Decreased bioavailability of vitamin D in obesity

Background: Obesity is associated with vitamin D insufficiency and secondary hyperparathyroidism. Objective: This study assessed whether obesity alters the cutaneous production of vitamin D-3 (cholecalciferol) or the intestinal absorption of vitamin D-2 (ergocalciferol). Design: Healthy, white, obese [body mass index (BMI; in kg/m(2)) greater than or equal to 30] and matched lean control subjects (BMI less than or equal to 25) received either whole-body ultraviolet radiation or a pharmacologic dose of vitamin D-2 orally. Results: Obese subjects had significantly lower basal 25-hydroxyvitamin D concentrations and higher parathyroid hormone concentrations than did age-matched control subjects. Evaluation of blood vitamin D-3 concentrations 24 h after whole-body irradiation showed that the incremental increase in vitamin D-3 was 57% lower in obese than in nonobese subjects, The content of the vitamin D-3 precursor 7-dehydrocholesterol in the skin of obese and nonobese subjects did not differ significantly between groups nor did its conversion to previtamin D-3 after irradiation in vitro. The obese and nonobese subjects received an oral dose of 50 000 IU (1.25 mg) vitamin D-2. BMI was inversely correlated with serum vitamin D-3 concentrations after irradiation (r = -0.55, P = 0.003) and with peak serum vitamin D-2 concentrations after vitamin D-2 intake (r = -0.56, P = 0.007). Conclusions: Obesity-associated vitamin D insufficiency is likely due to the decreased bioavailability of vitamin D-3 from cutaneous and dietary sources because of its deposition in body fat compartments.