Along with their many nutrients, soy foods contain proteins and isoflavones, compounds that have estrogen-like properties. High levels of estrogen can fuel some breast cancers, and these compounds are well studied for their link to cancer risk and survivorship.
A new study now adds to this research, suggesting that consuming soy and other foods containing isoflavones after diagnosis may help some breast cancer survivors live longer. The link with more soy foods and lower mortality was found among women diagnosed with tumors that do not have receptors for estrogen.
The study was published earlier this month in the journal Cancer.
Currently, the research on soy and breast cancer survivorship suggests that it is safe and healthy for breast cancer survivors to eat moderate amounts of soy.
Read the current and emerging research on soy and its link to cancer risk and survivorship, along with recipes, tips and frequently asked questions.
This latest study included data from more than 6,200 American and Canadian women with breast cancer. (Many previous studies have focused on Asian populations, where soy foods are more common.) The women filled out questionnaires about what they ate and other lifestyle habits when they entered the study. The researchers then estimated the amount of isoflavones the women were eating.
After an average of 9.4 years, 1,224 women had died. The women who had consumed the highest amounts of isoflavones had a 21 percent lower risk of having died from any cause when compared to those who ate the fewest isoflavones.
The association between isoflavone intake and reduced mortality was strongest in women with tumors that lacked receptors for the hormones estrogen and progesterone. Women who did not receive endocrine therapy as a treatment for their breast cancer had a weaker, but still significant association. No associations were found for women with hormone-receptor-positive tumors and for women who received hormone therapy.
While the study categorized women in the highest quartile as those who consumed 1.5 milligrams or more of isoflavones per day — equivalent to a few edamame — the authors caution that individuals tend to underestimate their food intake when filling out questionnaires.
And while the study adjusted for other factors related to risk, those factors may still have played a role. Women who were more likely to eat high amounts of isoflavones were more likely to be young, physically active, a healthy weight and drink no or little alcohol.
This study focused on foods containing isoflavones, not supplements. As a Emory University physician writes in an accompanying editorial: “Although dietary intake of soy foods is healthy and safe, the use of soy isoflavone supplements is another matter because it has not been evaluated in large, randomized clinical trials.”
This study was supported by the National Cancer Institute of the National Institutes of Health.
Source: Zhang, F. F., Haslam, D. E., Terry, M. B., Knight, J.A., Andrulis, I. L., Daly, M., Buys, S.S., and John, E. M. (2017). Dietary isoflavone intake and all-cause mortality in breast cancer survivors: the Breast Cancer Family Registry. Cancer. Published online: March 6, 2017. DOI: 10.1002/cncr.30615.