Are you taking too much vitamin D?

Many Americans have been warned that they’re deficient in vitamin D and need to take supplements to protect against all the purported ills of getting too little of the vitamin, such as heart disease, diabetes, fractures, and a variety of cancers. Yet the latest review of research, conducted by a Tufts Medical Center team, suggests that many of the supposed benefits of supplementation remain unproven and that taking too high a dose of D—a fat-soluble vitamin that has hormone-like effects—can increase the risk of problems it’s supposed to prevent.

The review, published Monday in the Annals of Internal Medicine, examined 19 clinical trials comparing supplements with placebos and found that fractures could be reduced in those who took calcium along with vitamin D but that the biggest benefits were conferred on elderly folks confined to nursing homes or hospitals. The data were mixed on cancer prevention. Some studies showed that taking 1,000 international units a day of vitamin D could reduce the risk of cancer, while others showed no benefit or an increased risk.

Of particular concern? “We found some potential harmful effects when vitamin D was administered as a megadose or bolus injection,” said study author Mei Chung, assistant director of the Tufts Evidence-based Practice Center. For example a clinical trial published last year in the Journal of the American Medical Association found that a single annual vitamin D shot containing 500,000 IUs—equivalent to 1,370 IUs per day—increased the risk of fractures and falls in elderly women who were already at high risk.

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Last year, a panel of experts convened by the Institute of Medicine set new guidelines for daily vitamin D intake, raising the recommended amount to 600 IUs for kids and adults up to age 70, and 800 IUs for those over 70—while setting 4,000 IUs a day as the safe upper limit.

“The recommendations cover the needs of 97 percent of [the] US population,” said Dr. JoAnn Manson, who served on the institute’s panel and is conducting a vitamin D clinical trial at Brigham and Women’s Hospital. “About one-third of Americans don’t have high enough levels of vitamin D and could benefit from increasing their intake through food or supplements.”

But the extent of the benefits conferred by vitamin D remains unknown. “The evidence is inconclusive that Vitamin D plays a role in nonskeletal outcomes,” said Manson. That was the same conclusion Chung and her colleagues reached in their review, which a government task force will use next month to issue recommendations on whether or not Americans should take vitamin D supplements to prevent disease.

“We’re still not clear what the optimal dose is for vitamin D,” Chung pointed out, since most of the studies didn’t test to see how much supplementation raised blood levels of vitamin D. And researchers still aren’t sure whether increasing the level of vitamin D in the body actually confers a wide range of health benefits. (Manson’s study, which will yield results in five years, is investigating whether taking 2,000 IU’s a day of vitamin D protects against cancer and heart disease better than a placebo, and it will monitor participants’ blood levels of the nutrient.)

A second research review published this week in the Annals concluded that studies are lacking to support the claim that vitamin D supplementation can protect against heart disease, hypertension, and stroke. While population studies have shown that those with high vitamin D levels get fewer heart attacks and strokes, “several possible scenarios, including the influence of confounders, may explain these associations,” write the Irish study authors.

“For example, vitamin D deficiency may only act as a surrogate marker for poor health status,” they added, reflecting an inability to get outdoors into the sun’s ultraviolet rays—which enables the body to manufacture vitamin D on its own. Those who don’t get outside much may be heavier and less likely to exercise, or they could have health conditions that keep them housebound.

In Boston and the rest of New England, however, it’s impossible to make any vitamin D from sunlight during the winter months, so supplements and a higher intake of vitamin D rich foods—tuna, salmon, and fortified foods including milk and orange juice—may be smart bets during this time of year, said Manson.

And given the negative research findings regarding megadose vitamin D shots, many experts agree that it’s probably a good idea to stay away from them.