When the Institute of Medicine announced last fall that it was changing the amount of vitamin D it recommends most adults get each day from 200 international units (IU) to 600 IU, some critics said the increase was too modest. They cited studies associating higher vitamin D levels with several disease-preventing benefits, in addition to its well-documented role in protecting bones.
But sometimes a conservative approach is justified. For example, it wasn't that long ago that researchers thought folic-acid supplements might protect the heart or prevent cancer, a promise that clinical trials have failed to confirm. Studies of antioxidant supplements have been equally disappointing. In an analysis of 67 trials involving about 230,000 patients, for example, the pills had no beneficial effect on mortality. And three antioxidant vitamins—A, E, and beta-carotene—actually increased the risk of death.
Even the good old multivitamin, far and away the supplement purchased most often in the U.S., has virtually no evidence to show that it improves the average person's health.
Supplements have their place, but "many of the claims are overblown and unsubstantiated," says JoAnn Manson, M.D., chief of preventive medicine at Brigham and Women's Hospital in Boston and a member of the institute's committee on vitamin D. "People are often given the impression that supplements work better than they really do."
Still, more than half of American adults have taken them. So we looked at the evidence for the top five vitamin supplements (excluding multivitamins), based on 2009 sales data from the Nutrition Business Journal, a trade publication. Below is our take on the potential risks and benefits of each, plus advice about who should consider taking them.
Note that the Food and Drug Administration doesn't require supplements to go through the same rigorous testing for safety and efficacy that drugs do.
And certain ingredients can cause serious side effects. If you take supplements, look for products with the "USP Verified" mark, which means they meet standards of quality, purity, and potency set by the nonprofit U.S. Pharmacopeia.