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

Journal Title (Medline/Pubmed accepted abbreviation): Med. Sci. Sports Exerc.
Year: 2011
Volume: 43
Volume: 5
Page numbers:793-799
doi (if applicable): 10.1249/MSS.0b013e31820065ea

Summary of background and research design

Background: Insulin-like growth factor (IGF-I) is a hormone that stimulates muscle growth.  There are several different forms of IGF-I that circulate in the blood, but there is only one that is active, the “bioavailable” form (i.e., not bound to IGF binding protein).  Many studies assessing IGF-I have historically measured total IGF-I levels, not the bioavailable form, and therefore these studies could provide a slightly distorted picture of the relationship between IGF levels and physiological responses. 

Because of their higher average fat mass and lower fat free mass percentages, women may benefit more than men from strategies designed to increase fat-free mass and strength, especially in physically-demanding occuprations such as military service.

Hypothesis: Increases in bioavailable IGF-I will have a greater association with exercise-induced increases in strength that total IGF-I during basic military training.

Subjects: 77 female Army recruits, age 21 ± 5 yrs old

Experimental design: Physiological and anthropometric measures were acquired pre- and post-basic training; observational study-no treatments assigned.

Protocol: Physical training was performed 6 days/wk for 8 wks.  Training involved military-relevant drills and conditioning.  Blood was collected during weeks 1 and 8 after an overnight fast and was assessed for free IGF-I and IGF binding proteins, which regulate bioavailability of IGF-I.  Body composition was assessed with skinfolds to determine fat percentage, with confirmation by dual energy x-ray absorptiometry (DEXA) in a subset of participants. 

Summary of research findings
  • On average, the females lost about 3.1 kg of fat mass and gained about 2.8 kg of fat free mass (p ≤ 0.05 when comparing week 1 to week 8). 
  • Change in bioavailable IGF-I explained 3 times as much of the variance in fat-free mass change as did total IGF.
  • Women who began the study with lower levels of bioavailable IGF-I than average were more likely to gain more than 7% fat-free mass.  The lower levels of baseline bioavailable IGF-1 allowed for an increase in bioavailable IGF-1 with training, which was associated with greater increases in fat-free mass.

Interpretation of findings/Key practice applications

Bioavailable IGF-I correlates better with increases in fat free muscle mass than total circulating IGF-I. This could be of clinical significance, because it showed that women who initiate training with lower than average concentrations of bioavailable IGF-I have greater potential to gain muscle mass. It would be interesting to see if dietary or pharmaceutical interventions that might raise bioavailable IGF-1 to a greater degree would correspond with greater increases in fat-free mass.

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