INCREASED PHYSICAL-ACTIVITY AND THE GROWTH-HORMONE IGF-I AXIS IN ADOLESCENT MALES

Insulin-like growth factor-I (IGF-I) is associated with muscle hypertr ophy, and circulating IGF-I levels are correlated with fitness. To tes t the hypothesis that IGF-I increases with increased physical activity in adolescent males, 38 subjects (16 +/- 0.7 yr old) were randomized to control (n = 18) or increased physical activity groups for 5 wk. Be fore and after the intervention, we measured thigh muscle volume using magnetic resonance imaging and serum levels of mean growth hormone (G H) by overnight multiple sampling, GH binding protein (GHBP), IGF-I, a nd IGFBPs 1-5 by standard assays. Energy expenditure was assessed with the doubly labeled water technique toward the end of the study. In th e training subjects there was 1) a significant increase in thigh muscl e volume (+3.6 +/- 1%), 2) 15.5 +/- 3.3% greater energy expenditure th an in controls, and 3) no evidence of weight loss (+1.44 +/- 0.4%). In contrast to our hypothesis, but similar to our recent observations in adolescent females, training decreased IGF-I (-12 +/- 4%, P < 0.005). Moreover, training substantially reduced GHBP (-21 +/- 4%, P < 0.0000 2) and increased IGFBP-2 (+40 +/- 16%, P < 0.008). Brief training incr eased muscle volume in weight-stable adolescent males and, surprisingl y, influenced not only IGF-I but GHBP and IGFBP-2 as well in a manner typically found in energy-deficient states.