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
The new findings appear online in the American Journal of Clinical
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
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.
So how could something that is supposed to be so good for you actually cause
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.