Multivitamins Linked to Breast Cancer

We don't normally report on vitamin supplement research. It is pretty clear that getting your vitamins from foods is better than taking them in pill form. Given that so many people take a multivitamin, however, we felt that this study was worth sharing with readers.

In 1997, researchers in Sweden recruited over 35,000 women to participate in their study of breast cancer risk factors (Am J Clin Nutr 2010;91(5):1268-72). The participating women filled out a detailed questionnaire of over 350 questions which asked about diet, supplement use, and other lifestyle factors. Those women who had a history of cancer of almost any kind were excluded from the study, and all participating women were between 49 and 83 years of age at the start of the study.

The study lasted through the end of 2007. At the close of the study, the researchers looked at those women who had developed breast cancer during the study and compared their use of multivitamins with those women who had not developed breast cancer.

Just over 25% of all of the participants reported that they took a multivitamin. The vast majority of those women took a multivitamin/multimineral supplement; only 1.6% of the participating women took a multivitamin without minerals. Those women who took multivitamins tended to have more education, have a history of benign breast disease, have taken oral contraceptives as well as postmenopausal hormones, and not have children. They also tended to exercise more, have a lower average Body Mass Index, and drink more alcohol than those women who did not take multivitamins.

Those women who took multivitamins on a daily basis were 19% more likely to develop breast cancer than those women who did not take multivitamins. That's not a typo: Nine. Teen. Percent. Taking multivitamins seemed to be more strongly associated with estrogen-receptor positive cancers, although all types were reported.

These results held true even when the researchers left out all cases of breast cancer that were reported within the first two years of the study, and when they took into account the women's intake of fruits and vegetables. Women who took daily multivitamins for over three years were 22% more likely to develop breast cancer, and those who took multivitamins and were also taking postmenopausal hormones were more than twice as likely to develop breast cancer.

What this means for you

Some previous studies have also shown an association between multivitamin use and breast cancer, while others have not. That said, we do know that taking multivitamins results in a 5% increase in breast density, which we know is associated with the risk of breast cancer. All in all, if you are already at risk for breast cancer, make sure you're getting your vitamins through a balanced, healthy diet - and not in a pill.

First posted: June 9, 2010