Do supplements really work?

Swallowing a dose of nutrients in pill form to improve your health makes sense­—in theory. And it's true that taking a supplement can help plug gaps in people who have or are at risk for specific nutrient deficiencies. 

Studies have often found that people whose diets include higher levels of certain nutrients (usually due to high intake of fruits and vegetables) have lower rates of various diseases, including cancer.

But clinical trials testing supplements of those same nutrients have turned up almost universally disappointing results.

For example:

1. Vitamin E does not prevent prostate cancer.

Supplements of vitamin E actually increased prostate-cancer risk in men age 50 and older who had low levels of the mineral selenium, according to a study published in the Journal of the National Cancer Institute. And the same research showed that taking selenium supplements with or without vitamin E boosted prostate cancer risk in men who already had high levels of the mineral.

2. Multivitamins do not prevent cancer or cardiovascular disease.

A 2014 report from the U.S. Preventive Services Task Force found no clear evidence that multivitamins do or don't help protect healthy people from developing cancer or cardiovascular disease. Nor were daily multivitamins any better than a placebo at helping men to fend off age-related declines in mental sharpness in a large study of 5,947 male physicians.

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3. Omega-3 pills do not protect the heart.

Omega-3 fatty acid supplements such as fish oil weren't associated with a statistically significant drop in risks for cardiovascular problems such as heart attack, stroke, or sudden death, according to a review of previous research in the Journal of the American Medical Association.

Why would nutrients proved to be good for us in our food not "work" when taken in pill form? "The biology is complicated," says Stephen P. Fortmann, M.D., senior investigator at Kaiser Permanente's Center for Health Research in Portland, Ore. In the case of antioxidants, for example, "hundreds of nutrients affect antioxidant function in the body, so it's not surprising that taking just a few isolated ones might have no effect or even mess up the system," he says.

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That's especially true when it comes to using supplements to reduce cancer risk, says Pieter Cohen, M.D., a physician at Harvard Medical School and the Cambridge Health Alliance in Massachusetts. In addition to vitamin E and selenium, he cites the example of beta-carotene—a form of vitamin A found in fruits and vegetables, especially orange-colored ones. Though observational studies have found that diets high in beta-carotene might reduce the risk of lung cancer, clinical trials testing supplements of beta-carotene found that it actually increased the risk of lung cancer in smokers. And it failed to protect against a wide range of other cancers, including those of the bladder, colon, kidneys, and pancreas.