Low plasma insulin-like growth factor-1 concentrations predict worsening of insulin-mediated glucose uptake in older people

OBJECTIVE: The relationship among insulin action, advancing age, and insuli n like growth factor-1 (IGF-1) is poorly understood. To gain further insigh t, the predictive role that low plasma IGF-1 concentration may have on insu lin-mediated glucose uptake in older persons was investigated. DESIGN: The study was designed as a longitudinal, observational trial. PARTICIPANTS: Fifty-eight healthy aged (73.1 +/- 9.4 years) subjects (31 ma les/27 females) were followed up for 12 months. MEASUREMENTS: At baseline and at the end of the follow-up, insulin-mediated glucose uptake was assessed by euglycemic glucose clamp and plasma total I GF-1 and IGF-binding protein 3 (IGF-BP-3) in each subject, and concentratio ns were determined. RESULTS: At baseline, plasma IGF-1 concentrations correlated with whale bod y glucose uptake (WBGD) (r = 0.39, P < .003),insulin-stimulated glucose oxi dation (GOX) (r = 0.35, P < .009)Sand non-oxidative glucose metabolism (r = 0.37, P < .007). Such correlations were also independent of age, sex, body fat, and waist/hip ratio. Fasting plasma total IGF-1 concentrations (84 +/ - 56 vs 63 +/- 44 mu g/L, P < .0.40), plasma IGF-1/IGF-BP3 molar ratio (0.1 3 +/- 0.05 vs 0.10 +/- 0.03 P < .050), and WBGD (34.8 +/- 5.0 vs 23.1 +/- 4 .6 mu mol/kg x min, P < .010) were more elevated at baseline than at the en d of the follow-up. Low baseline fasting plasma IGF-1 concentration (RR = 1 .5, 95% CI = 1.3-1.7) and plasma IGF-1/IGFBP3 molar ratio (RR = 1.4, 95% CT = 1.3-1.8) predicted a decline in WBGD. The predictive role of plasma IGF- 1 on age-related decline in WBGD was independent of age, sex, body fat, wai st/hip ratio, and degree of physical activity (model 1), or of fasting plas ma free fatty acid and triglyceride concentrations, LDL/HDL ratio, and basa l adjusted respiratory quotient (model 2). Finally, low plasma IGF-1 concen tration predicts a decline in WBGD independent of body fat, free fatty acid s, waist/hip ratio, and basal adjusted respiratory quotient (model 3). CONCLUSION: Our study demonstrates that fasting plasma IGF-1 concentration may have a modulatory role on insulin action in older people. This finding might prompt an evaluation of the direct effect of IGF-1 administration on insulin sensitivity in older adults.