Lack of an Association Between Insulin‐like Growth Factor‐I and Body Composition, Muscle Strength, Physical Performance or Self‐Reported Mobility Among Older Persons with Functional Limitations

OBJECTIVE: The hypothesis that decreased growth hormone (GH) secretion contributes to the functional decline that occurs with aging is far from substantiated. There have been few studies addressing the distribution and correlates of IGF-I, an indicator of GH activity, in nonclinical populations. As part of a growth hormone intervention trial, we examined the cross-sectional relations between IGF-I levels and multiple measures of physical function, body composition, and strength in a group of older men and women exhibiting mild to moderate reductions in measured physical performance. METHODS: Using a variety of advertising techniques, 155 older subjects were recruited from a metropolitan area to participate in a growth hormone and exercise intervention study. At baseline, all subjects had blood drawn for IGF-I and underwent testing of body composition using dual X-ray absorptiometry, muscle strength using isokinetic dynamometry, and functional assessment using timed performance measures and self-report. Associations between levels of IGF-I, body composition, strength, and physical function were assessed after dividing men and women separately into tertiles of IGF-I as well as treating IGF-I as a continuous variable. RESULTS: Men had higher IGF-I levels than women, and a significant inverse correlation was observed between age and IGF-I in men (r=-0.29, P=.009). There were no clear trends for associations between tertiles of IGF-I, and any of the variables tested. Linear regression models with IGF-I treated as a continuous measure were not associated significantly with any of the measures of physical function, body composition, or strength (all P > 0.05). CONCLUSIONS: In conclusion, although IGF-I levels declined with age in men, these data did not demonstrate an association between IGF-I and measures of muscle strength, body composition, or physical functioning. These findings support the growing body of evidence that IGF-I levels may not be an indicator of growth hormone activity in older persons.