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The Truth About Soy and Breast Cancer

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

Even for health-conscious people, telling facts from fiction can be tricky.

It’s important to know the real deal, especially now that soy is more common in the American diet. In addition to the 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, breakfast cereals, baked goods, energy bars and more.

Should you avoid or eat more of these foods? 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, find the truth behind these five common myths.

1. Myth: All soy products increase the risk of breast cancer.

There is no need to eliminate tofu and edamame from your diet.

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

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

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

But a steady stream of studies showed that a diet high in soy did not increase the chance of developing breast cancer and might actually reduce that risk.

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

“In Asian cultures, where people eat a lot of soy from a young age, there is less 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 products are safe,” says Denise Millstine, MD, director of integrative medicine at the Mayo Clinic in Scottsdale, AZ.

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 added to processed foods.

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

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

Researchers aren’t sure how high amounts of soy affect breast cancer risk. In an early study, soy supplements were shown to “turn on” genes that stimulate cancer growth in women with early-stage breast cancer.

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

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

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

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

“The results are promising, but there is not enough information yet,” says Meyers. Experts now believe that soy isoflavones may block estrogen’s attachment to breast cancer cells instead of stimulating growth as once thought.

Meyers notes that many of the landmark studies are being done in Asian countries, where people grow up eating soy in its traditional form. “That can affect the way their bodies process soy,” she says. “We need to see whether soy has the same effect later in life.”

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 doesn’t produce as much estrogen as a healthy 20-year-old,” says Millstine.

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

Just as eating a moderate amount of whole soy doesn’t make you more likely to develop breast cancer, it doesn’t seem to increase your risk of recurrence either.

“Still, I would advise breast cancer patients to avoid soy supplements,” says Millstine.

In one report, researchers analyzed data from nutrition 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 cancer recurrence compared to those who had the least.

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

The soy products included in the study were tofu, soy milk, and fresh soybeans. As you would expect, the Chinese women ate much more of it than those in the US. The results were still valid 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 greater role in estrogen receptor-positive breast cancers, early research links this 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 did not have the disease. All women answered questions about their diet, including how much soy they ate. Those who said they ate more soy were less likely to develop breast cancer than those who ate the least.

That finding does not prove that soy did not prevent breast cancer in any of the women. There may be other things at play.

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

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