Studies show that a lifelong diet rich in soy foods reduces the risk of breast cancer in women. This protective effect is less dramatic for women who eat less soy or who start eating soy later in life. Soy contains protein, isoflavones and fiber, all of which provide health benefits.
It was once thought that soy foods increase the risk of breast cancer. However, eating a moderate amount of soy foods does not increase risk of breast cancer — or other types of cancer. A moderate amount is one to two servings a day of whole-soy foods, such as tofu, soy milk and edamame.
So where did the idea come from that soy increases breast cancer risk? Isoflavones, which are found in soy, are plant estrogens. High levels of estrogen have been linked to an increased risk of breast cancer. However, food sources of soy don't contain high enough levels of isoflavones to increase the risk of breast cancer.
Soy or isoflavone supplements, on the other hand, generally contain higher levels of isoflavones. Some studies have suggested a link between soy or isoflavone supplements and an increased risk of breast cancer in women who have a family or personal history of breast cancer or thyroid problems.
Talk with your doctor or dietitian before taking supplements.
Dec. 07, 2022
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- AskMayoExpert. Integrative medicine (complementary and alternative medicine). Rochester, Minn.: Mayo Foundation for Medical Education and Research; 2018.
- Chen M, et al. Association between soy isoflavone intake and breast cancer risk for pre- and post-menopausal women: A meta-analysis of epidemiological studies. PLOS One. 2014;9:e89288. https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0089288. Accessed Oct. 10, 2018.
- Wu J, et al. Dietary protein sources and incidence of breast cancer: A dose-response meta-analysis of prospective studies. Nutrients. 2016;8:e730. https://www.mdpi.com/2072-6643/8/11/730. Accessed Oct. 10, 2018.
- Chen W, et al. Factors that modify breast cancer risk in women. https://www.uptodate.com/contents/search. Accessed Oct. 10, 2018.
- Baglia M, et al. The association of soy food consumption with the risk of subtype of breast cancers defined by hormone receptor and HER2 status. International Journal of Cancer. 2016;139:742.
- Soy. Natural Medicines. https://naturalmedicines.therapeuticresearch.com. Accessed Oct. 10, 2018.
- Zhao T-T, et al. Dietary isoflavones or isoflavone-rich food intake and breast cancer risk: A meta-analysis of prospective cohort studies. Clinical Nutrition. 2017. In press. Accessed Oct. 10, 2018.
- Rakel D, ed. Breast cancer. In: Integrative Medicine. 4th ed. Philadelphia, Pa.: Elsevier; 2018. https://www.clinicalkey.com. Accessed Oct. 10, 2018.
- Huser S, et al. Effects of isoflavones on breast tissue and the thyroid hormone system in humans: A comprehensive safety evaluation. Archives of Toxicology. 2018;92:2703.
- Ziaei S, et al. Isoflavones and breast cancer risk. Medicines. 2017;4:e18. https://www.mdpi.com/2305-6320/4/2/18. Accessed Oct. 20, 2018.
- Can I lower my risk of breast cancer progressing or coming back? American Cancer Society. https://www.cancer.org/cancer/breast-cancer/living-as-a-breast-cancer-survivor/can-i-lower-my-risk-of-breast-cancer-progressing-or-coming-back.html. Accessed Oct. 20, 2018.
- Li Y, et al. Dietary natural products for prevention and treatment of breast cancer. Nutrients. 2017;9:e728. https://www.mdpi.com/2072-6643/9/7/728. Accessed Oct. 10, 2018.
- Barbara Woodward Lips Patient Education Center. Healthy nutrition and cancer. Rochester, Minn.: Mayo Foundation for Medical Education and Research; 2018.