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 vitamin D-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 (21.3 +/- 6.7 ng/mL vs 29.6 +/- 11.7 ng/mL; P =.0004). Bioavailable 25-hydroxyvitamin D levels were not different among the two groups (3.1 +/- 1.6 ng/mL vs 2.6 +/- 1.2 ng/mL; P>.05). Insulin-resistant children showed higher bioavailable levels of 25-hydroxyvitamin D compared with non-insulin-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 concentration.