Is soy safe for breast cancer survivors?

When I meet with breast cancer survivors for the first time, typically they are still under active treatment with chemotherapy and/or radiation. A common side effect of treatment is fatigue; thus we discuss how good nutrition can combat fatigue. This usually leads to a review of sources of protein.  When I get to the very end of my inevitably rehearsed list, I name soy. If the woman has an estrogen-receptor positive tumor, they will always state “I can’t eat soy”.

But they can.  Numerous research studies show that soy is safe for breast cancer survivors, regardless of estrogen receptor status.

Some of the confusion lies in the term “phytoestrogen”. Soy foods contain isoflavones which are phytoestrogens. Phytoestrogens are similar in structure to human estrogen and can exert estrogen-like effects. Since high levels of human estrogen are linked to an increased risk of breast cancer, many healthcare providers previously cautioned estrogen receptor positive (ER+) patients to avoid soy. Phytoestrogens, however, do not appear to have the same effect of fueling the cancer growth. Studies have shown that soy foods do not increase breast cancer risk, and may in fact reduce the risk.

Initial research looking at the relationship between soy and breast cancer focused on the Asian populations where the incidence of breast cancer is lower compared to other cultures/ethnicities. Asian women also have a moderate intake of soy, consuming 1-2 servings daily. Early research proposed it was soy intake during childhood and adolescence that was associated with a decrease risk of breast cancer. Further studies, however, have shown that moderate intakes of soy can reduce the risk of death and cancer recurrence when added into the diet following a breast cancer diagnosis, including women who are postmenopausal.

A large study conducted in China, which included over 5,000 breast cancer survivors, found that soy intake decreased both risk of death and risk of recurrence among both women with either ER+ or estrogen receptor negative (ER-) breast cancer and in those who used and did not use Tamoxifen.

Over nineteen hundred female breast cancer survivors were included in the Life After Cancer Epidemiology (LACE) study. Results showed that soy intakes similar to Asian populations (1-2 servings/day) may decrease the risk of breast cancer recurrence. This reduction in risk was most evident among postmenopausal Tamoxifen users.

Another study of 524 patients found that soy was associated with a lower risk of recurrence among postmenopausal survivors with ER+, progesterone receptor positive (PR+) breast cancer and in those who received Anastrozole as endocrine therapy.

Data taken from the WHEL study analyzed soy intake in 3,088 breast cancer survivors. As isoflavone intake from soy increased, the risk of death decreased. While the decrease was not considered statistically significant, it adds to a body of research that shows no adverse effects of breast cancer survivors consuming soy.

Research has primarily focused on whole soy foods – not processed forms of soy or isoflavone powders. If you choose to include soy in your diet, as with all foods, less processed is better. And stick to a moderate intake of 1-2 servings per day. One serving of soy =

½ cup tofu

½ cup tempeh

½ cup edamame

1 cup soymilk

1 cup soy yogurt

¼ cup soy nuts

1 Tbsp miso


Written by
Dana H. Ingoglia, RD, CSO, LDN
Clinical Oncology Nutritionist
Florida Cancer Specialists




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