Bioavailable IGF-I is associated with fat-free mass gains after physical training in women.


UNLABELLED The contributions of systemic versus local insulin-like growth factor-I (IGF-I) action for mediating fat-free mass (FFM) accretion have yet to be fully clarified, but circulating IGF-I is the preferred measure in clinical practice, and its merits as a biomarker have been demonstrated for a number of physiological outcomes. PURPOSE To test the hypothesis that bioavailable IGF-I would have a stronger association with physical activity-induced FFM accretion than total IGF-I and would serve as a prognostic indicator of FFM accretion. METHODS Seventy-seven young healthy women (21 ± 5 yr, 62.7 ± 8.5 kg, 27.0% ± 6.0% body fat) participated in 8 wk of Army basic training involving intense physical activity. Total and bioavailable IGF-I; IGF binding proteins (IGFBP) 1, 2, 3, 4, 5, and 6; and body composition parameters were measured before and after the training. RESULTS There were significant (P≤0.05) increases in FFM (6%) and decreases in fat mass (-13%). Total IGF-I and IGFBP-4 to -6 increased, whereas IGFBP-1 and IGFBP-2 decreased. Bioavailable IGF-I (24%) explained three times the amount of variance in relative FFM changes than did total IGF-I (8%). Receiver operator characteristic curve analysis revealed that women with lower baseline bioavailable IGF-I were twice as likely to experience FFM gains >7%. Women gaining >7% FFM had greater increases in total IGF-I, maintained bioavailable IGF-I concentrations and experienced greater decreases in IGFBP-2 and increases in IGFBP-6 than those women gaining <7% FFM. CONCLUSIONS Circulating bioavailable IGF-I has a moderate association with physical activity-induced increases in FFM accretion in young, healthy women, and this association is greater than observed for total IGF-I.

DOI: 10.1249/MSS.0b013e31820065ea

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@article{Nindl2011BioavailableII, title={Bioavailable IGF-I is associated with fat-free mass gains after physical training in women.}, author={Bradley C. Nindl and James P. McClung and Jeremy K. Miller and J. Philip Karl and Joseph R. Pierce and Dennis E. Scofield and Andrew J. Young and Harris R. Lieberman}, journal={Medicine and science in sports and exercise}, year={2011}, volume={43 5}, pages={793-9} }