Vitamin D deficiency, which is common in children and adults, causes rickets, osteomalacia, and osteoporosis. Most organs and immune cells have a vitamin D receptor, and some also have the capacity to metabolize 25-hydroxyvitamin D to 1,25-dihydroxyvitamin D. 1,25-Dihydroxyvitamin D is a potent immunomodulator that also enhances the production and secretion of several hormones, including insulin. Vitamin D deficiency has been associated with increased risk of type 1 diabetes. Glycemic control and insulin resistance are improved when vitamin D deficiency is corrected and calcium supplementation is adequate. 25-Hydroxyvitamin D (measure of vitamin D status) of less than 20 ng/mL is vitamin D deficiency and 21 to 29 ng/mL is insufficiency. Children and adults need at least 1000 IU of vitamin D per day to prevent deficiency when there is inadequate sun exposure.