April 1, 2010 -- The message is hammered home every time we turn on the TV: Taking a daily multivitamin can help improve our overall health and well-being and may even protect against diseases like cancer. But now a new study suggests that this seemingly healthy habit may actually increase the risk of breast cancer.
The new findings appear online in the American Journal of Clinical Nutrition.
In the study of more than 35,000 Swedish women aged 49 to 83, 25.5% said they took multivitamins. None of the women had cancer when the study began. During about 10 years of follow-up, 974 women were diagnosed with breast cancer, and 293 of these diagnoses occurred among the 9,017 women who reported using multivitamins.
Overall, women who reported taking multivitamins were 19% more likely to develop breast cancer than their counterparts who said they did not take daily multivitamins. These findings held after the researchers adjusted for other risk factors including family history, advancing age, body mass index, smoking status, and alcohol use.
"The potential health benefits or adverse effects associated with multivitamin use are of great public health importance [and] the observed association is of concern and merits further investigation," conclude the researchers, who were led by Susanna C. Larsson, PhD, of the division of nutritional epidemiology at the National Institute of Environmental Medicine at the Karolinska Institute in Stockholm, Sweden.
The study could not establish cause and effect, but it did show an association of multivitamin use and increased risk of breast cancer.
There may be some biologically plausible reasons that multivitamins can increase breastcancer risk, the researchers say. For one, folic acid, an ingredient in many multivitamins, may increase breast density, which could potentially stimulate the development of cancer.
Some studies have also linked iron and zinc to increased cancer risk, though there have also been other studies that showed no association between these ingredients and cancer risk, the researchers say.