Changes in circulating insulin-like growth factor-binding protein-1 (IGFBP-1) during prolonged exercise: effect of carbohydrate feeding

Plasma levels of glucose, insulin, the insulin-like growth factor (IGF-I and -II) and IGFBP-1 were determined in four young healthy males performing cycle exercise to fatigue while being fed either placebo (trial C) or glucose polymer solution (trial G). There was a significant decline in glucose and insulin from rest to fatigue in C (P <0.01 and P <0.05, respectively), but not in G. IGF-I or IGF-II levels did not change significantly in either of the trials. IGFBP-1 levels increased 12-fold in C (11.4 +/- 1.6 ng/ml at rest to 136.5 +/- 19.7 ng/ml at fatigue P <0.01), and 5.6-fold in G (11.0 +/- 2.3 ng/ml to 62.2 +/- 15 ng/ml, P <0.05). In C a significant negative correlation was found between IGFBP-1 and glucose (r = 0.69, P <0.01) and IGFBP-1 and insulin (r = -0.612, P <0.05) in C, but not in G. These results suggest that during prolonged exercise factors other than insulin or glucose may regulate IGFBP-1 and that IGFBP-1 may serve a role other than to prevent the hypoglycaemic action of the IGFs.