Vitamin D and Skeletal Aging

INTRODUCTION Vitamin D is required for intestinal absorption of dietary calcium and for normal mineralization of bone. It fulfills the classic definition of a vitamin because it is an essential nutrient provided by natural foodstuffs that is needed in minute amounts to prevent deficiency diseases. Dietary sources are plants, which photosynthesize ergocalciferol (vitamin D2), fatty fishes such as salmon and mackerel, cod liver oil, and fortified foodstuffs (cereals, cereal bars, and juices). There are two reasons to consider vitamin D a hormone rather than a vitamin. First, it can be produced by the skin upon exposure to sunlight (Figure 1). Second, because its activity depends upon sequential hydroxylations in the liver and kidney, even dietary vitamin D must be enzymatically activated. The intermediate is 25-hydroxyvitamin D, the major circulating form, and the active metabolite is 1,25-dihydroxyvitamin D (Figure 2).