Circulating insulin-like growth factor (IGF-I) is positively correlated with fitness, but, paradoxically, training studies do not invariably demonstrate increases in IGF-I levels. We hypothesized that IGF-I would increase with training provided that the study was prospective, appropriately controlled, and target organ responses were assessed. 38 adolescent males(16±0.7 y.o.) were randomized to a control (n=18) and endurance type training intervention lasting 5 weeks. Training led to a significant increase in thigh muscle mass (3.6±1% by MRI) and body weight(1.44±0.4%). In contrast to our hypothesis, training led to a decrease in IGF-I and IGF binding protein-3 (BP-3) while IGFBP-2 increased. Finally, training led to a remarkable reduction in growth hormone binding protein (GHBP). These GH-IGF-I responses are typically found in energy-deficient states, even though muscle mass and body weight had increased. A reduction in GHBP may be one of the early mechanisms of this catabolic-type response. Previously observed positive correlations between IGF-I and fitness may reflect a chronic adaptation once a new steady-state of energy balance has been achieved.Figure

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