0 to 83.5 ml · kg
1 · min1)
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/70 kg body wt):
high-carbohydrate meal, 215 CHO, 26 P, 3 F; high-fat meal, 50 CHO, 14 P, 80 F. 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 maximal oxygen uptake, followed by a 10-km time trial. The high-carbohydrate meal compared with the high-fat meal resulted in significant decreases
(P < 0.05) in blood glucose, plasma
nonesterified 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 vs. no meal), plasma epinephrine, and plasma growth hormone
concentrations (each P < 0.05 vs.
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.
isoenergetic meals; fat oxidation; carbohydrate oxidation; performance
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