All multivitamins are not created alike which is the probable cause for opposing findings in studies regarding multivitamins and breast cancer. . Some studies show benefit, but most do not. The likely reason for this is that the majority of multivitamins contain a long list of ingredients, some of which are harmful. The conflicting results in studies on breast cancer risk or breast cancer mortality with respect to multivitamins demonstrate this point.
One large study that followed women for 9.5 years found those who took multivitamins regularly were 19 percent more likely to be diagnosed with breast cancer than those who did not.
However the Nurses’ Health Study found no effect overall of multivitamins on breast cancer risk., while another large U.S. study found an 18 percent increase in risk associated with multivitamins.2-3
A recent study on postmenopausal breast cancer survivors reported a contradictory finding. Among these women, those who regularly took multivitamins had a 30 percent reduced risk of death from breast cancer during the 7.1 year follow-up.2
The most probable reason is that multivitamins contain a number of beneficial components mixed with potentially harmful ones, such as folic acid. The authors of the studies mentioned above hypothesized that folic acid was a cause of the increase in breast cancer risk they observed with multivitamins (link)
Synthetic folic acid from a supplement is not the same as folate from natural foods. Synthetic folic acid acts very different than natural folate in our bodies. Synthetic folic acid may disrupt the normal, healthy actions of folate.
The explanation for the reduced risk in the most recent study is likely the beneficial components of the multivitamin; those that simply prevented deficiencies. Many Americans do not meet the recommended intake of several essential micronutrients.4-5Therefore, the decreased risk of breast cancer in this study might be a result of women getting a sufficient amount of these protective nutrients via multivitamins.
Low magnesium intake or blood level is associated with an increased risk of colon cancer and death from all causes.6-9 Also, adequate zinc, vitamin C and certain B vitamins help to protect against DNA damage that could lead to the development of cancer.10 Multivitamin supplements could help women reach adequacy for some of these vitamins and minerals.
The differing results of these studies give credence to the idea that vitamins are a mix of positive and negative elements. Therefore, supplement design and development should identify the positive elements to include and the negative to exclude.
To promote optimal health, multivitamin supplements should complement a healthful diet by filling potential nutritional gaps, avoiding excess of any nutrient, excluding any ingredients that may be harmful, and keeping up-to-date with the latest science.