No abstract

Insulin-like growth factor-I (IGF-I) is associated with muscle hypertrophy, and circulating IGF-I levels are correlated with fitness. To test the hypothesis that IGF-I increases with increased physical activity in adolescent males, 38 subjects (16 ± 0.7 yr old) were randomized to control ( n = 18) or increased physical activity groups for 5 wk. Before and after the intervention, we measured thigh muscle volume using magnetic resonance imaging and serum levels of mean growth hormone (GH) by overnight multiple sampling, GH binding protein (GHBP), IGF-I, and IGFBPs 1–5 by standard assays. Energy expenditure was assessed with the doubly labeled water technique toward the end of the study. In the training subjects there was 1) a significant increase in thigh muscle volume (+3.6 ± 1%), 2) 15.5 ± 3.3% greater energy expenditure than in controls, and 3) no evidence of weight loss (+1.44 ± 0.4%). In contrast to our hypothesis, but similar to our recent observations in adolescent females, training decreased IGF-I (−12 ± 4%, P < 0.005). Moreover, training substantially reduced GHBP (−21 ± 4%, P < 0.00002) and increased IGFBP-2 (+40 ± 16%, P < 0.008). Brief training increased muscle volume in weight-stable adolescent males and, surprisingly, influenced not only IGF-I but GHBP and IGFBP-2 as well in a manner typically found in energy-deficient states.

An acute insulin-like growth factor I (IGF-I) response to 10 min of above-lactate threshold cycle ergometer exercise was studied in 10 subjects (age 22-35 yr). Each subject exercised on three separate mornings after ingesting one of two isocaloric isovolemic liquid meals high in either fat or glucose or an isovolemic noncaloric placebo. The high-fat meal attenuated the growth hormone (GH) response (Cappon et al., J. Clin. Endocrinol. Metab. 76: 1418-1422, 1993). In contrast, IGF-I increased equally for all protocols [e.g., after the placebo meal IGF-I increased from 21,716 (SE) ng/ml preexercise to 25,316 ng/ml at 10 min of exercise; P < 0.05]. IGF-I peaked by the 10th min of exercise, like GH, and remained significantly elevated for only 20 min of recovery. We tested for possible GH-dependent mechanisms in which circulating IGF-I would increase 12-24 h after exercise. Ten subjects (age 23-32 yr) performed 10 min of above-lactate threshold exercise at 9, 10, and 11 A.M. GH was elevated after the first exercise bout (peak GH 6.05 +/- 1.45 ng/ml; P < 0.001) but was significantly reduced for the second and third bouts (peak GH 2.52 +/- 0.76 and 1.50 +/- 0.40 ng/ml, respectively). No increase in IGF-I was observed by 8 A.M. on the following day. Heavy ergometer exercise led to brief and small increases in circulating IGF-I that were independent of circulating GH.(ABSTRACT TRUNCATED AT 250 WORDS)