VDR and vitamin D deficiency: The causes of vitamin D deficiency are varied and include limited sunlight exposure, sunscreen use, inadequate intake of foods containing vitamin D, dark skin pigmentation, patients with intestinal malabsorption syndromes, genetic diseases of vitamin D metabolizing enzymes or the vitamin D receptor (VDR), drugs that can interfere with the vitamin D absorption or metabolism, hepatic disease, obesity, aging, and renal disease [18,19].