Decreased bioavailability of vitamin D in obesity

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Abstract
Background: Obesity is associated with vitamin D insufficiency and secondary hyperparathyroidism. Objective: This study assessed whether obesity alters the cutaneous production of vitamin D3 (cholecalciferol) or the intestinal absorption of vitamin D2 (ergocalciferol). Design: Healthy, white, obese [body mass index (BMI; in kg/m2) ≥ 30] and matched lean control subjects (BMI ≤ 25) received either whole-body ultraviolet radiation or a pharmacologic dose of vitamin D2 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 D3 concentrations 24 h after whole-body irradiation showed that the incremental increase in vitamin D3 was 57% lower in obese than in nonobese subjects. The content of the vitamin D3 precursor 7-dehydrocholesterol in the skin of obese and nonobese subjects did not differ significantly between groups nor did its conversion to previtamin D3 after irradiation in vitro. The obese and nonobese subjects received an oral dose of 50000 IU (1.25 mg) vitamin D2. BMI was inversely correlated with serum vitamin D3 concentrations after irradiation (r = −0.55, P = 0.003) and with peak serum vitamin D2 concentrations after vitamin D2 intake (r = −0.56, P = 0.007). Conclusions: Obesity-associated vitamin D insufficiency is likely due to the decreased bioavailability of vitamin D3 from cutaneous and dietary sources because of its deposition in body fat compartments.