Vitamin D: a hormone for all seasons.

Vitamin D is principally obtained from skin through the action of ultraviolet B irradiation on 7-dehydrocholesterol. It is further metabolized to 25-hydroxyvitamin D, the major circulating vitamin D compound, and then to 1,25-dihydroxyvitamin D, the hormonal form. The major function of vitamin D compounds is to enhance active absorption of calcium (and phosphate) from the gut, ensuring that bone does not need to be resorbed to maintain blood calcium concentrations despite obligatory urinary losses. Vitamin D compounds have direct effects to enhance bone and muscle function. Based on good evidence, target levels for 25-hydroxyvitamin D in blood are at least 50 nmol/l and there may be a case for higher targets of 75-80 nmol/l. Adequate calcium intakes help to reduce vitamin D degradation. Vitamin D and calcium together reduce the risk of falls and fractures in older people. There are vitamin D receptors in most nucleated cells and preliminary evidence that adequate vitamin D levels may be important in reducing the incidence of, or mortality from, some cancers, and in reducing autoimmune disease. Adequate vitamin D may also allow for a normal innate immune response to pathogens, improve cardiovascular function and mortality, and reduce type 2 diabetes. Supplemental vitamin D seems to work in a generally similar manner to skin-derived vitamin D.