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Breast Cancer Health Center

Soy and Breast Cancer: What's the Link?

What experts say about whether soy is helpful, harmful, or neutral when it comes to breast cancer.
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The Case for Soy continued...

Chen questions whether the findings have much relevance in the U.S. and other non-Asian countries where most people did not grow up eating foods with soy.

“These women had soy in their diets from childhood and they ate more of it as adults,” she says. “This really doesn’t tell us if adding soy to the diet later in life is protective.”

But another study suggests the findings may apply to people in Western countries. 

That study included nearly 2,000 California women who had been treated for breast cancer. Over an average of six years, those who ate the most soy isoflavones had fewer cancer recurrences than those who ate the least. 

The benefits were greatest among women taking the estrogen-blocking drug tamoxifen after their initial treatment.

More work is needed to see if eating soy foods may boost the benefits of tamoxifen and other treatments, notes researcher Bette Caan, DrPh, of Kaiser Permanente Northern California. 

“I think we can say with confidence that eating soy foods is not harmful for breast cancer survivors,” Caan tells WebMD. “The next question is, ‘Is it beneficial?’ and that is certainly worth pursuing.”

Soy's Form May Matter

Bill Helfreich, PhD, a nutrition professor at the University of Illinois, has been studying soy for more than a decade. His studies in mice were among the first to show that soy isolates stimulate breast cancer cell growth. He bristles at the suggestion that the new positive research on soy invalidates his own earlier work.

“The science is clear," Helfreich tells WebMD. “Isoflavone supplements are estrogenic. They do almost everything estrogens do, including stimulating estrogen-responsive tumors.”

Helfreich's studies didn't show that whole soy promoted tumor cell growth. He agrees that eating minimally processed soy may be good for everyone, including breast cancer survivors.

“I’ve been called anti-soy, but that’s not right,” he says. “Soy is a fine legume, but there is nothing magical about it and it should not be promoted as medicine.”

Helfreich says Westerners who eat minimally processed soy foods, as Asians tend to do, will probably derive similar health benefits. “But you can’t take a soy supplement and eat biscuits and gravy and call it an Asian diet,” he says.

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