Despite the anticancer benefits of lifelong consumption of dietary soy, adding soy to diet for the first time while taking tamoxifen for breast cancer may increase the risk of tumor recurrence, suggested study findings published in Clinical Cancer Research.1

For this study, female Sprague-Dawley rats were fed either a control diet free of genistein, a soy phytochemical, or 500 ppm genistein throughout their lives. Mammary tumors were chemically induced in the animals, after which genistein was added to the diet of a subset of the control-diet rats. Expression of autophagy-related genes and immunosuppression genes was diminished within the lifetime genistein-diet group, compared with rats in the other study arms.

“Lifetime genistein intake reduced de novo resistance to tamoxifen, compared with postdiagnosis genistein groups,” the authors reported. Whether these findings will apply to humans is not know.

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Most women with breast cancer have estrogen-receptor positive tumors; for many patients, endocrine treatments such as tamoxifen reduce estrogen-fueled tumor growth and recurrence after patients achieve remission.

Longtime dietary soy consumption is associated with improved tamoxifen outcome and reduced recurrence rates for breast cancer, possibly by improving the body’s overall anticancer immunity, the authors suggested. Genistein also appears to inhibits cellular autophagy.

However, why adding soy to diet after initiating tamoxifen therapy would diminish treatment effectiveness remains unclear.


1. Zhang X, Cook KL, Warri A, et al. Lifetime genistein intake increases the response of mammary tumors to tamoxifen in rats. Clin Cancer Res. 2017;23(3):814-824. doi: 10.1158/1078-0432.CCR-16-1735