Ann Intern Med 174: 145 - 156, 2021. doi: 10.7326/M20 - 3812 Etiology of Vitamin D Deficiency and Dependency Vitamin D deficiency may result from the following: Inadequate exposure to sunlight Inadequate intake of vitamin D Reduced absorption of vitamin D Abnormal metabolism of vitamin D Resistance to the effects of vitamin D Inadequate exposure or intake Inadequate direct sunlight exposure or sunscreen use and inadequate intake usually occur simultaneously to result in clinical deficiency.