This study determined the effect of carbohydrate ingestion during exercise on the lipolytic rate, glucose disappearance from plasma (R_{d Glc}), and fat oxidation. Six moderately trained men cycled for 2 h on four separate occasions. During two trials, they were fed a high-glycemic carbohydrate meal during exercise at 30 min (0.8 g/kg), 60 min (0.4 g/kg), and 90 min (0.4 g/kg); once during low-intensity exercise [25% peak oxygen consumption (O_{2 peak})] and once during moderate-intensity exercise (68% O_{2 peak}). During two additional trials, the subjects remained fasted (12-14 h) throughout exercise at each intensity. After 55 min of low-intensity exercise in fed subjects, hyperglycemia (30% increase) and a threefold elevation in plasma insulin concentration (P < 0.05) were associated with a 22% suppression of lipolysis compared with when subjects were fasted (5.2 ± 0.5 vs. 6.7 ± 1.2 µmol · kg¹ · min¹, P < 0.05), but fat oxidation was not different from fasted levels at this time. Fat oxidation when subjects were fed carbohydrate was not reduced below fasting levels until 80-90 min of exercise, and lipolysis was in excess of fat oxidation at this time. The reduction in fat oxidation corresponded in time with the increase in R_{d Glc}. During moderate-intensity exercise, the very small elevation in plasma insulin concentration (~3 µU/ml; P < 0.05) during the second hour of exercise when subjects were fed vs. when they were fasted slightly attenuated lipolysis (P < 0.05) but did not increase R_{d Glc} or suppress fat oxidation. These findings indicate that despite a suppression of lipolysis after carbohydrate ingestion during exercise, the lipolytic rate remained in excess and thus did not limit fat oxidation. Under these conditions, a reduction in fat oxidation was associated in time with an increase in glucose uptake.

insulin; lipolysis; glucose uptake; exercise intensity; glycogen; stable isotopes

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