Introduction: Free hormones are biologically more active in the target tissues. So we wanted to evaluate the vitamin D adequacy with bioavailable and free vitamin D. In order to calculate the bioavailable and free vitamin D according to the previously reported formula, VDBP level was measured. VDBP polymorphisms were also evaluated as they can affect the binding affinity.
Methods: Eighty-four obese and 78 healthy children included in the study. Anthropometry, calcium, phosphorus, alkaline-phosphatase, parathyroid hormone, 25 (OH) vitamin D, bioavailable-free vitamin D, VDBP level and polymorphism were evaluated in the whole group.
Results: When the girls compared within themselves, PTH values were found to be higher in the obese group (p=0.001). When we compared obese and control subjects without gender discrimination, VDBP and PTH levels were found to be statistically higher in the obese group(respectively p=0.008, p=0.002). When only the cases included in the winter period were analyzed, PTH and VDBP were found to be higher and bioavailable and free vitamin D lower in the obese group. There was no difference in terms of total vitamin D between groups during the winter season.
Discussion/conclusion: While total, free, bioavailable vitamin D in obese group was similar to the control group in autumn, free and bioavailable vitamin D in the winter was lower than the control group. Also PTH was higher in obese group than the control group in both autumn and winter. Therefore, many more studies are needed to evaluate the variability of free, bioavailable vitamin D according to the seasons and its effects.
Keywords: 25- hydroxyvitamin D; bioavailable vitamin D; free vitamin D; obesity; polymorphism; vitamin D binding protein.