Soy Food and Breast Cancer
A recent article in JAMA contradicts everything we have been told for years about breast cancer and soy. This study from Shanghai found that women with breast cancer who ate soy-containing foods had a lower risk of recurrence and death. The usual suggestion has been that women who have an ER positive breast cancer should avoid anything but small amounts of soy since it is a plant estrogen. If you enjoy tofu or soy milk or other soy foods, you might want to discuss this finding with your doctor. Here is a summary and the link if you are interested in the whole article:
Soy Food Intake and Breast Cancer Survival
Xiao Ou Shu, MD, PhD
Ying Zheng, MD, MSc
Hui Cai, MD, PhD
Kai Gu, MD
Zhi Chen, MD, PhD
Wei Zheng, MD, PhD
Wei Lu, MD, PhD
Estrogen is believed to play a central role in breast cancer development and progression. Blocking the effect of estrogen, either by inhibiting estrogen action or by reducing estrogen production, has been widely used in breast cancer treatment as an adjuvant therapy. Soy foods are rich in phytoestrogens, mainly in the form of isoflavones, which are natural estrogen receptor modulators that possess both estrogen-like and antiestrogenic properties.
Soy constituents have also been shown to have other anticancer effects, including the inhibition of DNA topoisomerase I and II, proteases, tyrosine kinases, inositol phosphate,and angiogenesis and may also boost immune response and possess antioxidative effects.
Consumption of soy food has been inversely related to the risk of breast cancer in many epidemiological studies. However, genistein, a major form of isoflavone, has been shown to enhance the proliferation of breast cancer cells in vitro and to promote estrogen-dependent mammary tumor growth in ovariectomized rats. In addition, breast cancer treatments often lead to a decrease in the endogenous estrogen supply of survivors, and a concern has been raised as to whether soy isoflavones may exert their estrogenic effects, promote cancer recurrence, and, thus, negatively influence overall survival. Furthermore, both in vivo and in vitro studies have suggested that soy isoflavones may interact with tamoxifen, although both synergistic and antagonistic interactions have been reported.
For editorial comment see p 2483.
Author Affiliations: Division of Epidemiology, Department< of Medicine, Vanderbilt Epidemiology Center and Vanderbilt-Ingram Cancer Center, Vanderbilt University Medical Center, Nashville, Tennessee
(Drs Shu, Cai, Chen, and W. Zheng); and Shanghai
Institute of Preventive Medicine, Shanghai, China
(Drs Y. Zheng, Gu, and Lu).
Corresponding Author: Xiao Ou Shu, MD, PhD, Department of Medicine, Vanderbilt Epidemiology Center, 2525 West End Ave, Ste 600, Nashville, TN 37203-1738 (firstname.lastname@example.org).
Context 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.
Objective To evaluate the association of soy food intake after diagnosis of breast cancer with total mortality and cancer recurrence.
Design, Setting, and Participants The Shanghai Breast Cancer Survival Study, a large, population-based cohort study of 5042 female breast cancer survivors in China. Women aged 20 to 75 years with diagnoses 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 3 follow-up interviews conducted at 18, 36, and 60 months after diagnosis. Annual record linkage with the Shanghai Vital Statistics Registry database was carried out to obtain survival information for participants who were lost to follow-up. Medical charts were reviewed to verify disease and treatment information.
Main Outcome Measures Total mortality and breast cancer recurrence or breast cancer-related deaths. Cox regression analysis was carried out with adjustment for known clinical predictors and other lifestyle factors. Soy food intake was treated as a time-dependent variable.
Results During the median follow-up of 3.9 years (range, 0.5-6.2 years), 444 deaths and 534 recurrences or breast cancer-related deaths were documented in 5033 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. The hazard ratio associated with the highest quartile of soy protein intake was 0.71 (95% confidence interval [CI], 0.54-0.92) for total mortality and 0.68 (95% CI, 0.54-0.87) for recurrence compared with the lowest quartile of intake. The multivariate-adjusted 4-year mortality rates were 10.3% and 7.4%, and the 4-year recurrence rates were 11.2% and 8.0%, respectively, for women in the lowest and highest quartiles 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.
Conclusion Among women with breast cancer, soy food consumption was significantly associated with decreased risk of death and recurrence.
JAMA. 2009;302(22):2437-2443 www.jama.com
©2009 American Medical Association. All rights reserved. (Reprinted) JAMA, December 9, 2009—Vol 302, No. 22 2437