You may have heard that you shouldn’t eat soy if you’re at risk of breast cancer. But then you see headlines saying it could protect against the disease. So what is the truth?
Even for health-conscious people, it can be difficult to separate fact from fiction.
It’s important to know the real deal, especially now that soy is becoming more common in the American diet. Along with the traditional forms of edamame, tofu, tempeh, and miso, soy is also a popular low-fat protein source. It’s found 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 “holistically” – meaning as close to nature as possible – so you don’t get too much.
For more clarity, learn the truth behind these five common myths.
There’s no need to eliminate tofu and edamame from your diet.
“For years, soy had 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 estrogen-sensitive (or, as doctors say, “estrogen receptor-positive” or “ER-positive”), meaning that estrogen fuels their growth.
“So there was a concern that soy could act as an estrogen in the body and stimulate cancer cells,” says Meyers. “It was circulating on blogs, and people were telling each other to avoid soy.”
But a steady stream of studies have shown that a diet high in soy does not increase your chance of developing breast cancer and may 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 per day, about one to two servings, were 11% less likely to develop breast cancer than those who ate less than 5 grams took.
“There are lower rates of breast cancer in Asian cultures, where people eat a lot of soy from a young age,” says Meyers. And in these societies, people still eat soy in its traditional form.
Meanwhile, another analysis of eight studies showed that those who ate the most soy isoflavones — about the amount in a serving of tofu — had a 29% lower risk of developing the disease than those who ate the least took.
“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.
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 does not typically contain 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 from eating protein shakes and soy hot dogs than from eating edamame.”
Researchers aren’t sure how large amounts of soy affect breast cancer risk. In an early study, soy supplements were shown to “turn on” genes that promote cancer growth in women with early-stage breast cancer.
Experts recommend sticking with a moderate amount, or about 1-2 servings of soy per day. One portion includes:
While eating a moderate amount of soy is fine, it’s too early 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 actually block estrogen from attaching to breast cancer cells, rather than boosting growth as was previously thought.
Meyers notes that many of the outstanding studies have been conducted in Asian countries, where people are raised on soy in its traditional form. “It can affect the way your body processes soy,” she says. “We need to see if eating soy has the same effect later in life.”
More research also needs to be done on how much soy you eat at different ages. “Soy may have a greater effect on a postmenopausal woman who doesn’t produce as much estrogen as a healthy 20-year-old,” says Millstine.
Just as eating a moderate amount of whole soy doesn’t make you more likely to get breast cancer, it doesn’t appear to increase your risk of recurrence either.
“Nevertheless, I would recommend that breast cancer patients avoid soy supplements,” says Millstine.
In a report, researchers analyzed data from diet surveys conducted by more than 9,500 American and Chinese women. Those who reported eating the most soy were 25% less likely that the cancer would return than those who had the least.
Some experts feared soy might interfere with breast cancer drugs that lower estrogen levels, such as B. Tamoxifen. However, the same study showed that soy also protected against recurrence in patients taking tamoxifen.
The soy foods included in the study were tofu, soy milk, and fresh soybeans. As might be expected, Chinese women ate far more than those in the US. The results were still holding when the researchers took this fact into account.
While it’s true that soy isoflavones play a greater role in estrogen receptor-positive breast cancer, early research links this to a lower risk of other types of breast cancer.
This 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 of the women answered questions about their diet, including how much soy they ate. Those who reported eating more soy were less likely to have some type of breast cancer compared to those who ate the least.
This finding does not prove that soy prevented breast cancer in any of the women. Other things might be involved.
“More research needs to be done,” says Meyers. “It could be that people who eat more soy have healthier lifestyles in general.”
Stay tuned to see if this proves helpful across the board, whether you regularly eat tofu, pour soy milk on your breakfast cereal, or munch on edamame.