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Soy and breast cancer

A woman in a green sleeveless sweater stirs pasta on the stovetop and smiles.

April 21, 2019—You're a breast cancer survivor—or you're at high risk for this common cancer. And you wonder: Should I shy away from soy?

It's a good question, because our understanding of this topic has evolved in the last several years. Soy foods, like tofu, tempeh, soy milk and edamame, contain plant chemicals called isoflavones. These compounds look similar to the hormone estrogen and act like estrogen in some ways. At high levels, estrogen is known to spur the growth of some breast cancers. In years past, there was some concern that eating soy foods could raise estrogen levels and increase breast cancer risk.

So what does science say? While the isoflavones in soy are called "phytoestrogens," soy does not actually contain estrogen. Multiple studies—in which thousands of women's health data was tracked for many years—show that women who regularly eat soy don't have a heightened breast cancer risk. Research also shows that eating a moderate amount of soy foods (one or two daily servings) doesn't raise the risk of breast cancer coming back.

Soy may be protective

What's more, emerging evidence suggests that soy foods may actually lower the risk of hormone-dependent cancers, including those of the breast, prostate and endometrium (lining of the uterus). This may be because isoflavones can block natural estrogens in the blood. In the major studies mentioned above, women who regularly ate soy foods had a lower risk of breast cancer than those who ate none.

The bottom line: Soy foods are safe, and possibly even protective, for breast cancer survivors and women at risk.

And if you're concerned about your breast health? It's best to stick to actual foods—not supplements like soy pills and powders enriched with isoflavones. That's because the current research on soy and breast health has examined only soy foods.

Sources: American Cancer Society; American Institute for Cancer Research; Academy of Nutrition and Dietetics

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