Millions of people take a multivitamin in the hopes of averting disease, but the supplements seem to offer no defense against cancer or heart disease, researchers reported Monday.

In a study that followed more than 160,000 older U.S. women, the researchers found that the 41 percent who used multivitamins were neither less likely to develop cancer or heart disease over eight years nor to have a lower overall death rate.

About half of Americans routinely use a dietary supplement, often a multivitamin, and studies show that one of the primary motivations is the belief that supplements will protect them from chronic diseases.

However, the current findings suggest that, at least for postmenopausal women, multivitamin use “does not confer meaningful benefit or harm” when it comes to cancer and heart disease, the investigators report in the Archives of Internal Medicine.

The team, led by Dr. Marian L. Neuhouser of the Fred Hutchinson Cancer Research Center in Seattle, asks, “Why do millions of Americans use a daily multivitamin for chronic disease prevention when the supporting scientific data are weak?”

One reason, they say, “may be the varied health messages received by the public.” Position statements from medical organizations that multivitamins do not prevent disease are mixed with messages to take a multivitamin if dietary intake is less than optimal - leaving the public confused, Neuhouser and her colleagues note.

Until clinical trials prove otherwise, the researchers write, multivitamins should not be seen as a way to prevent chronic disease.

The findings are based on data from the Women’s Health Initiative, a large U.S. study of postmenopausal women’s risk factors for cancer, heart disease and osteoporosis. It included 161,808 women who enrolled between 1993 and 1998.

At the outset, the women reported on their dietary supplement use and other lifestyle and health factors. Their rates of several common cancers, heart disease, stroke and death were tracked through 2005.

Overall, the researchers found no clear differences between multivitamin users and non-users in rates of death, cancer or cardiovascular problems.

There is an ongoing clinical trial of U.S. male doctors looking at whether multivitamins lower the risks of cancer, heart disease and other chronic ills after the age of 50, Neuhouser and her colleagues point out.

Such clinical trials are considered the “gold standard” for proving cause-and-effect; the current study, in contrast, was an observational one - looking at women’s reported behaviors and their subsequent rates of disease.

“The scientific community might consider whether a randomized controlled trial of multivitamins in women could definitively resolve whether benefit or harm ensues from the routine use of multivitamins,” Neuhouser and her colleagues write.

SOURCE: Archives of Internal Medicine, February 9, 2009.