VDR and secondary hyperparathyroidism: In addition, the following reasons may also explain the low vitamin D status in obesity: (1) lower sun exposure; (2) reduced hepatic synthesis of 25(OH)D due to obesity-associated secondary hyperparathyroidism; (3) negative feedback from an increased 1,25(OH)D concentration; (4) VDR polymorphisms; (5) lower prevalence of vitamin D supplement use (45, 50, 51).