Exogenous glucose oxidation during exercise in endurance-trained and untrained subjects

To investigate the effect of training status on the fuel mixture used during exercise with glucose ingestion, seven endurance-trained cyclists (Tr; maximum O2 uptake 67 ± 2.3 ml · kg-1 · min-1) and eight untrained subjects (UTr; 48 ± 2 ml · kg-1 · min-1) were studied during 120 min of exercise at ~60% maximum O2 uptake. At the onset of exercise, 8 ml · kg-1 · min-1 of an 8% naturally enriched [13C]glucose solution was ingested and 2 ml/kg every 15 min thereafter. Energy expenditure was higher in Tr subjects compared with UTr subjects (3,404 vs. 2,630 kJ; P < 0.01). During the second hour, fat oxidation was higher in Tr subjects (37 ± 2 g) compared with UTr subjects (23 ± 1 g), whereas carbohydrate oxidation was similar (116 ± 8 g in Tr subjects vs. 114 ± 4 g in UTr subjects). No differences were observed in exogenous glucose oxidation (50 ± 2 g in Tr subjects and 45 ± 3 g in UTr subjects, respectively). Peak exogenous glucose oxidation rates were similar in the two groups (0.95 ± 0.07 g/min in Tr subjects and 0.96 ± 0.03 g/min in UTr subjects). It is concluded that the higher energy expenditure in Tr subjects during exercise at the same relative exercise intensity is entirely met by a higher rate of fat oxidation without changes in the rates of exogenous and endogenous carbohydrates