Patients with class III obesity (BMI > 40) present with low levels of 25(OH)D for various reasons, including nutritional factors, psychological reasons leading to less sun exposure, decreased hepatic expression of CYP2R1, and sequestration of the vitamin in the excess adipose tissue (15, 72). The gene discussed is CYP2R1; the disease is obesity due to melanocortin 4 receptor deficiency.