Insulin-Like Growth Factor-I System Response To Increased Energy Flux

Abstract

Initial fitness level has been shown to influence the response of the insulin-like growth factor-I (IGF-I) system to exercise training (more extensive changes observed in lower fit individuals, Rosendal, JAP, 2002). While the literature reports equivocal results for the effect of exercise on IGF-I, few studies have tightly controlled for energy balance. PURPOSE: To determine if aerobic fitness level alters the response of IGF-I and the IGF binding proteins (IGFBPs) to an acute increase in physical activity when energy balance is maintained (state of increased energy flux). METHODS: Seven low fit (VO2peak: 39±3 ml.kg-1.min-1) and seven high fit (VO2peak: 56±7 ml.kg-1.min-1) men completed a 4 day (D1-D4) baseline period of normal activity and controlled energy balance followed by 7 days (D5-D11) of a 1,000 kcal.d-1 increase in physical activity. Energy balance was maintained by increasing caloric intake during this period. Morning fasting blood samples were taken on days 3, 5, 6, 11, and 12 and analyzed for IGF-I, IGFBP-1, IGFBP-2, IGFBP-3, and the acid labile subunit (ALS). A repeated measures ANOVA (p < 0.05) was used for statistical analysis. RESULTS: Time effects (see Table) were observed for IGF-I, IGFBP-2, IGFBP-3, and ALS; however, no interaction or group effects occurred. IGFBP-1 exhibited no significant differences.Table: Values are mean±SD. Superscripts denote significant differences between days (p < 0.05).CONCLUSION: Increasing physical activity while maintaining energy balance decreased circulating IGF-I, IGFBP-3, and ALS and increased IGFBP-2 concentrations by the same magnitude in both low and high fit individuals. We conclude that an increase in energy flux influences the IGF-I system and suggests that yet, unidentified exercise-associated mechanisms, not related to energy deficit, regulate the IGF-I system.