What You Need to Know
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You may have heard that you shouldnât eat soy if you are at risk for breast cancer. But then you see headlines saying that it could protect against the disease. So whatâs 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. Itâs 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 âwholeâ — as in, as close to nature as possible — so you donât 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.
Thereâs no need to banish tofu and edamame from your diet.
âFor years, soy got a bad rap because of its isoflavones,â 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, âestrogen-receptor-positiveâ or âER-positive,â) which means that estrogen fuels their growth.
âSo there was a fear that soy could act as estrogen in the body and stimulate cancer cells,â Meyers says. âIt was spread on blogs, and people would tell each other to avoid soy.â
But a steady stream of studies showed that a diet high in soy didnât 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.
âIn Asian cultures, where people eat a lot of soy from a young age, there are lower rates of breast cancer,â 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.
âAs part of a healthy diet, whole soy foods are safe,â 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 thatâs added to processed foods.
The soy protein isolate found in supplements, protein powders, and meat substitutes is usually stripped of nutrients, such as fiber.
âItâs also a more concentrated form of soy,â Millstine says. âSo youâre much more likely to get a high dose if youâre having protein shakes and soy hot dogs than if youâre eating edamame.â
Researchers arenât certain how large amounts of soy affect breast cancer risk. In one early study, soy supplements were shown to âswitch onâ 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, itâs too soon to suggest eating more to protect your breasts.
âThe results are promising, but thereâs still not enough information,â 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. âThat may influence the way their body processes soy,â she says. âWe need to look at if having soy later in life has the same effect.â
More research also needs to be done on how much soy you get at different ages. âSoy may have more of an impact on a postmenopausal woman whoâs not producing as much estrogen as a healthy 20-year-old,â Millstine says.
4. Myth: If you have or had breast cancer, avoid all soy foods.
Just as eating a moderate amount of whole soy doesnât make you more likely to get breast cancer, it also doesnât seem to raise your risk for recurrence.
âStill, Iâd recommend that breast cancer patients avoid soy supplements,â 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 itâs 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 didnât 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 doesnât prove that soy prevented breast cancer in any of the women. Other things could be involved.
âMore research still needs to be done,â Meyers says. âIt could be that people who eat more soy have healthier lifestyles in general.â
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.
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