A. Eliakim et al., Fitness, training, and the growth hormone -> insulin-like growth factor I axis in prepubertal girls, J CLIN END, 86(6), 2001, pp. 2797-2802
We recently demonstrated that a brief endurance type training program led t
o increases in thigh muscle mass and peak oxygen uptake ((V) over dot O-2)
in prepubertal girls. In this study, we examined the effect of training on
the GH-->insulin-like growth factor I(GH-->IGF-I) axis, a system known to b
e involved both in the process of growth and development and in the respons
e to exercise. Healthy girls (mean age 9.17 +/- 0.10 yr old) volunteered fo
r the study and were randomized to control (n = 20) and training groups (n
= 19) for 5 weeks. Peak (V) over dot O-2, thigh muscle volume, and blood sa
mples [for IGF-I, IGF-binding proteins (IGFBP)-1 to -6, and GHBP] were meas
ured. At baseline, IGF-I was significantly correlated with both peak (V) ov
er dot O-2 (r = 0.44, P < 0.02) and muscle volume (r = 0.58, P < 0.004). IG
FBP-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 v
olume. 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 (V) ov
er dot O-2, per kilogram were noted, suggesting decreases in the following:
1) GH function, 2) insulin sensitivity, and 3) fitness. Following the trai
ning 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 GH-->IGF-I axis, but, pa
radoxically, early in a training program, children first pass through what
appears to be a neuroendocrine state more consistent with catabolism.