Context  Both obesity (body mass index, BMI ≥ 30 kg/m2) and Black race are associated with a higher risk of vitamin D deficiency and secondary hyperparathyroidism. We hypothesized the risk of hypovitaminosis D would therefore be extraordinarily high in obese Black adults.

Objective  To study the effects of race and adiposity on 25-hydroxyvitamin D [25(OH)D] and parathyroid hormone (iPTH).

Design, Setting and Participants  Cross-sectional study of 379 Black and White adults from the Washington D.C. area. BMI ranged from 19·9 to 58·2 kg/m2.

Main Outcome Measures  Prevalence of hypovitaminosis D [25(OH)D < 37·5 nmol/l] and secondary hyperparathyroidism [25(OH)D < 37·5 nmol/l with iPTH > 4·2 pmol/l].

Results  Obese Black subjects had lower mean 25(OH)D, 40·3 (SD, 20·3) nmol/l, compared with obese Whites, 64·5 (29·7), P < 0·001, nonobese Blacks, 53·3 (26·0), P = 0·0025 and nonobese Whites, 78·0 (33·5), P < 0·001. The prevalence of hypovitaminosis D increased with increasing BMI, and was greater (P < 0·001) in Blacks than Whites within all BMI categories examined. Among subjects with BMI ≥ 35 kg/m2, 59% of Blacks vs 18% of Whites had hypovitaminosis D (odds ratio 6·5, 95% confidence interval 3·0–14·2). iPTH was negatively correlated with 25(OH)D (r = 0·31, P < 0·0001), suggesting those with hypovitaminosis D had clinically important vitamin D deficiency with secondary hyperparathyroidism. For secondary hyperparathyroidism 35·2% of Blacks met the criteria, compared to 9·7% of Whites (OR 3·6, CI 1·5–98·8).

Conclusions  Obese Black Americans are at particularly high risk for vitamin D deficiency and secondary hyperparathyroidism. Physicians should consider routinely supplementing such patients with vitamin D or screening them for hypovitaminosis D.