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The Soy - Breast Cancer Controversy

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By: Richard Lee, M.D., and Lorenzo Cohen, Ph.D.

Cancer patients often take herbs and supplements with the hope of improving their outcomes, and utilization is especially high in women with breast cancer.

One controversial area is the use of soy and soy-related products during and after treatment. Natural soy is known to contain phytoestrogens such as isoflavones, and these compounds in preclinical studies promote estrogen-dependent breast cancer growth. Thus, breast cancer patients are commonly advised to avoid all dietary soy.  

soybeans.jpgFindings from two recent studies are starting to question whether soy may in fact be helpful. A study published at the end of 2009 in The Journal of the American Medical Association by Shu et al., investigated the association between soy food intake and breast cancer survival among a prospective, population-based cohort of 5,042 Chinese women diagnosed with breast cancer between 2002 and 2006. In-person interviews were conducted at baseline, and 12 and 36 months about food consumption, including soy. Soy intake was inversely correlated with breast cancer recurrence and mortality (i.e., higher soy intake was associated with increased survival). The findings remained when comparing estrogen positive/negative and tamoxifen users/non-users subgroups.

Earlier in 2009, results from the Life After Cancer Epidemiology (LACE) study appeared. In this prospective cohort trial involving 1,954 women with breast cancer, those who consumed soy isoflavones at levels comparable to Asian populations actually had a reduced risk of recurrence, especially if they were on concurrent tamoxifen therapy. The authors concluded: "Soy isoflavones consumed at levels comparable to those in Asian populations may reduce the risk of cancer recurrence in women receiving tamoxifen therapy and, moreover, appears not to interfere with tamoxifen efficacy."

These studies provide further information that regular dietary soy as part of a normal diet is probably safe and, in fact, may be beneficial for women with breast cancer. These studies, as well as others, create a growing body of evidence that the current advice that all soy foods should be removed from the diet of a woman diagnosed with breast cancer is probably untrue.

The role of non-dietary soy products such as soy supplements, powders or pills and the use of heavily processed soy items such as soy cheese, soy hot dogs or soy turkey remain unclear and should be avoided.

As the research to date is observational in nature, future randomized trials are needed to verify if a beneficial effect truly exists for the use of dietary soy. 


1.  Shu XO, et al. Soy and intake and breast cancer survival.  JAMA 302(22):2437-2443, 2009.
2.  Guha N, et al. Soy isoflavones and risk of cancer recurrence in a cohort of breast cancer survivors: the Life After Cancer Epidemiology study.  Breast Cancer Res Treat 118:395-405, 2009.


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