Soy and Breast Cancer: What's the Link?

To be safe, Jennifer Mukai all but eliminated soy from her diet after being told she had breast cancer in May 2009.

Being of Japanese descent and also health conscious, the Seattle interior designer says she was eating a lot of soy in various forms before her diagnosis.

“I drank about three-quarters of a cup of soy milk in my coffee twice a day and ate tofu and edamame [soy beans] pretty regularly,” the 44-year-old tells WebMD. “I was also probably getting quite a bit of soy in the meat-substitute products I was eating.”

Infection Risk During Chemotherapy

When she asked her oncologist about it, he advised her to cut way back on the soy foods, saying the jury was still out on its safety for women with a history of breast cancer.

“His thinking was that a little was probably safe, but he said some other oncologists on the staff told their patients to avoid soy completely,” she says.

Is soy safe or even beneficial for women who have had breast cancer and women who might get it? Or is the jury still out, as Mukai’s doctor told her, on soy's role in breast cancer or other cancers?

Experts who spoke to WebMD agreed on some points and disagreed on others, including whether recent research has cleared the air or muddied the waters with regard to soy and breast cancer.

Soy Foods Probably Safe

Once found only in health food stores and Asian markets, soy is now a fixture in the American diet, even among people who have never tried tofu, tempeh, or miso soup.

A cheap source of protein, soy is used in the manufacture of a wide range of highly processed foods, including breads, cookies, crackers, breakfast cereals, soy ‘milk’, non-dairy creamers, imitation cheeses, and even some yogurts.

The experts and researchers interviewed by WebMD say that breast cancer survivors probably have little to fear from eating soy foods in moderation.

They also said that soy supplements, a widely used alternative to hormone therapy for menopause-related hot flashes, should be avoided.

The studies have not found these compounds to be very helpful in terms of reducing hot flashes,” Dana Farber Cancer Center oncologist Wendy Chen, MD, tells WebMD. “And they have not been studied in women with breast cancer, so the risk is not known.”

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.”

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.

Helferich says breast cancer survivors can safely eat soy foods in moderation.

Despite the new research, Jennifer Mukai says she won’t be adding much more soy to her diet in the near future.

Mukai is taking aromatase inhibitors, prescription drugs that help lower the amount of estrogen in the body.

Because the whole point of the treatment is to eliminate estrogen from her body, Mukai says it doesn’t make sense to eat foods that contain estrogen, even in very small amounts.