Soy and Breast Cancer
Are soy foods safe for breast cancer survivors, including women who were treated for estrogen receptor positive breast cancer?
Summary: The current consensus among health experts who study soy is that breast cancer survivors can safely eat these foods. Emerging research suggests that soy foods may decrease the likelihood of breast cancer recurrence in women with a history of the disease.
Most health experts agree that the evidence is not strong enough to recommend that all women with a history of breast cancer eat more soy. However, soy foods do appear to be safe, and possibly beneficial for female breast cancer survivors.
Confusion about soy arises from the term "phytoestrogens." Some soy nutrients—the isoflavones—have chemical structures that look a bit like the estrogen found in a woman's body. This is where the term phytoestrogen originated. However, phytoestrogens are not the same thing as female estrogens. Soy foods do not contain estrogen.
Several large, human studies—in which thousands of women have been followed for many years—consistently show that compared with women who do not eat soy, women who regularly eat soy have lower breast cancer risk. Some of these studies also suggest that breast cancer survivors who consume soy foods have a lower risk of breast cancer recurrence compared with survivors who avoid soy.
These studies have been conducted in both Asian and US populations. This is important because soy has long been a part of many Asian cuisines, but it is a relatively new introduction to the American diet.
These studies are observational. This means researchers collect diet information from women, then follow them for many years to see who gets breast cancer. In an observational study, it is always possible that the true connection with better breast health is not soy, but something else that is related to eating soy.
For example, women who eat soy foods also may eat less fried food and more vegetables. They may exercise more and maintain a healthier body weight. Any one of these other things could be the reason why soy-eating women have lower breast cancer risk.
This means observational studies can't conclusively prove that soy protects against breast cancer. However, these studies are reassuring in affirming that soy foods do not increase breast cancer risk. They point toward a protective effect of soy on breast health, regardless of other lifestyle and diet choices.
Soy foods are a healthy option, while soy dietary supplements may not be. The research on soy and breast health has looked at soy foods, not dietary supplements. If you require extra calories during cancer treatment from a medical food supplement, the soy protein in this type of product is not a problem. However, soy pills and isoflavone-enriched powders should be avoided.
If you're a woman concerned about breast health and you like soy, stick to healthy, whole soy foods, such as tofu, tempeh, soymilk, and edamame. The occasional soy protein bar or snack food is fine, but as with all plant foods, less processed is better.
Stop Soy Fear
In the end, feel confident in whatever choice you make about soy foods. Eat these foods if you enjoy them, or skip them altogether if soy isn't to your liking.
The original question and answer were generously donated by Diana Dyer, MS, RD a cancer survivor, registered dietitian, organic garlic farmer, and the author of "A Dietitian's Cancer Story: Information & Inspiration for Recovery & Healing from a 3-time Cancer Survivor.
Question and Answer updated by Suzanne Dixon, MPH, MS, RD on behalf of the ON DPG
- Caan BJ, Natarajan L, Parker B, Gold EB, Thomson C, Newman V, Rock CL, Pu M, Al-Delaimy W, Pierce JP. Soy food consumption and breast cancer prognosis. Cancer Epidemiol Biomarkers Prev. 2011;20:854-58.
- Guha N, Kwan ML, Quesenberry CP Jr, Weltzien EK, Castillo AL, Caan BJ. Soy isoflavones and risk of cancer recurrence in a cohort of breast cancer survivors: the Life After Cancer Epidemiology study. Breast Cancer Res Treat. 2009;118:395-405.
- Kang X, Zhang Q, Wang S, Huang X, Jin S. Effect of soy isoflavones on breast cancer recurrence and death for patients receiving adjuvant endocrine therapy. CMAJ. 2010;182:1857-62.
- Dong JY, Qin LQ. Soy isoflavones consumption and risk of breast cancer incidence or recurrence: a meta-analysis of prospective studies. Breast Cancer Res Treat. 2011;125:315-23.
- ACS Guidelines on Nutrition and Physical Activity for Cancer Prevention.
- American Institute for Cancer Research, Soy Update.
Page Update: April 2013