Abstract
Acute physical exercise increases growth hormone (GH) secretion, and GH regulates the expression of insulin-like growth factor I (IGF-I) and IGF-binding protein (IGFBP) 3. IGFBP-1 is a local modulator of IGF activity with rapid dynamic regulation that is downregulated by insulin. The IGF system mediates the metabolic actions of GH, and possibly it regulates glucose metabolism. We hypothesize that strenuous exercise causes changes in the IGF system. We studied the effects of the marathon run on the circulating levels of IGF-I, IGFBP-1, IGFBP-3, and insulin in 23 participants. Immediately after the run, the most striking change was an 11.6-fold median increase in serum IGFBP-1 level (from 63.7 +/- 50.5 to 736 +/- 408 micrograms/l; P < 0.001). Because the insulin level remained unchanged, the elevation of serum IGFBP-1 level cannot be explained by changes in insulin. One day after the run, the IGFBP-1 level had returned to baseline. The physiological role of this increment could be the inhibition of hypoglycemic effects of IGF-I and/or regulation of glucose availability to the muscles. The changes in IGF-I and IGFBP-3 levels were less dramatic: the IGF-I and IGFBP-3 levels were lower 1 and 3 days after the run. This report provides an important basis for authentic effects of strenuous exercise on the IGF-system.
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