Abstract
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.The steroid hormone vitamin D is required for
normal calcium and phosphorus metabolism and is thus an important contributor to musculoskeletal health. Recent
data have linked low vitamin D levels to a wide range of diseases, including cancer, cardiovascular disease,
autoimmune disease and infection. Adequate levels of vitamin D are maintained through its cutaneous
photosynthesis and oral ingestion. By some estimates, one billion people worldwide have vitamin D deficiency
or insufficiency. A number of factors influence the photosynthesis and bioavailability of vitamin D and contribute
to risk of impaired vitamin D status. These factors include variation in sun exposure due to latitude, season, time
of day, atmospheric components, clothing, sunscreen use and skin pigmentation, as well as age, obesity and the
incidence of several chronic illnesses. This review will focus on factors that influence vitamin D status and
contribute to the prevalence of low vitamin D levels.
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