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Ask Tufts Experts September 2017 Issue

Q. I've been drinking soy milk daily for years. Since it contains phytoestrogens, I wonder if it puts me at higher risk of developing breast cancer?

A. Fang Fang Zhang, MD, PhD, an associate professor at Tufts' Friedman School and a cancer epidemiologist, responds: "Soy foods contain phytoestrogens (plant-based estrogens) called isoflavones. These are structurally similar to the hormone estrogen. So, they bind to estrogen receptors, although less strongly than estrogen, and exert estrogen-like effects.

"Because a high level of circulating estrogen is a known risk factor for breast cancer, there has been concern that consumption of soy foods may increase women’s risk of developing breast cancer. However, the existing epidemiologic evidence (observational research) does not support that. On the contrary, some evidence suggests that a high intake of soy foods is associated with a reduced risk of breast cancer. The exact mechanisms for an anti-cancer effect are unclear but may include isoflavones inhibiting cell proliferation (an increase in cell number) and reducing inflammation and oxidative stress.

"Overall, epidemiologic studies consistently suggest no harm of soy food consumption in either women with or without breast cancer. So, continue to enjoy soy foods (preferably minimally processed ones, like unsweetened soy milk, tofu and edamame/green soybeans) as part of an overall healthy and balanced diet."

To learn more: Cancer, June 2017

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