Abstract

Objectives—To determine whether consecutive bouts of intense endurance exercise over a three week period alters serum concentrations of insulin-like growth factor I (IGF-I) and/or its binding proteins.

Methods—Seventeen professional cyclists (mean (SEM) Vo₂max, 74.7 (2.1) ml/kg/min; age, 27 (1) years) competing in a three week tour race were selected as subjects. Blood samples were collected at each of the following time points: t₀ (control, before the start of competition), t₁ (end of first week), and t₃ (end of third week). Serum levels of both total and free IGF-I and IGF binding proteins 1 and 3 (IGFBP-1 and IGFBP-3) were measured in each of the samples. Cortisol levels were measured in nine subjects.

Results—A significant (p<0.01) increase was found in total IGF-I and IGFBP-1 at both t₁ and t₃ compared with t_o (IGF-I: 110.9 (17.7), 186.8 (12.0), 196.9 (14.7) ng/ml at t₀, t₁, and t₃ respectively; IGFBP-1: 54.6 (6.6), 80.6 (8.0), and 89.2 (7.9) ng/ml at t₀, t₁, and t₃ respectively). A significant (p<0.01) decrease was noted in free IGF-I at t₃ compared with both t_o and t₁ (t₀: 0.9 (0.1) ng/ml; t₁: 0.9 (0.1) ng/ml; t₃: 0.7 (0.1) ng/ml); in contrast, IGFBP-3 levels remained stable throughout the race.

Conclusions—It would appear that the increase in circulating levels of both IGF-I and its binding protein IGFBP-1 is a short term (one week) endocrine adaptation to endurance exercise. After three weeks of training, total IGF-I and IGFBP-1 remained stable, whereas free IGF-I fell below starting levels.