Vitamin D is not strictly a vitamin, rather it is the precursor of one of the hormones involved in the maintenance of calcium homeostasis and the regulation of cell proliferation and differentiation, where it has both endocrine and paracrine actions. Dietary sources are relatively unimportant compared with endogenous synthesis in the skin by photolysis of 7-dehydrocholesterol; problems of deficiency arise when there is inadequate exposure to sunlight. The deficiency diseases (rickets in children and osteomalacia in adults) are therefore largely problems of temperate and subarctic regions, although cultural factors that result in little exposure to sunlight may also cause problems in subtropical and tropical areas. There are few foods that are rich sources of vitamin D. It is generally accepted that, for people with inadequate exposure to sunlight (young children and the house-bound elderly), supplements are necessary to maintain adequate status. Excessively high intakes of vitamin D are associated with hypercalcemia and calcinosis.

Although the pioneering studies of Chick and others during the 1920s clarified the dual roles of sunlight exposure to promote endogenous synthesis and dietary sources of the vitamin, it was not until high specific activity [3H]vitamin D became available in the 1960s that the onward metabolism of vitamin D to the active metabolite, calcitriol, was discovered, and its mechanism of action elucidated, largely by Kodicek and coworkers in Cambridge and DeLuca and coworkers in Wisconsin. Calcitriol acts as a steroid hormone, binding to a nuclear receptor protein in target tissues and regulating gene expression.