"Often the enthusiasm for these vitamins and supplements outpaces the evidence. And when the rigorous evidence is available from randomized controlled trials, often the results are at odds with the findings of the observational studies," explains Dr. JoAnn Manson, chief of preventive medicine at Brigham and Women's Hospital, professor of medicine at Harvard Medical School, and principal investigator of a large randomized trial known as VITAL (Vitamin D and Omega-3 Trial).

Because observational studies may not fully control for dietary factors, exercise habits, and other variables, they can't prove whether the treatment is responsible for the health benefits. "People who take supplements tend to be more health conscious, exercise more, eat healthier diets, and have a whole host of lifestyle factors that can be difficult to control for fully in the statistical models," Dr. Manson says.

Some supplements that were found to have health benefits in observational studies turned out, with more rigorous testing, to be not only ineffective but also risky. Vitamin E, which was initially thought to protect the heart, was later discovered to increase the risk for bleeding strokes. Folic acid and other B vitamins were once believed to prevent heart disease and strokes—until later studies not only didn't confirm that benefit but actually raised concerns that high doses of these nutrients might increase cancer risk.

How to get your nutrients

We need a variety of nutrients each day to stay healthy, including calcium and vitamin D to protect our bones, folic acid to produce and maintain new cells, and vitamin A to preserve a healthy immune system and vision.

Yet the source of these nutrients is important. "Usually it is best to try to get these vitamins and minerals and nutrients from food as opposed to supplements," Dr. Manson says.

Fruits, vegetables, fish, and other healthy foods contain nutrients and other substances not found in a pill, which work together to keep us healthy. We can't get the same synergistic effect from a supplement. Taking certain vitamins or minerals in higher-than-recommended doses may even interfere with nutrient absorption or cause side effects.