The major function of vitamin D (either vitamin D2 or D3) is to maintain healthy bone. Most humans obtain their vitamin D requirement through casual exposure of the skin to solar ultraviolet B and from dietary intake. The cutaneous synthesis of vitamin D is a function of 7-dehydrocholesterol concentration in epidermis, melanin pigmentation, and the solar zenith angle which depends on latitude, season, and time of day. Our recent study also indicates that altitude may influence the production of previtamin D3. One area which has shown more progress during the past decade is the use of simulated sunlamp to improve vitamin D production in patients with intestinal malabsorption and elderly who were infirmed or living in northern latitude. Vitamin D deficiency is common in infants, children, and adults worldwide. The major cause of vitamin D deficiency globally is an underappreciation of the crucial role of sunlight in providing humans with their vitamin D requirement. The association between vitamin D deficiency and the increased risk of cancers, autoimmune diseases, infectious diseases, and cardiovascular disease indicates the importance of sunlight, vitamin D, and overall health and well-being of the general population.
Chen TC, Lu Z, and Holick MF. Photobiology of Vitamin D. In Vitamin D: Physiology, Molecular Biology and Clinical Applications by Holick MF. Humana Press, 2010.