Muscle net glucose uptake and glucose kinetics after endurance training in men.

We evaluated the hypotheses that alterations in glucose disposal rate (Rd) due to endurance training are the result of changed net glucose uptake by active muscle and that blood glucose is shunted to working muscle during exercise requiring high relative power output. We studied leg net glucose uptake during 1 h of cycle ergometry at two intensities before training [45 and 65% of peak rate of oxygen consumption (V˙o 2 peak)] and after training [65% pretrainingV˙o 2 peak, same absolute workload (ABT), and 65% posttrainingV˙o 2 peak, same relative workload (RLT)]. Nine male subjects (178.1 ± 2.5 cm, 81.8 ± 3.3 kg, 27.4 ± 2.0 yr) were tested before and after 9 wk of cycle ergometer training, five times a week at 75%V˙o 2 peak. The power output that elicited 66.0 ± 1.1% ofV˙o 2 peak before training elicited 54.0 ± 1.7% after training. Whole body glucose Rd decreased posttraining at ABT (5.45 ± 0.31 mg ⋅ kg-1 ⋅ min-1at 65% pretraining to 4.36 ± 0.44 mg ⋅ kg-1 ⋅ min-1) but not at RLT (5.94 ± 0.47 mg ⋅ kg-1 ⋅ min-1). Net glucose uptake was attenuated posttraining at ABT (1.87 ± 0.42 mmol/min at 65% pretraining and 0.54 ± 0.33 mmol/min) but not at RLT (2.25 ± 0.81 mmol/min). The decrease in leg net glucose uptake at ABT was of similar magnitude as the drop in glucose Rd and thus could explain dampened glucose flux after training. Glycogen degradation also decreased posttraining at ABT but not RLT. Leg net glucose uptake accounted for 61% of blood glucose flux before training and 81% after training at the same relative (65%V˙o 2 peak) workload and only 38% after training at ABT. We conclude that 1) alterations in active muscle glucose uptake with training determine changes in whole body glucose kinetics; 2) muscle glucose uptake decreases for a given, moderate intensity task after training; and 3) hard exercise (65%V˙o 2 peak) promotes a glucose shunt from inactive tissues to active muscle.