Differential IGF-I System Responses During 8 Weeks of Resistance vs. Generalized Physical Training in Men

Abstract

Mechanical load is known to consistently induce increased local (i.e. muscle) expression of IGF-I. The extent to which exercise training results in changes in the systemic circulation for the IGF-I system (inclusive of it's family of binding proteins (BPs)) has been far less definitive. PURPOSE: To test the hypotheses that 1) the family of IGFBPs might be more responsive to exercise training than IGF-I itself and 2) that a training program emphasizing greater localized mechanical load might also result in greater systemic changes in the IGF-I system. METHODS: 32 subjects were randomly divided into a resistance training (RT) group (n = 15, 27±5 yr, 174±6 cm, 81±12 kg) and a generalized physical training group (PT) (n= 17, 29±5yr, 179±8 cm, 85±10 kg) and all underwent 8 weeks of training (1.5 hr/day, 5 days/wk). The RT group engaged in conventional weight lifting exercises, running/hiking and interval/agility drills, while the PT group engaged in low-impact callisthenic-type and interval/agility running drills. Blood was sampled at pre, mid-and post training and analyzed by immunoassay for total IGF-I, free IGF-I, IGFBPs 1, 2 and 3, and the acid labile subunit (ALS). A repeated measures ANOVA (p <0.05) was used for statistical analysis. RESULTS: No interaction or time effects were observed for total IGF-I, free IGF-I, or IGFBP1. Time effects were observed for IGFBP2, ng/ml (pre; 569±295 <mid; 819±497 = post; 833±481) and ALS, μg/ml (pre; 16±1 = mid; 17±1 >post; 14±1). An interaction effect was observed for the RT group as IGFBP3 increased from pre to mid (3462±216 vs. 3962±227 μg/ml), but was no longer significant post training (3770±228 μg/ml). CONCLUSION: The results support the contention that some IGFBPs are more responsive to exercise stress than IGF-I itself. The changes observed were not specific to exercise modality (i.e. high-impact resistance vs. low-impact generalized callisthenic exercises) suggesting that changes in the systemic IGF-I system during exercise training may be a generalized metabolic response, rather than a response specific to exercise modality or muscle overload.