Women with breast cancer who consume soy food have lower risk of cancer recurrence

"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 write.

Xiao Ou Shu, M.D., Ph.D., of Vanderbilt University Medical Center, Nashville, Tenn., and colleagues examined the association between soy isoflavone intake with breast cancer recurrence and survival. The researchers analyzed data from the Shanghai Breast Cancer Survival Study, a large, population-based study of 5,042 female breast cancer survivors in China. Women ages 20 to 75 years with diagnoses of breast cancer between March 2002 and April 2006 were recruited and followed up through June 2009. Information on cancer diagnosis and treatment, lifestyle exposures after cancer diagnosis, and disease progression was collected at approximately 6 months after cancer diagnosis and was reassessed at three follow-up interviews conducted at 18, 36, and 60 months after diagnosis. A Shanghai Vital Statistics Registry database was used to obtain survival information for participants who were lost to follow-up.

After a median (midpoint) follow-up of 3.9 years, 444 total deaths and 534 recurrences or breast cancer-related deaths were documented among the group of 5,033 surgically-treated breast cancer patients. Soy food intake, as measured by either soy protein or soy isoflavone intake, was inversely associated with mortality and recurrence. Patients in the group with the highest intake of soy protein had a 29 percent lower risk of death during the study period, and a 32 percent lower risk of breast cancer recurrence compared to patients with the lowest intake of soy protein. The adjusted 4-year mortality rates were 10.3 percent and 7.4 percent and the 4-year recurrence rates were 11.2 percent and 8.0 percent, respectively, for women with the lowest and highest groups of soy protein intake.

"The inverse association was evident among women with either estrogen receptor-positive or -negative breast cancer and was present in both users and nonusers of tamoxifen," the researchers write.

"In summary, in this population-based prospective study, we found that soy food intake is safe and was associated with lower mortality and recurrence among breast cancer patients. The association of soy food intake with mortality and recurrence appears to follow a linear dose-response pattern until soy food intake reached 11 grams/day of soy protein; no additional benefits on mortality and recurrence were observed with higher intakes of soy food. This study suggests that moderate soy food intake is safe and potentially beneficial for women with breast cancer."

Editorial: Challenges in Design and Interpretation of Observational Research on Health Behaviors and Cancer Survival

Rachel Ballard-Barbash, M.D., M.P.H., of the National Cancer Institute, Bethesda, Md., and Marian L. Neuhouser, Ph.D., of the Fred Hutchinson Cancer Research Center, Seattle, write in an accompanying editorial that while this study provides important information, there are several concerns, including differences in the quality, type and quantity of soy food intake between China and the U.S. (47 mg/d vs. 1 to 6 mg/d, respectively, average isoflavone intake). Also, they point out the relatively short median follow-up time of the study (4 years); that there likely are differences in screening rates in China compared with the U.S.; and a number of factors may make it difficult to compare stage- and treatment-specific results in China with outcomes in the U.S.

"Even though the findings by Shu et al suggest that consumption of soy foods among breast cancer patients is probably safe, studies in larger cohorts are required to understand the effects of these foods among diverse clinical subgroups of breast cancer patients and survivors. In the meantime, clinicians can advise their patients with breast cancer that soy foods are safe to eat and that these foods may offer some protective benefit for long-term health. Moreover, the potential benefits are confined to soy foods, and inferences should not be made about the risks or benefits of soy-containing dietary supplements. Patients with breast cancer can be assured that enjoying a soy latte or indulging in pad thai with tofu causes no harm and, when consumed in plentiful amounts, may reduce risk of disease recurrence."