Repeated prolonged whole-body low-intensity exercise: effects on insulin sensitivity and limb muscle adaptations

This study investigates the effect of prolonged whole-body low-intensity exercise on insulin sensitivity and the limb muscle adaptive response. Seven male subjects (weight, 90.2 +/- 3.2 kg; age, 35 +/- 3 years) completed a 32-day unsupported crossing of the Greenland icecap on cross-country skies pulling sleighs. The subjects were studied before and 3 to 4 days after the crossing of the icecap. Subjects came in overnight fasted, and an intravenous glucose tolerance test (IVGTT) was done. A biopsy was obtained from the vastus lateralis and deltoid muscle. On a separate day, a progressive test was performed to establish maximal oxygen uptake. During the crossing, subjects skied for 342 +/- 41 min/d. Peak oxygen uptake (4.6 +/- 0.2 L/min) was decreased (P < .05) by 7% after the crossing and body mass decreased (P < .05) by 7.1 +/- 0.2 kg, of which 4.4 +/- 0.5 kg was fat mass and 2.7 +/- 0.2 kg lean body mass. Glycosylated hemoglobin (5.6% +/- 0.01%) was not affected by the crossing. The IVGTT data revealed that insulin sensitivity (7.3 +/- 0.6 mU x L-1 x min-1) and glucose effectiveness (0.024 +/- 0.002 min-1) were not changed after the crossing. Similarly, the IVGTT data, when expressed per kilogram of lean body mass or body mass, were not affected by the crossing. Citrate synthase activity was higher (P < .05) in the leg (29 +/- 1 micromol x g-1 x min-1) than in the arm muscle (16 +/- 2 micromol x g-1 x min-1) and was unchanged after the crossing. Muscle GLUT4 protein concentration was higher (P < .05) in the leg (104 +/- 10 arbitrary units) than in the arm (54 +/- 9 arbitrary units) and was not changed in the leg, but was increased (P < .05) by 70% to 91 +/- 9 arbitrary units in the arm after the crossing. In conclusion, the increased glucose transporter expression in arm muscle may compensate for the loss of lean body mass and the decrease in aerobic fitness and thereby contribute to the maintenance of whole-body insulin sensitivity after prolonged low-intensity exercise training.