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Contradictory Findings on Multivitamins and Breast Cancer

The science on the potential benefits of multivitamins is quite unclear. Some studies show benefit, but most do not. The likely reason is that most multivitamins contains a long list of ingredients, of which some are likely health-promoting and others harmful. Divergent findings on breast cancer risk or breast cancer mortality with respect to multivitamins demonstrates this point.

In 2010, one large study that followed women for 9.5 years found that those who took multivitamins regularly were 19 percent more likely to be diagnosed with breast cancer than those that didn’t take multivitamins.1 The Nurses’ Health Study found no effect overall of multivitamins on breast cancer risk, and another large U.S. study found an 18 percent increase in risk associated with multivitamins.2,3 More recently, a 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

Why are we seeing these contradictory results? Most likely because 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. Synthetic folic acid from a supplement is not the same as folate from natural foods; it is more highly bioavailable and may disrupt the normal, healthy actions of folate. More information about the connection between folic acid and breast cancer can be found in these articles:

The Dangers of Folic Acid in Multivitamins and Prenatal Vitamins
Is Supplemental Folic Acid Harmful?
Position Paper: Folate vs. Folic Acid

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.4,5 Therefore the decreased risk of breast cancer in this study might be a result of women ingesting 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 eating the standard American diet 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, and 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.

1. Larsson SC, Akesson A, Bergkvist L, et al: Multivitamin use and breast cancer incidence in a prospective cohort of Swedish women. Am J Clin Nutr 2010, 91:1268-1272.
2. Zhang S, Hunter DJ, Hankinson SE, et al: A prospective study of folate intake and the risk of breast cancer. JAMA 1999, 281:1632-1637.
3. Stolzenberg-Solomon RZ, Chang SC, Leitzmann MF, et al: Folate intake, alcohol use, and postmenopausal breast cancer risk in the Prostate, Lung, Colorectal, and Ovarian Cancer Screening Trial. Am J Clin Nutr 2006, 83:895-904.
4. Burnett-Hartman AN, Fitzpatrick AL, Gao K, et al: Supplement use contributes to meeting recommended dietary intakes for calcium, magnesium, and vitamin C in four ethnicities of middle-aged and older Americans: the Multi-Ethnic Study of Atherosclerosis. J Am Diet Assoc 2009, 109:422-429.
5. Cogswell ME, Zhang Z, Carriquiry AL, et al: Sodium and potassium intakes among US adults: NHANES 2003-2008. Am J Clin Nutr 2012, 96:647-657.
6. Reffelmann T, Ittermann T, Dorr M, et al: Low serum magnesium concentrations predict cardiovascular and all-cause mortality. Atherosclerosis 2011, 219:280-284.
7. Guasch-Ferre M, Bullo M, Estruch R, et al: Dietary magnesium intake is inversely associated with mortality in adults at high cardiovascular disease risk. J Nutr 2014, 144:55-60.
8. Chen GC, Pang Z, Liu QF: Magnesium intake and risk of colorectal cancer: a meta-analysis of prospective studies. Eur J Clin Nutr 2012, 66:1182-1186.
9. Rosanoff A, Weaver CM, Rude RK: Suboptimal magnesium status in the United States: are the health consequences underestimated? Nutr Rev 2012, 70:153-164.
10. Ames BN: Prevention of mutation, cancer, and other age-associated diseases by optimizing micronutrient intake. J Nucleic Acids 2010, 2010.

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