A new Swedish prospective study has reported that multivitamin use is associated with a 19% higher risk of breast cancer. Multivitamins are frequently taken in the belief that they will prevent cancer and cardiovascular disease. However, the influence of multivitamins on breast cancer risk has not been established. The study included 35,329 initially cancer-free women in the Swedish Mammography Cohort. In 1997, the women completed a self-administered questionnaire concerning multivitamin use and other breast cancer risk factors.

During an average follow-up period of 9.5 years, 974 of the study participants were diagnosed with invasive breast cancer. Taking multivitamins was found to be associated with a statistically significant 19% increased risk of breast cancer compared to no multivitamin use. Breast cancer risk and multivitamin use did not vary significantly according to the hormone receptor status of the cancer. The authors conclude that multivitamin use may be associated with increased risk of breast cancer.

Implications of the study

This is the first study that has found clear-cut evidence that multivitamin use may be associated with increased risk of breast cancer among a well-educated and well-nourished population. Other studies have found a benefit for multivitamin or antioxidant intake among populations who might otherwise be at risk of a deficiency. There is also some evidence that dietary intakes of vitamin C and beta-carotene have different influences on cancer risk than taking vitamin C or beta-carotene supplements (which might actually promote some forms of cancer).

Based on the available evidence, we conclude that multivitamin use is not helpful for breast cancer survivors and those at high risk for breast cancer, and should be avoided by breast cancer patients undergoing radiation treatment or chemotherapy. This assumes that such survivors and high risk women (and men) consume a healthy, well-rounded diet abundant in vegetables, fruits and healthy fats (see our recommended food list). Radiation and chemotherapy patients should consume a diet of healthy foods that have been specifically found to enhance the effects of treatment. Please see our web pages on these topics.

On the other hand, we think that vitamin D supplements have a place for most U.S. residents since it appears to be extremely difficult to obtain adequate levels through exposure to sunlight and in the diet. Adequate levels of vitamin D are associated with reduced risk of breast cancer and better survival outcomes.

Please see our article on how to optimize your breast cancer diet for information on what to eat during all stages of treatment and recovery.