Improved insulin action in muscle, liver, and adipose tissue in physically trained human subjects

The present studies were initiated to assess the effect of insulin on muscle, liver, and adipose tissue in eight control and eight physically trained individuals matched for age and body mass index. Results indicated that percent body fat was 53% lower and maximal oxygen consumption 50% higher in physically trained subjects. Although the plasma glucose response to a standard oral glucose challenge was similar in the two groups, the insulin response was significantly lower in the trained individuals (P < 0.001). Mean (±SE) insulin-stimulated glucose uptake, quantified in vivo by the euglycemic hyperinsulinemic clamp technique, was significantly greater in physically trained individuals at steady-state plasma insulin concentrations of ~10 μU/ml (3.41 ± 0.14 vs. 2.73 ± 0.22 mg·kg fat free mass^-1·min^-1, P < 0.05) and 50 μU/ml (13.58 ± 0.75 vs. 9.82 ± 0.53 mg·kg fat free mass^-1·min^-1, P < 0.001). In addition, mean (±SE) hepatic glucose production rate was lower in physically trained subjects at insulin levels of 10 μU/ml (0.63 ± 0.19 vs. 1.19 ± 0.22 mg·kg body wt^-1·min^-1, P < 0.05) and 50 Υ/min (0.18 ± 0.14 vs. 0.60 ± 0.17 mg·kg body wt^-1, P < 0.05). Finally, the ability of insulin to stimulate mean (±SE) glucose uptake above basal levels were greater in adipocytes isolated from trained individuals (94 ± 10 vs. 56 ± 14 fl·cell^-1·s^-1, P < 0.01). On the other hand, no difference in specific binding of insulin to its receptor on monocytes was noted between the two groups. These findings indicate that in vivo insulin-stimulated glucose uptake was enhanced in physically trained human subjects, and this effect was seen in all three of insulin's target tissues.