Soy and Breast Cancer: 5 Myths and Facts

You may have heard that you shouldn’t eat soy if you are at risk for breast cancer. But then you see headlines saying that it could protect against the disease. So what’s the truth?

Even for health-savvy people, telling fact from fiction can be tricky.

Knowing the real deal is important, especially now that soy is more common in the American diet. Along with its traditional forms of edamame, tofu, tempeh, and miso, soy is also a popular low-fat source of protein. It’s in soy milk, meat substitutes, cereal, baked goods, energy bars, and more.

Should you avoid these foods or eat more of them? The simplest answer is to think “whole” -- as in, as close to nature as possible -- so you don’t get too much.

For more clarity, get the truth behind these five common myths.

1. Myth: All soy foods raise your risk for breast cancer.

There’s no need to banish tofu and edamame from your diet.

“For years, soy got a bad rap because of its isoflavones,” says Marleen Meyers, MD, director of the Perlmutter Cancer Center Survivorship Program at NYU Langone Medical Center.

These plant-based chemicals are similar in structure to estrogen. Most breast cancers are sensitive to estrogen (or, as doctors say, “estrogen-receptor-positive” or “ER-positive,”) which means that estrogen fuels their growth.

“So there was a fear that soy could act as estrogen in the body and stimulate cancer cells,” Meyers says. “It was spread on blogs, and people would tell each other to avoid soy.”

But a steady stream of studies showed that a diet high in soy didn’t increase the chances of developing breast cancer and may even reduce that risk.

In one study of more than 73,000 Chinese women, researchers found that those who ate at least 13 grams of soy protein a day, roughly one to two servings, were 11% less likely to develop breast cancer than those who got less than 5 grams.

“In Asian cultures, where people eat a lot of soy from a young age, there are lower rates of breast cancer,” Meyers says. And in those societies, people still eat soy in its traditional forms.

Meanwhile, another analysis of eight studies showed that those who got the most soy isoflavones -- about the amount in a serving of tofu - were 29% less likely to get the disease compared to those who got the least.

“As part of a healthy diet, whole soy foods are safe,” says Denise Millstine, MD, director of integrative medicine at the Mayo Clinic in Scottsdale, AZ.

Continued

2. Myth: All types of soy have the same effect on the body.

Your body may process the natural soy in tofu, miso, and soy milk differently than the kind that’s added to processed foods.

The soy protein isolate found in supplements, protein powders, and meat substitutes is usually stripped of nutrients, such as fiber.

“It’s also a more concentrated form of soy,” Millstine says. “So you’re much more likely to get a high dose if you’re having protein shakes and soy hot dogs than if you’re eating edamame.”

Researchers aren’t certain how large amounts of soy affect breast cancer risk. In one early study, soy supplements were shown to “switch on” genes that encourage cancer growth in women with early-stage breast cancer.

Experts recommend sticking with a moderate amount, or about one to two servings, of whole soy a day. One serving includes:

  • Half a cup of cooked edamame
  • 1 cup of soy milk
  • 1 ounce of soy nuts
  • 3 ounces of tofu

3. Myth: Eat soy to protect against breast cancer.

While eating a moderate amount of soy is fine, it’s too soon to suggest eating more to protect your breasts.

“The results are promising, but there’s still not enough information,” Meyers says. Experts now believe that soy isoflavones may actually block estrogen from attaching to breast cancer cells instead of spurring growth like once thought.

Meyers notes that many of the hallmark studies are done in Asian countries, where people grow up eating soy in its traditional forms. “That may influence the way their body processes soy,” she says. “We need to look at if having soy later in life has the same effect.”

More research also needs to be done on how much soy you get at different ages. “Soy may have more of an impact on a postmenopausal woman who’s not producing as much estrogen as a healthy 20-year-old,” Millstine says.

4. Myth: If you have or had breast cancer, avoid all soy foods.

Just as eating a moderate amount of whole soy doesn’t make you more likely to get breast cancer, it also doesn’t seem to raise your risk for recurrence.

Continued

“Still, I’d recommend that breast cancer patients avoid soy supplements,” Millstine says.

In one report, researchers analyzed data from diet surveys completed by more than 9,500 American and Chinese women. Those who said they ate the most soy were 25% less likely to have their cancer return compared to those who had the least.

Some experts worried that soy might interfere with breast cancer drugs that lower estrogen levels, such as tamoxifen. But the same study showed that soy also protected against recurrence in patients who took tamoxifen.

The soy foods that the study included were tofu, soy milk, and fresh soybeans. As you might expect, the Chinese women ate far more of it than those in the U.S. The results still held when the researchers considered that fact.

5. Myth: Soy only affects breast cancers that are sensitive to estrogen.

While it’s true that soy isoflavones play a bigger role in estrogen-receptor positive breast cancers, early research links it to a lower risk of other types of breast cancer.

That finding comes from a study of 756 Chinese women who had breast cancer and about 1,000 others who didn’t have the disease. All of the women answered questions about their diets, including how much soy they ate. Those who said they ate more soy were less likely to have any type of breast cancer, compared to those who ate the least.

That finding doesn’t prove that soy prevented breast cancer in any of the women. Other things could be involved.

“More research still needs to be done,” Meyers says. “It could be that people who eat more soy have healthier lifestyles in general.”

Stay tuned to see if that proves to be helpful across the board, whether you eat tofu regularly, pour soy milk on your breakfast cereal, or snack on edamame.

WebMD Feature Reviewed by Jennifer Robinson, MD on March 22, 2016

Sources

SOURCES:

Denise Millstine, MD, director of integrative medicine, Mayo Clinic; consultant, women’s health, Mayo Clinic Breast Clinic.

Marleen Meyers, MD, assistant professor of medicine, NYU School of Medicine; director of survivorship, Perlmutter Cancer Center Survivorship Program, NYU Langone Medical Center.

UCSF Medical Center: “A Guide to Foods Rich in Soy.”

Hilakivi-Clarke, L. Journal of Nutrition, December 2010.

Lee, S. American Journal of Clinical Nutrition, June 2009.

Wu, A. British Journal of Cancer, January 2008.

United States Department of Agriculture: “USDA Database for the Isoflavone Content of Selected Foods.”

Shike, M. Journal of the National Cancer Institute, September 2014.

American Institute for Cancer Research: “AICR’s Foods That Fight Cancer.”

University of Idaho Extension: “What’s On Your Plate Today?”

Nechuta, S. American Journal of Clinical Nutrition, July 2012.

Zhang, M. Breast Cancer Research and Treatment, December 2009.

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