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Fitness, Training, and the Growth HormoneInsulin-Like Growth Factor I Axis in Prepubertal Girls¹

Connecticut Children’s Medical Center (A.E., T.P.S.), University of Connecticut Health Center, Farmington, Connecticut 06030; Department of Pediatrics (R.N., D.M.C.), University of California, Irvine, College of Medicine, Irvine, California 92612; and Departments of Medicine, Biochemistry, and Physiology (S.M.), Loma Linda University, Pettis Veteran’s Administration Medical Center, Loma Linda, California 92354

Address all correspondence and requests for reprints to: Dan M. Cooper, M.D., Professor of Pediatrics, University of California, Irvine, College of Medicine, Clinical Research Center, Building 25, ZOT 4094-03, 101 The City Drive, Orange, California 92868. E-mail: dcooper{at}uci.edu

Abstract

We recently demonstrated that a brief endurance type training program led to increases in thigh muscle mass and peak oxygen uptake (O₂) in prepubertal girls. In this study, we examined the effect of training on the GHinsulin-like growth factor I (GHIGF-I) axis, a system known to be involved both in the process of growth and development and in the response to exercise. Healthy girls (mean age 9.17 ± 0.10 yr old) volunteered for the study and were randomized to control (n = 20) and training groups (n = 19) for 5 weeks. Peak O₂, thigh muscle volume, and blood samples [for IGF-I, IGF-binding proteins (IGFBP)-1 to -6, and GHBP] were measured. At baseline, IGF-I was significantly correlated with both peak O₂ (r = 0.44, P < 0.02) and muscle volume (r = 0.58, P < 0.004). IGFBP-1 was negatively correlated with muscle volume (r = -0.71, P < 0.0001), as was IGFBP-2. IGFBP-4 and -5 were significantly correlated with muscle volume. We found a threshold value of body mass index percentile (by age) of about 71, above which systematic changes in GHBP, IGFBP-1, and peak O₂ per kilogram were noted, suggesting decreases in the following: 1) GH function, 2) insulin sensitivity, and 3) fitness. Following the training intervention, IGF-I increased in control (19.4 ± 9.6%, P < 0.05) but not trained subjects, and both IGFBP-3 and GHBP decreased in the training group (-4.2 ± 3.1% and -9.9 ± 3.8%, respectively, P < 0.05). Fitness in prepubertal girls is associated with an activated GHIGF-I axis, but, paradoxically, early in a training program, children first pass through what appears to be a neuroendocrine state more consistent with catabolism.

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