I have a love/hate relationship with vitamins. This study nudges me away from taking them:
Multivitamin Use And Breast Cancer Incidence In A Prospective Cohort Of Swedish Women, American Journal of Clinical Nutrition, 2010
The Swedish Mammography Cohort, 35,329 women, mean follow-up 9.5 years. Results:
Multivitamin use was associated with a statistically significant increased risk of breast cancer.
The possibility that multivitamin use may increase the risk of breast cancer is biologically plausible. A recent study (2) showed that the current use of multivitamins and minerals in premenopausal women was associated with a significant 5.3% higher mean mammographic breast density (after adjustment for potential confounders), which is strongly and positively related to breast cancer risk. Folic acid in supplements could possibly increase the risk of breast cancer. High doses of folic acid from supplements or fortified foods are of concern because synthetic folic acid is more bioavailable than folate from natural food sources and, hence, potentially more potent in promoting cancer growth .
If folic acid is responsible for the observed association between multivitamin use and a risk of breast cancer, the association may not be seen in the US population because breakfast cereals and grain products have been fortified with folic acid since 1998 in the United States.
Other constituents of multivitamins with minerals that may be associated with cancer risk include iron and zinc. Experimental studies showed that a diet low in iron (19) or zinc (20) can suppress carcinogen-induced mammary cancer in rats. In a case-control study (21) nested in a cohort of women with benign breast disease, iron and zinc concentrations in benign breast tissue were positively associated with breast cancer risk. … In the current study, there was a nonsignificant 55% increase in breast cancer risk associated with zinc supplement use among nonusers of multivitamins.
Folic acid is a real contender for the problem. Unfortunately, we started adding folic acid to flour products in the US in the late 1990s so that will mask any studies trying to determine folic acid’s contribution. Sweden doesn’t fortify with folic acid; perhaps that’s why they found an association.
Interesting that bit about diets low in iron and zinc being beneficial vis-a-vis breast cancer. Vegan diets are naturally low in iron and zinc.