(USA TODAY) -- Dashing the hopes of those who hope to pop a pill to prevent heart disease, doctors announced Monday that daily multivitamins don't stave off cardiovascular problems, such as heart attacks, stroke or death.

The findings, published in The Journal of the American Medical Association and presented at the American Heart Association's Scientific Sessions in Los Angeles, come from the only large-scale, long-term trial of its kind, called the Physicians' Health Study II.

"Vitamin supplements will never be a substitute for a healthy diet," says study co-author JoAnn Manson, chief of preventive medicine at Boston's Brigham and Women's Hospital and a professor at Harvard Medical School. "While some people may need supplementation, it's not necessary for most of the population."

The study, which followed 14,661 male doctors for more than 11 years, had slightly more positive results for cancer.

That part of the study, published last month, found that men who took multivitamins lowered their risk of cancer by 8%.

Vitamins are increasingly popular. About 39% of Americans took a multivitamin in 2003-06, up from 30% in 1988-94, according to an accompanying editorial.

Recent studies have largely failed to find much benefit from dietary supplements. Those results have frustrated and confused many consumers, partly because earlier studies suggested so many benefits for vitamins.

Studies such as the one published Monday show the importance of conducting rigorous tests -- in which patients are randomly assigned to one health intervention or another, while neither patients nor their doctors know which group they're in -- says Christopher Cannon, a Harvard professor of medicine and a cardiologist at Brigham and Women's Hospital.

These randomized, controlled trials -- the gold standard of medical evidence -- often disprove trends suggested by "observational" studies, in which researchers follow patients over time, says Cannon, who wasn't involved in the new study.

"We're not surprised by these results, but they don't discount the many other benefits that multivitamins provide, including filling nutrient gaps, helping prevent neural-tube birth defects and serving in combination with other healthy habits as a basic and affordable insurance policy for overall wellness," says Duffy MacKay, vice president for scientific affairs at the Council for Responsible Nutrition, an industry group.

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