Body mass index, free insulin-like growth factor I, and physical function among older adults: results from the ilSIRENTE study.

The aim of the present study was to evaluate the mediating role played by obesity on the relationship of free insulin-like growth factor I (IGF-I) and IGF-binding protein-3 (IGFBP-3) with muscle strength and physical performance. Data were from baseline evaluation of the ilSIRENTE Study. Muscle strength was measured by hand grip strength. Physical performance was assessed using the walking speed and the 0-3 Short Physical Performance Battery (SPPB) score. Based on its median value, free IGF-I was categorized in the following two groups: low IGF-I (IGF-I <0.65 ng/ml; n = 174) and high IGF-I (IGF-I > or =0.65 ng/ml; n = 175). Similarly, IGFBP-3 was categorized in the following two groups: low IGFBP-3 (IGFBP-3 <4,319.9 ng/ml; n = 174) and high IGFBP-3 (IGFBP-3 > or =4,319.9 ng/ml; n = 175). Body mass index (BMI) was categorized as follows: <25 kg/m(2) (n = 160), 25-29.9 kg/m(2) (n = 133), > or =30 kg/m(2) (n = 56). Mean age of the 349 participants was 85.8 yr, and 234 (67%) were women. After adjusting for potential confounders, no significant association of IGF-I and IGFBP-3 with study outcomes was observed. After the study sample was stratified by BMI groups, compared with participants with low IGF-I level, those with high IGF-I level had a significantly better grip strength [35.2 +/- 1.6 vs. 29.2 +/- 2.0 (SE) kg, P = 0.03], walking speed (0.55 +/- 0.04 vs. 0.40 +/- 0.04 m/s, P = 0.01), and SPPB score (1.9 +/- 0.1 vs. 1.5 +/- 0.1 m/s, P = 0.01) but only in the group with BMI > or =30 kg/m(2) and not in other BMI groups. A statistically significant interaction between BMI and IGF-I level was observed on all study outcomes. By contrast, no association was observed between IGFBP-3 and study outcomes, independently of BMI. In conclusion, high IGF-I level is associated with better physical function in older persons with obesity, but not in nonobese subjects.