Collaborative Study Clears Confusion

After years of confusion, a new, large study found eating soy foods did not increase the risk of cancer recurrence or death among breast cancer survivors. Investigators from several research institutions in the United States and China examined the association between soy food intake and breast cancer outcomes, using data from the After Breast Cancer Pooling Project, a multi-institution collaborative study.

“There has been widespread concern about the safety of soy foods for women with breast cancer,” said lead investigator Xiao Ou Shu, MD, PhD, professor of Medicine at Vanderbilt-Ingram Cancer Center (VICC). “Soy foods contain large amounts of isoflavones that are known to bind to estrogen receptors in cells and have estrogen-like and anti-estrogenic effects. There are concerns that isoflavones may increase the risk of cancer recurrence among breast cancer patients by compromising the effect of tamoxifen, since both tamoxifen and isoflavones bind to estrogen receptors.”

To address these concerns, investigators pooled data from three prospective cohorts of breast cancer patients: Life After Cancer Epidemiology (LACE); Women’s Healthy Eating and Living Study (WHEL), both in the United States; and the Shanghai Breast Cancer Survival Study (SBCSS) in China.

The study included 9,515 breast cancer patients who answered food frequency questionnaires to assess what kinds of food they ate, including soy foods. The two major soy products common across all studies were tofu and soy milk. The average daily soy isoflavones intake among U.S. women was 3.2 mg. In the Shanghai group the amount was significantly higher, at 45.9 mg. 

Soy isoflavones intake was assessed 14.6 months after the breast cancer diagnosis and the average follow-up time was approximately 7.4 years after diagnosis. Women who ate the most soy food, at more than 11.83 mg isoflavones per day, had a 27 percent reduced risk for breast cancer recurrence, compared to those with the lowest intake level (3.68 mg per day or lower). High soy food intake was also associated with a slightly reduced risk of mortality, although these results did not reach statistical significance.

“Our results indicate it may be beneficial for women to include soy food as part of a healthy diet, even if they have had breast cancer,” Shu said. “This cannot be directly generalized to soy supplements, however, as supplements may differ from soy foods in both the type and amount of isoflavones.”

Sarah Nechuta, PhD, MPH, postdoctoral research fellow at VICC, is first author of the study, which included investigators from Kaiser Permanente, Oakland, Calif., Brigham and Women’s Hospital, Harvard Medical School, and the Dana-Farber Cancer Institute, Boston, Moores UCSD Cancer Center, University of California San Diego, and the Shanghai Center for Disease Control and Prevention.

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