EXOGENOUS GLUCOSE OXIDATION DURING EXERCISE IN ENDURANCE TRAINED AND UNTRAINED SUBJECTS 576

Abstract

To investigate the effect of training on the oxidation of orally ingested glucose during exercise 7 endurance trained cyclists (T, VO2max 67±2.3 ml/kg.min) and 8 untrained subjects (UT, 46±2 ml/kg.min) were studied during 120 min of exercise at -60% VO2max. At the onset of exercise, 8 ml/kg of an 8% naturally labeled [13C]-glucose solution was ingested and 2 ml/kg every 20 min thereafter. Body weights of the T and UT were comparable (75.2±3.8 kg vs 72.3±1.2 kg). Breath samples were collected at rest and every 15 min during exercise and analyzed for13 CO2/12CO2 ratio by IRMS (Finnigan MAT 252). To allow background correction, all subjects performed a second test in which glucose of low natural 13C abundance was ingested. Tests were performed in random order. All results were calculated for the 60-120 min period. Since exercise was performed at the same relative intensity trained subjects had a higher energy expenditure (3404 kJ versus 2630 kJ; P<0.01). Fat contributed significantly more to energy expenditure (EE) in T (45%) compared to UT (33%) while CHO contributed 55% and 67% respectively. No differences were observed in exogenous glucose oxidation which contributed 24% in T and 26% in UT to EE respectively. As a consequence, the relative contribution of endogenous carbohydrates (CHO), as calculated by total minus exogenous CHO oxidation, was significantly less in T (31%) compared to UT (41%). In absolute terms, exogenous glucose oxidation was slightly, but not significantly higher in T compared to UT (50±2 g versus 45±3 g). In addition, peak exogenous glucose oxidation rates were not significantly different among the 2 groups (0.91 g/min in T and 0.86 g/min in UT). It was concluded that although the state of training had a major effect on endogenous substrate utilization, it had no influence on exogenous glucose utilization during submaximal exercise at the same relative intensity.