VDR and vitamin D deficiency: The interaction of 1,25-dihydroxyvitamin D with the vitamin D receptor increases the efficiency of absorption of intestinal calcium by up to 40% and phosphorus up to 80%.11, 12 Vitamin D deficiency is defined as a 25-hydroxyvitamin D (25[OH] D) level of less than 20 ng/mL.13, 14 There are many causes of vitamin D deficiency, including heritable disorders like obesity,15 dark skin16, 17 and acquired disorders like lack of sunlight, drugs,18 and malabsorption.