Introduction: This study hypothesized that insulin-like growth factor-binding protein (IGFBP), rather than insulin-like growth factor I (IGF-I) itself, would be more responsive to acute exercise stress in a dose-dependent fashion.
Methods: Eight men (24 ± 5 yr, 87 ± 9 kg, 182 ± 6 cm, 21 ± 5% body fat) had blood drawn every 4 h after exercise for 24 h and assayed for IGF-I, IGFBP-1, -3, -6, the acid labile subunit (ALS), insulin, glucose, and nonesterified free fatty acids on five occasions: no exercise (control, C), moderate-duration resistance exercise (MDRE; 25, 5-10 repetition maximum (RM) sets), long-duration resistance exercise (LDRE; 50, 5-10 RM sets), moderate-duration aerobic exercise (MDAE; three 15-min cycling bouts at ∼70% V˙O2peak), and long-duration aerobic exercise (LDAE; six 15-min cycling bouts at ∼70% V˙O2peak). Energy requirements were determined from resting metabolic rate, age, and a physical activity factor. Dietary control was implemented by providing all meals during the experimental trials. A two-way ANOVA with repeated measures (P < 0.05) was used for statistical analysis.
Results: Significant exercise effects were observed for IGFBP-1 (C: 14.0 ± 2.7 < MDRE: 35.9 ± 8.6 = LDRE: 45.2 ± 10.6 = MDAE: 34.2 ± 7.4 = LDAE: 47.0 ± 11.8 ng·mL−1) and insulin (C: 26.0 ± 9 < LDRE: 13.2 ± 6 ng·mL−1). In addition, a dose-response relationship was observed for the IGFBP-1 response (long-duration exercise (46 ± 10 ng·mL−1) > moderate-duration exercise (35 ± 7 ng·mL−1). There were no exercise effects for total IGF-I, IGFBP-3, and ALS. Effects of time of day were observed for all variables except ALS.
Conclusions: For the circulating IGF-I system components measured, only IGFBP-1 seems to be a sensitive biomarker capable of assessing the physiological strain of acute physical exercise.