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.
Why the Concern?
“We tell women with breast cancer to definitely avoid the [soy] supplements,” Chen says. “Our message to the general public is that we really don’t know if supplements are safe because they haven’t been tested.”
The concern about soy stems from the fact that most breast cancers are fueled by the female sex hormone estrogen.
Just as the body produces estrogen, so do plants, and soy contains high amounts of estrogen-like chemicals called isoflavones. The research is unclear about how these plant-based estrogens impact the body’s own estrogen levels and breast cancer growth.
One theory is that they boost overall estrogen levels; another theory is that they compete with the body’s natural estrogen and actually reduce levels of the hormone.
Some animal studies have shown that they cause breast cancer cells to grow. But more recent studies, done in people, suggest that they may reduce the risk of recurrence in women with estrogen-positive breast cancers.
The Case for Soy
The most widely reported of these studies involved about 5,000 breast cancer survivors in Shanghai, China. Over almost four years, people who ate the most soy foods had the highest survival rate and lowest risk of recurrence.
When the women were divided into four groups based on soy consumption, the quarter that ate the most soy had a 29% lower risk for death and a 32% lower risk of having their breast cancer return than the quarter of women who ate the least.
“We studied soy foods, not supplements,” says researcher Xiao Ou Shu, MD, PhD, of Vanderbilt University Medical Center in Nashville, Tenn.
That distinction is important, she says, because it's not clear if the isoflavones in soy or some other nutrient in soy foods was responsible.
“I think we can say with confidence that moderate intake of soy foods is not harmful and may be beneficial for breast cancer survivors,” Shu tells WebMD. “But we are talking about moderate intake, not mega doses.”
In the study, women who ate about 11 grams a day of soy protein -- about equal to a cup of soymilk or a quarter of a block of tofu -- had the lowest risk. Higher soy consumption did not lower risk further.
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.”