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


OBJECTIVE
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.
Military Performance Division, US Army Research Institute of Environmental Medicine, Natick, MA 01760, USA. Bradley.nindl@us.army.mil
Bradley C
BC
Nindl
James P
JP
McClung
Jeremy K
JK
Miller
J Philip
JP
Karl
Joseph R
JR
Pierce
Dennis E
DE
Scofield
Andrew J
AJ
Young
Harris R
HR
Lieberman
D000293:Adolescent; D000328:Adult; D001682:Biological Availability; D001823:Body Composition / Q000502:physiology*; D005260:Female; D006801:Humans; D018970:Insulin-Like Growth Factor Binding Protein 1 / Q000097:blood; D018971:Insulin-Like Growth Factor Binding Protein 2 / Q000097:blood; D007334:Insulin-Like Growth Factor I / Q000378:metabolism*; D008889:Military Personnel; D010809:Physical Fitness / Q000502:physiology*; D012372:ROC Curve; D014481:United States; D055815:Young Adult
D018970:Insulin-Like Growth Factor Binding Protein 1; D018971:Insulin-Like Growth Factor Binding Protein 2; D007334:Insulin-Like Growth Factor I