The largest study ever of multivitamin use in older women found they did nothing to prevent common cancers or heart disease.
The eight-year study in 161,808 postmenopausal women echoes recent disappointing vitamin studies in men.
Millions of Americans spend billions of dollars on vitamins to boost their health. Research has focused on cancer and heart disease in particular because of evidence that diets full of vitamin-rich foods may protect against those illnesses. But that evidence doesn't necessarily mean pills are a good substitute.
The study's lead author, researcher Marian Neuhouser of the Fred Hutchinson Cancer Research Center in Seattle, offered this advice: "Get nutrients from food. Whole foods are better than dietary supplements," Neuhouser said.
The study appears in Monday's Archives of Internal Medicine.
Co-author Dr. JoAnn Manson said despite the disappointing results, the research doesn't mean multivitamins are useless.
For one thing, the data are observational, not the most rigorous kind of scientific research. And also, it's not clear if taking vitamins might help prevent cancers that take many years to develop, said Manson, chief of preventive medicine at Harvard's Brigham & Women's Hospital.
She said multivitamins may still be useful "as a form of insurance" for people with poor eating habits.
The study involved an analysis of data on women in their 50s and up who participated in long-running government studies on postmenopausal women. Almost 42 percent of the women said they used multivitamins regularly.
After about eight years, roughly equal numbers of vitamin users and nonusers developed common cancers, heart attacks and other cardiovascular problems. Overall, there were 9,619 cases of cancer, including cancers of the breast, lung, ovary, colon and stomach; and 8,751 cardiovascular ailments including heart attacks and strokes. In addition, 9,865 women died, also at similar rates in multivitamin users and nonusers.
Alice Lichtenstein, a Tufts University nutrition professor who was not involved in the research, said the study is important because it involved so many women.
"All the evidence keeps pointing in the same direction," Lichtenstein said.
Eric Jacobs, an American Cancer Society epidemiologist, said while his group doesn't advise vitamins to prevent cancer, it does recommend maintaining a healthy weight and eating at least five servings of fruits and vegetables daily while limiting red meat. Similar habits are also thought to help reduce heart disease risks.