Insulin Resistance Indices Are Inversely Associated With Vitamin D Binding Protein Concentrations

Context: We hypothesized that, similar to the coordinated homeostatic regulation of most hormones, the concentration of free and bioavailable 25-hydroxyvitamin D [25(OH) D] will be tightly controlled by total 25(OH) D and vitamin D binding protein (VDBP) and that the VDBP concentrations will be associated with insulin resistance status. Objective: Our primary objective was to investigate associations between total, free, and bioavailable 25(OH) D and VDBP. We also evaluated the relationships of VDBP with insulin resistance indices. Study Design: The study design was cross-sectional in the setting of a university children's hospital. The relative concentration of bioavailable 25(OH) D to total 25(OH) D [bioavailable 25(OH) D/total 25(OH) D was expressed as a percentage [percentage bioavailable 25(OH) D]. Results: Subjects were 47, postmenarchal, female adolescents, with a mean age of 15.8 +/- 1.4 years, a mean body mass index of 23.1 +/- 4.0 kg/m(2). The total 25(OH) D was strongly associated with VDBP (rho = 0.57, P < .0001). At lower total 25(OH) D concentrations, the concentration of bioavailable 25(OH) D relative to total 25(OH) D was higher (23.8% vs 14.9%, P < .0001), whereas the relative concentration of free 25(OH) D was similar (P = .44). VDBP was inversely associated with fasting insulin (rho = -0.51, P = .0003) and homeostatic model assessment of basal insulin resistance (rho = -0.45, P= .002) and positively with whole-body insulin sensitivity (rho = 0.33, P = .02); these relationships persisted after adjusting for percentage fat and attenuated after adjusting for race. Conclusion: Our data suggest that VDBP concentrations are regulated by total 25(OH) D levels to maintain adequate concentrations of bioavailable 25(OH) D. VDBP concentrations are inversely associated with hyperinsulinemia and insulin resistance.