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Does eating soy affect the risk of death in breast cancer survivors?


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Checked Special populations:  Of special interest to (pull down list): women who consume soy and women who have been diagnosed with ER-/PR- breast cancers

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Is eating soy safe for people who have had breast cancer? This topic has been controversial among health care providers, patients, and survivors for many years because research has yielded mixed results. Some studies suggest people who have been diagnosed with breast cancer should eat more soy products, while other studies recommend they eat less or avoid it altogether. Which should it be? Adding to this research is a new study that asked breast cancer survivors about their soy consumption before and after diagnosis. (4/27/17)


STUDY AT A GLANCE

This study is about:

Whether eating soy is associated with a higher risk of death in breast cancer survivors.

Why is this study important?

Whether women who have had breast cancer should eat foods that contain soy has been debated for many years. Research examining soy’s effect on breast cancers has been mixed; it is still unclear whether women with breast cancer should avoid soy products or eat more soy than they did before their diagnosis.                                                                  

Study findings: 

  1. Women diagnosed with breast cancer whose diet contained the highest levels of soy products (more than 1.5 mg daily) before or after diagnosis had a reduced risk of dying from any cause (not just breast cancer) compared to breast cancer survivors who had the lowest level of soy in their diets (less than 0.3 mg daily).
    • This association was limited to women who did not previously receive hormone therapy as part of their breast cancer treatment, including those with hormone-receptor-negative (ER/PR-negative) breast cancers.

What does this mean for me?

This study suggests that breast cancer survivors who had a higher dietary intake of soy had less risk of dying from any cause than breast cancer survivors who ate less soy. However, researchers do not completely understand how soy affects breast cancer growth or risk for recurrence.  More work needs to be done in this area, especially for women who have ER/PR-positive breast cancer. 

It is important to remember that this study adds to what we know about soy consumption and breast cancer but it does not provide definitive conclusions for specific patients.  Not all breast cancers are the same, so research that applies to one subtype may not apply to all subtypes. A report by American Cancer Society written by experts in nutrition, physical activity, and cancer survivorship and published in 2012 assessed available research regarding nutritional intake (including soy) for breast cancer survivors; the report stated that, “Current evidence does not suggest that consuming soy foods is likely to have adverse effects on risk of recurrence or survival.”  Ultimately, health care providers know their patients’ cancers and situations best. Women should discuss their concerns with their health care providers, and continue to follow their recommendations regarding soy consumption.

Questions to ask your health care provider:

  • Is it safe for me to eat soy products?
  • Should I consider consulting a nutritionist about dietary changes that will improve my health and reduce my risk of recurrence?

IN DEPTH REVIEW OF RESEARCH

Study background:

Soy consumption by people with breast cancer is controversial among researchers and health care providers, and that controversy often confuses patients and survivors. Some research shows that dietary soy acts against breast cancer development by decreasing the amount of estrogen made by the body. On the other hand, other research shows that dietary soy can act like estrogen, activating the signaling pathways that encourage breast cancer growth.

While studies from China have consistently found a lower risk of breast cancer recurrence and/or risk of death associated with higher soy consumption in women, studies conducted in the United States have yielded mixed results.

Fang Fang Zhang and colleagues from Tufts University and other institutions published work in the journal Cancer in March 2017 looking at soy consumption and risk of death from any cause (not just breast cancer) in women who have been diagnosed with breast cancer.

Researchers of this study wanted to know:

How does eating soy products affect the risk of death in women who have had breast cancer?  

Population(s) looked at in the study:

The researchers used the Breast Cancer Family Registry to survey women from San Francisco, New York City, Philadelphia, Utah, and Ontario (Canada) who had invasive breast cancer. Information on the women’s tumor hormone status (estrogen and progesterone receptors) was taken from pathology reports or from cancer registry records.  Researchers obtained mortality data in various ways, including annual telephone contacts and questionnaires.

These women completed questionnaires that included information on their diets. Of the 6,235 women who participated, 4,769 returned the questionnaire within 5 years before they were diagnosed (pre-diagnosis), while 1,466 returned the questionnaire within 5 years after they were diagnosed (post-diagnosis). The women were asked how frequently they consumed certain foods (including soy) and the portion size.  Average follow-up was 113 months (9.4 years), during which 1,224 deaths occurred.

Study findings: 

  1. Women with breast cancer whose diets contained the highest levels of soy products (more than 1.5 mg daily), whether pre- or post-diagnosis, had a reduced risk of dying from any cause (not just breast cancer), compared to breast cancer patients who had the lowest levels of soy in their diets (less than 0.3 mg daily).
    • This association was limited to women who had hormone-receptor-negative (ER-/PR-) breast cancers and women who did not previously receive hormone therapy as part of their breast cancer treatment.   

Limitations:

Because the researchers used a self-reporting survey to measure soy intake, the results are subject to certain data collection errors, including the possibility that participants incorrectly remembered how often they ate a particular food or how much of it they ate. (Data from women whose reported total calorie intake was much higher or much lower than the average caloric intake of other participants were considered to be unreliably reported and were excluded from the results.) Women who had higher soy intake were more likely to be Asian American, young, premenopausal, physically active, more educated, not overweight or obese, nonsmokers, and either did not drink alcohol at all or drank less than 7 alcoholic drinks per week. To ensure that the effect on risk was due to soy consumption and not any of these other factors, the researchers adjusted for this information in their mathematical models. Finally, the researchers looked only at all-cause mortality, meaning deaths that were not breast cancer specific; they were unable to look at breast cancer recurrence. The researchers did not have additional information on any other diseases that the women may have had, which may have also increased the risk of death.

Conclusions:

This study suggests that women who have breast cancer and regularly consume soy products have a lower risk of dying from any cause. However, more work needs to be done to understand and confirm these findings. Breast cancer patients should continue to follow their doctor’s recommendations regarding soy consumption during and after treatment. Women who are worried about their dietary intake of soy or any other nutrients should discuss their concerns with their health care providers.

Posted 4/27/17

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References

Rock CL, Doyle C, Demark-Wahnefried W, et al. “Nutrition and physical activity guidelines for cancer survivors.CA: A Cancer Journal for Clinicians, 62: 242–274.  

Zhang FF, Haslam DE, Terry MB, et al. “Dietary Isoflavone Intake and All-Cause Mortality in Breast Cancer Survivors: The Breast Cancer Family Registry.” Cancer. Published online first in March 2017. 

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