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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 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.
So how could something that is supposed to be so good for you actually cause breast cancer?
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 breast cancer 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.
"There may be some components within a multivitamin that could potentially increase breast cancer risk, but the problem is we don't know which component," says Katherine Lee, MD, an assistant professor of medicine at Cleveland Clinic in Ohio.
In the new study, women did not provide information on what brands of vitamins they took; they simply reported whether or not they took them. There is a chance that recall bias may have affected their ability to accurately remember whether, or how often, they took multivitamins.
The new research did show that vitamin E, C, and B-6 did not appear to be responsible for the increased breast cancer risk. Calcium also appeared to provide protection from breast cancer, the new study shows.
"If you have a normal healthy diet, you probably don't need to take a multivitamin," says Lee. "Have a discussion with your physician about your diet and what food or food groups you avoid, and maybe consider adding supplements that address these deficiencies over a multivitamin," she suggests.
"I hope women don't toss all their multivitamins yet," she says. "We have to get at the heart of the matter."