Plasma C-peptide and insulin in trained and untrained subjects.

Plasma insulin and C-peptide were simultaneously determined under various conditions in 11 endurance-trained athletes and 12 nonathletes. Both groups performed an exhaustive ergometer test and an endurance test with 38% of the maximal achieved work load for 45 min. An intravenous glucose tolerance test was also performed. In the basal state, athletes had low plasma insulin and C-peptide concentrations. During exercise, insulin and C-peptide decreased similarly in both groups. In the recovery period, insulin and C-peptide rose within a few minutes. There were differences between the extent as well as the time course of this "rebound" effect after exhaustive or endurance exercise that might be related to glucose alterations. The insulin response but not the C-peptide response after glucose injection was blunted in trained subjects. Results indicate that basal plasma insulin concentrations are lower in athletes due to reduced insulin secretion. During exercise, insulin secretion is diminished independent of the training state. The blunted response of insulin after glucose administration in athletes is due to an enhanced plasma clearance.