Questions remain about vitamin D benefits and risks

Vitamin D has emerged as something of a wonder supplement, according to the claims of dozens of studies published in the past few years. Researchers have suggested that adequate levels of the nutrient not only strengthen bones, but reduce the risk of cancer, heart disease and other conditions. Several studies also suggest Americans are not getting enough vitamin D, which is synthesized in the skin from sun exposure and found in certain foods, such as fortified milk and cereal, some types of fish and legumes.

Not so fast, says a report published Monday by an expert panel of scientists. A summary of evidence by the U.S. Preventive Services Task Force found vitamin D alone does not prevent bone fractures and that too little evidence exists to draw a link between the supplement and cancer prevention.

Moreover, a second study published Monday finds scant evidence to bolster claims that vitamin D lowers the risk of heart disease.

The U.S. Preventive Services Task Force report, which is a draft recommendation that will soon be open for public comment, was based on a review of 19 clinical trials and 28 observational studies. The report found strong evidence that vitamin D (300 to 1,100 international units per day) combined with calcium (500 to 1,200 milligrams per day) reduced the risk of fracture in older people.

Determining whether vitamin D helps prevent some types of cancer or heart disease is trickier, the U.S. Preventive Services Task Force report notes, because it is difficult to quantify how much vitamin D people get from sun exposure and dietary intake.

The report on vitamin D and cardiovascular disease reviewed previous research and found much of it inconclusive or contradictory. “[N]o clear evidence indicates that vitamin D supplementation has a role to play in the prevention of cardiovascular disease, outside of clinical studies,” the authors wrote.

In short, both reports, published in the Annals of Internal Medicine, conclude that more research is needed to clarify how much vitamin D people need to maximize the supplement’s benefits. Too much vitamin D can be harmful, causing renal and kidney stones.

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