ENDURANCE TRAINING DECREASES PLASMA-GLUCOSE TURNOVER AND OXIDATION DURING MODERATE-INTENSITY EXERCISE IN MEN

To assess the effects of endurance training on plasma glucose kinetics during moderate-intensity exercise in men, seven men were studied before and after 12 wk of strenuous exercise training (3 days/wk running, 3 days/wk cycling). After priming of the glucose and bicarbonate pools [U-13C]glucose was infused continuously during 2 h cycle ergometer exercise at 60% of pretraining peak O2 uptake (V̇O2) to determine glucose turnover and oxidation. Training increased cycle ergometer peak V̇O2 by 23% and decreased the respiratory exchange ratio during the final 30 min of exercise from 0.89 ± 0.01 to 0.85 ± 0.01 (SE) (P < 0.001). Plasma glucose turnover during exercise decreased from 44.6 ± 3.5 μmol · kg fat-free mass (FFM)-1 · min-1 before training to 31.5 ± 4.3 after training (P < 0.001), whereas plasma glucose clearance (i.e., rate of disappearance/plasma glucose concentration) fell from 9.5 ± 0.6 to 6.4 ± 0.8 ml · kg FFM-1 · min-1 (P < 0.001). Oxidation of plasma-derived glucose, which accounted for ~90% of plasma glucose disappearance in both the untrained and trained states, decreased from 41.1 ± 3.4 μmol · kg FFM-1 · min-1 before training to 27.7 ± 4.8 after training (P < 0.001). This decrease could account for roughly one-half of the total reduction in the amount of carbohydrate utilized during the final 30 min of exercise in the trained compared with the untrained state.