Soy contains high levels of plant isoflavones that exert a variety of anticancer activities in laboratory studies. Perhaps because soy has the potential to produce both estrogenic and antiestrogenic effects, studies on soy and breast carcinogenesis have produced conflicting results. For the breast cancer survivor, current epidemiologic and laboratory evidence suggests there are unlikely to be harmful effects when soy is provided in the diet consistent with amounts in a typical Asian diet; whether such levels of soy intake may result in beneficial effects is also unclear. This amount would be provided by as many as three servings per day of soy foods, such as tofu and soy milk. However, because higher doses of soy may have estrogenic effects and because higher levels of estrogens clearly increase the risk for breast cancer progression, it is prudent for breast cancer survivors to avoid the high doses of soy and soy isoflavones that are provided by more concentrated sources such as soy powders and isoflavone supplements.

As mentioned earlier, research on this subject is continuing. A large 2009 California study²⁰ by Guha and colleagues followed women with breast cancer undergoing treatment with the widely-utilized chemotherapeutic agent, tamoxifen. These investigators found that the patients whose diets included the highest amounts of the soy isoflavone, daidzein, had approximately a 60% reduction in breast cancer recurrence compared to those with the lowest levels. They 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. Further confirmation is required in other large prospective studies before recommendations regarding soy intake can be issued to breast cancer survivors.

At a bare minimum, this study debunks the claim that soy has been proven to be harmful for women with breast cancer. It may be helpful. We don’t yet know for certain. At this point, whether a breast cancer patient should eat soy, and in what amounts, is a personal decision to be undertaken with care.

In our next issue: Soy Controversies in the Media

Daniel Redwood, DC, is an Associate Professor at Cleveland Chiropractic College – Kansas City and Editor-in-Chief of Health Insights Today.

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