Journal of Applied Physiology
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J Appl Physiol 87: 1413-1420, 1999;
8750-7587/99 $5.00
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Vol. 87, Issue 4, 1413-1420, October 1999

INVITED REVIEW
Effect of training status on fuel selection during submaximal exercise with glucose ingestion

Luc J. C. van Loon, Asker E. Jeukendrup, Wim H. M. Saris, and Anton J. M. Wagenmakers

Department of Human Biology, Maastricht University, 6200 MD Maastricht, The Netherlands

In this study, an oral glucose load was enriched with a [U-13C]glucose tracer to determine differences in substrate utilization between endurance-trained (T) and untrained (UT) subjects during submaximal exercise at the same relative and absolute workload when glucose is ingested. Six highly trained cyclists/triathletes [maximal workload (Wmax), 400 ± 9 W] and seven UT subjects (Wmax, 296 ± 8 W) were studied during 120 min of cycling exercise at 50% Wmax (~55% maximal O2 consumption). The T subjects performed a second trial at the mean workload of the UT group (148 ± 4 W). Before exercise, 8.0 ml/kg of a 13C-enriched glucose solution (80 g/l) was ingested. During exercise, boluses of 2.0 ml/kg of the same solution were administered every 15 min. Measurements were made in the 90- to 120-min period when a steady state was present in breath 13CO2 and plasma glucose 13C enrichment. Energy expenditure was higher in T than in UT subjects (58 vs. 47 kJ/min, respectively; P < 0.001) at the same relative intensity. This was completely accounted for by an increased fat oxidation (0.57 vs. 0.40 g/min; P < 0.01). At the same absolute intensity, fat oxidation contributed more to energy expenditure in the T compared with the UT group (44 vs. 33%, respectively; P < 0.01). The reduction in carbohydrate oxidation in the T group was explained by a diminished oxidation rate of muscle glycogen (indirectly assessed by using tracer methodology at 0.72 ± 0.1 and 1.03 ± 0.1 g/min, respectively; P < 0.01) and liver-derived glucose (0.15 ± 0.03 and 0.22 ± 0.02 g/min, respectively; P < 0.05). Exogenous glucose oxidation rates were similar during all trials (±0.70 g/min).

exogenous glucose; endurance training; carbohydrate metabolism; stable isotopes; substrate metabolism


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