Many U.S. women take multivitamin with mineral supplements ("multivitamins") as part of a strategy to maintain or improve overall health. Such supplements typically include vitamins and minerals at levels that are at minimum high enough to prevent deficiencies in them. In theory, since oxidative stress plays an important role in the development of many cancers, antioxidants might reduce cancer risk. The question arises whether such supplements may be beneficial in lowering the risk of breast cancer recurrence.
Do multivitamins improve health and survival in healthy populations?
To investigate this question, we first examined the premise that taking multivitamins promotes good health in the general population. In fact, based on the available evidence, multivitamin use does not appear to reduce the risks of developing cardiovascular disease, cancer, or premature death (from any cause):
- A prospective study that included 161,808 postmenopausal women in the Women's Health Initiative clinical trials tallied cardiovascular disease events, various cancers, and deaths during eight years of follow up. Approximately 42% of the study participants used multivitamins. No associations were found between multivitamin use and the risk of cancer (including breast cancer), heart attacks, stroke, blood clots, or overall mortality.
- Another prospective study designed to investigate the associations between multivitamin/mineral supplement use and risk of premature death or cancer included 182,099 participants enrolled in the Multiethnic Cohort Study in Hawaii and California. Study participants averaged 60 years of age and were followed for an average of 11 years. No associations were found between multivitamin use and death (from any cause), cardiovascular disease, or cancer (including breast cancer).
- Still another prospective study that included 38,772 women in the Iowa Women's Health Study was designed to investigate the associations between the use of vitamin and mineral supplements and death from any cause in older women. The average age at baseline was 61.6 years. The women reported their supplement use at baseline and in 1997 and 2004 and were followed through year-end 2008. Use of multivitamins was found to be associated with a slight (6%) increase in risk of death compared with nonuse.
Do multivitamins help prevent breast cancer?
A number of studies have examined the question as to whether multivitamin use reduces the risk of breast cancer. Generally speaking, most studies have found no association between multivitamin use and breast cancer risk. A 2011 meta-analysis of previous studies concluded that multivitamin use is likely not associated with a significant increased or decreased risk of breast cancer.
On the other hand, a 2010 Swedish prospective study reported that multivitamin use was associated with increased risk of breast cancer. The study included 35,329 initially cancer-free women in the Swedish Mammography Cohort who were followed for an average of 9.5 years. Taking multivitamins was found to be associated with a 19% increased risk of breast cancer compared to no multivitamin use. The association between breast cancer risk and multivitamin use did not differ significantly by hormone receptor status of the cancer.
Do multivitamins help prevent breast cancer recurrence?
The issue of whether multivitamin use could prevent breast cancer recurrence has also been investigated, primarily using data from women in the Kaiser Permanente Northern California Cancer Registry. One study included 2,236 women who were diagnosed with early-stage breast cancer between 1997 and 2000. Participants were enrolled approximately two years after diagnosis. Multivitamin use both before and after diagnosis was assessed via mailed questionnaire. Breast cancer outcomes were determined yearly. Compared to never use, multivitamin use after diagnosis was not associated with breast cancer outcome. Compared to never use, persistent use of multivitamins from that extended from pre- to post-diagnosis was associated with a reduced risk of recurrence, but this was not statistically significant.
However, a major 2013 U.S. study of participants in the Women’s Health Initiative reported that postmenopausal women with invasive breast cancer who took multivitamins with minerals supplements had a lower risk of breast cancer-specific death than those who did not.
Based on the limited available evidence, multivitamin use may or may not prevent recurrence, and generally does not appear to promote it. It is plausible that supplementation might prevent cancer in women who are deficient in a given micronutrient, whereas the same supplement could promote cancer in women who are not deficient, especially in high doses. For example, vitamin A, folate and copper supplements have all been associated with increased risks of breast cancer.
We conclude that breast cancer survivors should attempt to obtain most of their vitamin and mineral requirements through nutrition rather than taking multivitamins. However, we realize that adequate levels of certain nutrients such as vitamin D and omega-3 fats (found in walnuts, flaxseed and their oils, as well as fatty fish) may be difficult to obtain through diet and supplements may be appropriate. Women should take advantage of the blood tests that are available to determine their levels of important vitamins and minerals and correct any suboptimal levels through diet and supplements. In addition, note that taking multivitamins during radiation treatment or chemotherapy could potentially serve to protect cancer cells from the treatments and we would not recommend it.