April 12, 2011

Soy foods not a risk for breast cancer survivors


After years of confusion about the safety of soy food consumption by breast cancer survivors, a large new study found that eating soy foods did not increase the risk of cancer recurrence or death among breast cancer survivors. The study was presented at the American Association for Cancer Research (AACR) annual meeting in Orlando, Fla.

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.

Xiao Ou Shu (Joe Howell / Vanderbilt)
Xiao Ou Shu (Joe Howell / Vanderbilt)

“There has been widespread concern about the safety of soy foods for women with breast cancer,” said lead investigator Xiao Ou Shu, professor of medicine, Vanderbilt-Ingram Cancer Center.

“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.”

Estrogen is a hormone that has been linked to cancer growth, especially in breast cancer. Breast cancer patients whose tumors are found to be estrogen receptor positive are often treated with the estrogen-blocking drug tamoxifen.

“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,” Shu explained.

To address these concerns, the 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 Shanghai, 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 like tofu, soy milk, cooked soybeans, miso soup, soybean sprouts and breakfast/protein shakes with added soy.

The U.S. studies assessed those soy foods commonly consumed by U.S. women and the China study used a validated soy questionnaire designed for women in Shanghai. The two major soy products that were 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.”

Women can get the level of soy isoflavone that is similar to the highest consumption level found in the U.S. study population by consuming a cup of soy milk or two ounces of tofu per day.

Sarah Nechuta, 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, Mass., Moores UCSD Cancer Center, University of California San Diego, and the Shanghai Center for Disease Control and Prevention, Shanghai, China.

The research was funded by the American Recovery and Reinvestment Act of 2009, which combines the resources of several National Cancer Institute-funded studies.