There had been a concern that soy foods could have an adverse effect on outcomes among breast cancer patients. "Soy foods are rich in isoflavones, a major group of phytoestrogens that have been hypothesized to reduce the risk of breast cancer. However, the estrogen-like effect of isoflavones and the potential interaction between isoflavones and tamoxifen have led to concern about soy food consumption among breast cancer patients,” the authors, from Vanderbilt [University]-Ingram Cancer Center (Nashville, TN, USA), wrote in their study, published in the December 9, 2009, issue of the Journal of American Medial Association (JAMA).
Tamoxifen, which is designed to block estrogen, is a widely used treatment for breast cancer patients. Xiao Ou Shu, M.D., Ph.D., professor of medicine and her colleagues analyzed data from the Shanghai Breast Cancer Survival Study, a large, population-based study of 5,042 female breast cancer survivors in China, which Vanderbilt University Medical Center and the Shanghai Institute of Preventive Medicine (China) have carried out since 2001.
Women ages 20 to 75 years, diagnosed between March 2002 and April 2006, were studied through June 2009. Trained interviewers using structured questionnaires asked the women about demographic characteristics, reproductive and disease history, medication use, diet, lifestyle factors and use of complementary and alternative medicine.
Researchers used a food frequency questionnaire designed to measure soy foods commonly consumed in Shanghai, including tofu, soymilk, fresh soybeans and other soy products, as well as meat, fish and cruciferous vegetables.
After a median follow-up of 3.9 years, there were 444 total deaths and 534 breast cancer recurrences in the study group. Soy food consumption after cancer diagnosis, measured as soy protein intake, was inversely associated with mortality and recurrence. The associations of soy protein/isoflavones intake with mortality and recurrence appear to follow a dose-response pattern until soy protein intake reaches 11 grams per day or soy isoflavones intake reaches 40 mg/day. After these points, the association appears to level off or rebound. "We found that women in the highest soy food intake groups had the lowest mortality and recurrence rates rate compared with women in the lowest soy food intake group, regardless of tamoxifen use status,” said Dr. Shu.
The associations of soy food intake with mortality and recurrence were observed for women with either ER-positive or ER-negative breast cancer. The association between soy food intake and overall mortality did not appear to vary by menopausal status. "It is important to note that we studied soy food intake and not the use of soy capsule supplements,” explained Dr. Shu. "These capsules frequently contain only soy isoflavones, while soy foods contain other nutrients, as well. So we cannot infer that the isoflavones alone would provide the same protective benefits.”
Soy isoflavones compete with estrogens in the binding of estrogen receptors, reduce estrogen synthesis, and help clear steroids from the body. These antiestrogenic effects may be one of the underlying mechanisms through which soy foods are associated with better breast cancer outcomes. Other constituents of soy foods, such as folate, protein, protease inhibitors, calcium, or fiber, also may be responsible for the survival benefits of soy food consumption.
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