Many people take multivitamins in the hopes of thwarting disease, but a new study finds that older women who use multivitamins may be more likely than non-users to develop breast cancer.

The study, reported in the American Journal of Clinical Nutrition, points only to an association between multivitamin use and breast cancer. It does not prove that the supplements directly contribute to the disease.

However, the researchers say, it’s biologically plausible that multivitamins could have such an effect, and the potential link “merits further investigation.”

The findings come from a decade-long study of more than 35,000 Swedish women who were between the ages of 49 and 83 and cancer-free at the outset. Over an average of 10 years, 974 women were diagnosed with breast cancer.

Researchers found that women who reported multivitamin use at the study’s start were 19 percent more likely than non-users to develop breast cancer. That was with factors like age, family history of breast cancer, weight, fruit and vegetable intake, and exercise, smoking and drinking habits taken into account.

Still, the large majority of multivitamin users did not develop breast cancer during the study period. Of 9,017 users, 293 were diagnosed with the disease, as were 681 women among the 26,000-plus who did not use multivitamins.

And while the study points to a generally higher risk of breast cancer among multivitamin users as a whole, the risks to any individual woman would likely be small.

“If the association is causal, using multivitamins would have a modest effect on breast cancer risk for any one woman,” lead researcher Dr. Susanna C. Larsson, of the Karolinska Institute in Stockholm, told Reuters Health in an email.

But given the widespread use of multivitamins, any potential risks are of “great public health importance,” the researchers say.

In the U.S., for example, it’s estimated that half of adults 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.

But a recent study of more than 160,000 older U.S. women found that over eight years, those who took multivitamins were no less likely than non-users to die of heart disease or cancer, with all cancers lumped together in a group.

The current study included more than 35,000 women who were surveyed about their multivitamin use, as well as a number of other health and lifestyle factors. It’s possible, according to Larsson, that factors the study did not measure could explain the association between multivitamins and breast cancer.

On the other hand, there are biologically plausible reasons that multivitamins themselves could be to blame, the researcher said. A recent study found that among premenopausal women, multivitamin users tended to have greater breast density than non-users - meaning the breasts have relatively less fat and more glandular and connective tissue. Greater breast density is linked to a relatively higher risk of breast cancer.

It’s not clear from that study, however, whether multivitamins themselves somehow boost breast density.

Another possibility, according to Larsson’s team, could be the B vitamin folic acid, which animal research has linked to breast cancer. Human studies, however, have come to various conclusions; while one found a higher risk of breast cancer among women who took folic acid supplements, others have linked the vitamin to either no effect on breast cancer risk, or a decreased risk.

Since multivitamins are, by definition, a mix of vitamins and minerals, it is difficult to pinpoint which nutrient, of combination of nutrients, may be particularly tied to breast cancer risk, the researchers point out.

Until more is known, a woman’s best bet is to get her vitamins and minerals from a well-balanced diet rather than pills, Larsson advised.

“If you eat a healthy and varied diet,” she said, “there is no need to use multivitamins.”

SOURCE: American Journal of Clinical Nutrition, online March 24, 2010.