Bioavailable Vitamin D in Obese Children: The Role of Insulin Resistance.

Context: Studies examining vitamin D levels in association with childhood obesity usually do not consider the effect of insulin on vitamin D–binding protein and do not calculate the unbound, bioavailable vitamin D. Objective: This study aimed to evaluate in a group of children 1) the concentrations of both total 25-hydroxyvitamin D and bioavailable fraction, and 2) the potential role of insulin resistance in modulating the concentrations of bioavailable vitamin D. Design, Setting, and Patients or Other Participants: This was a cross-sectional study at a University Pediatric Department in which 63 obese children and 21 lean controls were enrolled. Main Outcome Measures: Total 25-hydroxyvitamin D and vitamin D–binding protein were measured, twosingle-nucleotide polymorphisms in the coding region of the vitaminD–binding protein (rs4588 and rs7041) were studied, and the vitamin D bioavailable fraction was calculated. Results: Obese children showed total 25-hydroxyvitamin D levels lower compared with nonobese children (21.36.7 ng/mL vs 29.611.7 ng/mL; P.0004). Bioavailable 25-hydroxyvitaminDlevels were not different among the two groups (3.1 1.6 ng/mL vs 2.6 1.2 ng/mL; P .05). Insulinresistant children showed higher bioavailable levels of 25-hydroxyvitamin D compared with noninsulin- resistant children (3.4 1.4 ng/mL vs 2.0 0.9 ng/mL; P .013) and an inverse correlation between insulin resistance and vitamin D–binding protein was found (r: 0.40; P .024). Conclusions: Obese children present levels of bioavailable 25-hydroxyvitamin D similar to those of normal-weight children due to reduced concentration of vitamin D–binding protein. The insulin resistance could play a role in this reduced concentrati