McCollum Award Lecture, 1994: vitamin D--new horizons for the 21st century.

Vitamin D is absolutely essential for the maintenance of a healthy skeleton throughout our lives. There is mounting evidence that vitamin D insufficiency and vitamin D deficiency in elderly people is a silent epidemic that results in bone loss and fractures. It is casual exposure to sunlight that provides most humans with their vitamin D requirement. Seasonal changes, time of day, latitude, aging, sunscreen use, and melanin pigmentation can substantially influence the cutaneous production of vitamin D. Although the recommended dietary allowance for vitamin D in adults is 5 micrograms (200 IU), there is mounting evidence that in the absence of exposure to sunlight the vitamin D requirement is at least 15 micrograms (600 IU)/d. The skin is a target tissue for the active form of vitamin D (1,25-dihydroxycholecalciferol). 1,25-Dihydroxycholecalciferol inhibits the proliferation of cultured keratinocytes and induces them to differentiate. 1,25-Dihydroxycholecalciferol and its analogs have been developed as an effective new therapy for the treatment of the hyperproliferative skin disease psoriasis.