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Soy and Breast Cancer Recurrence: Calculating the benefit or risk
Jacob Schor ND FABNO
November 29, 2010
Over the years many of you have asked whether eating soy beans and soy derived foods was a good or a bad idea for women with a history of breast cancer. Utterly confused by the published data, I generally suggested to avoid soy, saying something like, “Soy is either good for you or bad for you, it’s unlikely neutral, but since we don’t know which, best to stay away.”
A recently published study seems to have sorted the situation out so that I can give a more accurate answer. Now when women with a history of breast cancer asks about eating soy, we can accurately answer, “Depends.”
Kang et al, in a study published in the November 23, 2010 issue of the Canadian Medical Association Journal, compared dietary intake of soy isoflavones in 524 women who underwent breast cancer surgery with their risk of cancer recurrence or death. In total 524 women were followed for an average of 5.1 years.
Postmenopausal women eating the most soy had a lower risk of recurrence than women eating less soy. Those in the upper quartile of soy isoflavone consumption compared to the lowest quartile were at about 33% lower risk.
But it wasn’t really this simple. The risk varied with a number of factors including the hormone receptor status of their initial tumors and what type of hormone therapy the women received after surgery. In certain situations, soy appears to increase risk of recurrence.
A free download of the Kang paper is available at: http://www.cmaj.ca/cgi/reprint/182/17/1857?maxtoshow=&hits=10&RESULTFORMAT=&fulltext=soy+breast+cancer+recurrence&searchid=1&FIRSTINDEX=0&sortspec=date&resourcetype=HWCIT
I would suggest if you are interested in this topic, that you download the full paper and turn to figure 1 on page 4. This chart summarizes their results in more detail. Here’s what it tells us:
a. Consumption of soy isoflavones was protective for women who were estrogen receptor and progesterone receptor positive (ER+/PR+). Women in the upper versus lower quartiles of soy isoflavone consumption had a 34% decreased risk of recurrence.
b. Consumption of soy may increase risk of recurrence in women who were ER+ and PR-. These numbers were not statistically significant but strongly suggestive.
c. Similar results were seen for women who were ER- and PR+ and those who were treated with tamoxifen. High soy appears to increase risk for recurrence slightly, 10-20%, but these data again were not statistically significant so that we can’t make conclusive statements.
d. The numbers were statistically significant for the women taking the aromatase inhibitor anastrozole (Arimidex). Women in the highest quartile of soy isoflavone consumption had a 35% lower risk of recurrence.
Thus, at this point, we can tentatively provide a more nuanced answer to the soy and breast cancer question.
Women whose cancers were both ER and PR positive and those being treated with anastrozole, may benefit from eating soy. Women who were negative for either hormone receptor or who are taking tamoxifen should continue to avoid soy.
This is unlikely to be the last study on this subject and we will not be surprised to find ourselves amending these recommendations as time goes on and new studies are published. Frustrating as that statement may seem, it is still far more accurate than prior opinions we might have voiced.
Kang X, Zhang Q, Wang S, Huang X, Jin S. Effect of soy isoflavones on breast cancer recurrence and death for patients receiving adjuvant endocrine therapy. CMAJ. 2010 Nov 23;182(17):1857-62.