Around one-third of North Americans take multivitamin supplements that contain B vitamins.
B6 helps maintain healthy immune function, and B12 is important for the brain and nervous system.
Another B vitamin is folic acid, commonly taken by pregnant women to prevent birth defects.
It's very important that women have adequate intake of folic acid before and throughout pregnancy.
But with all their health benefits, does taking supplements of these three vitamins help prevent cancer in women? Researchers at Harvard University decided to explore that question.
"We were interested in the effect of the folic acid and B vitamins on cancer in general, total cancer, as well as specifically the risk of breast cancer and colorectal cancer," says Dr. Joanne Manson.
The study appears in the Journal of the American Medical Association (JAMA).
For more than seven years, it followed 5,000 women age 40 and over. These women were all at high risk for cardiovascular disease. Half of the group took large doses of the three vitamins, the other half took placebos. To date, it's the largest B vitamin study of women.
"Women have been underrepresented in previous B vitamin trials," says Dr. Shumin Zhang.
But this study showed no evidence of reducing cancer risk.
"It was surprising that there was no evidence of any cancer benefit because some previous studies had suggested that at least a diet that is high in foliate and B vitamins might be protective against cancer," says Manson.
So although the vitamins provide other health benefits, the jury is still out on whether they reduce the risk of cancer.
With a report from CTV British Columbia's Dr. Rhonda Low
