Background: Studies examining vitamin D levels in association with childhood obesity usually do not concurrently measure levels of vitamin D-binding protein and do not calculate the unbound, bioavailable vitamin D, that is considered the fraction of 25-hydroxyvitamin D able to exert biological activity.
Objective and hypotheses: To evaluate in a group of children for the most part obese i) the concentrations of both total 25-hydroxyvitamin D and of the bioavailable fraction ii) the potential role of insulin resistance in modulating the concentrations of bioavailable vitamin D.
Method: 63 obese children and 21 lean controls were enrolled and the main metabolic parameters were investigated. Total 25-hydroxyvitamin D and vitamin D-binding protein were measured, two SNPs in the coding region of the vitamin D-binding protein (rs 4588 and rs 7041) were studied and, using these data, the vitamin D bioavailable fraction was calculated.
Results: Obese children showed total 25-hydroxyvitamin D levels lower compared to not-obese children (21.3±6.7 ng/ml vs 29.6±11.7 ng/ml; P: 0.0004). Bioavailable 25-hydroxyvitamin D levels, on the contrary, were not different among obese and not obese children (3.1±1.6 ng/ml vs 2.6±1.2 ng/ml; P>0.05). Insulin resistant children showed higher bioavailable levels of 25-hydroxyvitamin D compared to not insulin resistant children (3.4±1.4 ng/ml vs 2.0±0.9 ng/ml; P: 0.013) and an inverse correlation between insulin resistance and vitamin D-binding protein was found (r: −0.40; P: 0.024).
Conclusion: Our data i) show that obese children, although have low concentrations of total 25-hydroxyvitamin D, present levels of bioavailable 25-hydroxyvitamin D similar to those of normal weight children ii) demonstrate that this finding is due to a reduced concentration of vitamin D-binding protein iii) suggest that the increased insulin resistance usually present in obesity may be associated to this reduction.
01 Oct 2015 - 03 Oct 2015