SUPPLEMENT: Vitamin D
ALSO KNOWN AS: Calciferol, ergocalciferol, calcitriol, cholecalciferol
USES: To treat psoriasis, scleroderma, seasonal affective disorder, and to prevent osteoporosis and cancer
BACKGROUND: Vitamin D is a fat-soluble substance found in dairy products, fish, and mushrooms. Sunlight, the major source of vitamin D for humans, promotes the endogenous synthesis of its active form. Multivitamins and supplements with various amounts of vitamin D are commercially available. The two forms used in humans are ergocalciferol (D2) and cholecalciferol (D3)—they are bioequivalent, but D3 may be preferable because it has a longer half-life.
RESEARCH: Vitamin D was shown to improve bone density and prevent fractures, but such benefits were not observed in the Women’s Health Initiative (WHI) study, perhaps due to the use of inadequate doses. Vitamin D effectively treats psoriasis. Its ability to improve seasonal affective disorder is unclear. It has also been examined as a preventive agent and as a treatment for cancer. Observational, prospective, and retrospective studies suggest that it decreases the risk of developing cancer. A recent double-blind, randomized controlled trial found that supplementation with vitamin D plus calcium significantly reduced cancer incidence.
Conversely, a recent large prospective study showed that, with the exception of colorectal cancer, vitamin D did not protect against cancer, and a study of Finnish smokers showed that subjects with higher 25(OH)D levels were at greater risk for pancreatic cancer compared with those with lower concentrations. Controlled clinical trials studying vitamin D supplementation are required to determine optimal levels for cancer prevention.
ADVERSE REACTIONS: High doses of vitamin D can cause hypercalcemia. Toxicity associated with elevated serum levels of vitamin D includes depression, headache, drowsiness, weakness, calcium and bone loss, and metastatic calcification of soft tissues in the kidney, heart, blood vessels, and lungs.
DR. CASSILETH: The recommendation to minimize sun exposure to prevent skin cancer has produced a pandemic of vitamin D deficiency. Vitamin D has generated considerable interest in the past decade, as accumulating evidence from both retrospective and prospective epidemiologic studies suggests an association between vitamin D deficiency and increased risk of autoimmune, infectious, and cardiovascular diseases, as well as cancer. Vitamin D supplementation assists repletion in deficient individuals. Major medical centers typically recommend 1,000 IU/d for an average adult.
Vitamin D maintains serum calcium and phosphorus levels by regulating their absorption and excretion. It is also important for bone formation, as deficiency can cause rickets and other bone disorders, is a potent antiproliferative agent, and a pro-differentiation hormone.
Currently, the level of the major circulating form of vitamin D, 25-hydroxyvitamin D—resulting from vitamin D metabolism in the liver—is used to measure vitamin D status. Optimal concentration, including that for cancer prevention, is not yet known. Vitamin D lowers the incidence of colorectal cancer but not that of other cancers. This plus the Finnish pancreatic cancer study suggest that response to vitamin D may vary by tissue type. Additional research is needed to fully understand the mechanism of action and optimal dosing of vitamin D.
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