Soy Foods Found Harmless to Breast Cancer Patients

Soy is a food typically associated with a health-conscious diet, but medical researchers have wondered whether it actually could be bad for women who have had breast cancer.

Soy is a concern because of its potential impact on a woman's estrogen levels. Estrogen is known to promote the growth of breast cancer cells .

A new study, however, finds that those who have survived breast cancer do not increase their risk of recurrence or death by eating soy-based food.

Soy and estrogen

Soy foods contain large amounts of compounds called isoflavones that are known to bind to estrogen receptors in the body and can mimic estrogen as well as block its effects.

"There has been widespread concern about the safety of soy food for women with breast cancer," said study researcher Dr. Xiao-Ou Shu, professor of medicine at Vanderbilt Epidemiology Center, Vanderbilt University Medical Center in Tennessee.

The worry that isoflavones could increase the risk of cancer recurrence among breast cancer patients was based on the fact that their cancer treatment reduced the patients' estrogen levels, Shu explained. Researchers were particularly concerned that isoflavones may have an impact on the effect of tamoxifen , a breast cancer treatment, because both tamoxifen and isoflavones bind to estrogen receptors, she said.

However, in women without breast cancer, consumption of soy isoflavones has been linked with a reduced risk of developing breast cancer .

No differences

For Shu's study, 16,048 women with breast cancer answered questions about their intake of soy isoflavones, including consumption of tofu and soy milk. On average, the women filled out the questionnaires 13 months after their breast cancer diagnosis. The participants' breast cancer outcomes, including recurrence and death from cancer, were assessed on average nine years later. Women in the study were from the United States and China.

Those who ate the highest amount of soy isoflavones more than 23 mg per day were compared with those who ate the lowest amount, 0.48 mg per day. Shu found no difference between these two groups in terms of who did and who didn't see a recurrence of their breast cancer. There was also no difference in the death rate from breast cancer between these groups.

The average daily intake of soy isoflavones among the U.S. women was 3.2 mg; and the average intake among Chinese women was 45.9 mg.

Researchers are still examining the interaction of soy isoflavones and tamoxifen.

The study was presented this week in Orlando, Fla., at the annual American Association for Cancer Research meeting. The research was funded by the American Recovery and Reinvestment Act of 2009.

Pass it on: Soy foods do not increase the risk of breast cancer recurrence in women, researchers say.

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Rachael Rettner

Rachael is a Live Science contributor, and was a former channel editor and senior writer for Live Science between 2010 and 2022. She has a master's degree in journalism from New York University's Science, Health and Environmental Reporting Program. She also holds a B.S. in molecular biology and an M.S. in biology from the University of California, San Diego. Her work has appeared in Scienceline, The Washington Post and Scientific American.