More than half of American women (and men) reach for a supplement bottle to get the nutrition insurance they think they need. But nutritional supplements rarely live up to their hype, reports the January 2013 issue of Harvard Women's Health Watch.

Over the years there has been a lot of highly positive news about supplements. Antioxidants such as vitamin E were once seen as a promising silver bullet against heart disease, cancer, and Alzheimer's disease. Omega-3 fatty acids were once touted for warding off strokes and other cardiovascular events. The latest supplement in the spotlight, vitamin D, is being eyed as a possible defense against a long list of diseases, including cancer, diabetes, depression, and even the common cold.

But here's the big caveat: much of the most exciting research included observational studies, which can't show cause and effect. Observational studies follow large groups of people who chose to take supplement. While that strategy can yield useful information, it isn't nearly as reliable as testing a particular supplement against a placebo (inactive pill) in a controlled setting. When that was done, the more stringent randomized controlled trials often found no effect for the supplement.

"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 results 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).

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, vitamin A to preserve a healthy immune system and vision, and many, many more. Yet the source of these nutrients is important. "Usually it is best to try to get vitamins, minerals, and nutrients from food as opposed to supplements," Dr. Manson says.

Read the full-length article: "Dietary supplements: Do they help or hurt?"