No, soy doesn’t cause cancer: Researchers dispel persistent myth in new study

Despite its culinary uses stretching back to the 11th century BCE in China, contemporary myths about soy foods like tofu have proven hard to shake. A new University of Toronto study published in Advances in Nutrition adds to the body of evidence about the safety of soy foods, dispelling the myth that postmenopausal women should avoid them.

As the researchers point out, health advocacy groups such as the Canadian Cardiovascular Society recommend people incorporate soy foods like tofu, tempeh and edamame, and government agencies such as Health Canada and the U.S. Food and Drug Administration recognize them as a source of protein. Even so, myths about soy foods and cancer abound.

“For years, I’ve been trying to dispel the myth about soy and breast cancer. And it seems that the message still isn’t out because almost every day, every patient I see is worried about consuming soy products,” according to Dawn Mussallem, a lifestyle medicine expert at Mayo Clinic who wasn’t involved in the new research.

The U of T study, funded by the United States Department of Agriculture’s United Soybean Board and the Canadian Institutes of Health Research, analyzed 40 randomized controlled trials with more than 3,000 participants. It found that eating soy foods had “no effect on key markers of estrogen-related cancers,” supporting the safety of soybeans “as both a food and potential therapy.” (Namely, as a possible alternative to hormone replacement therapy, or HRT.)

Previous research suggests that soy foods may reduce the risk of heart disease and alleviate menopause symptoms such as hot flashes and night sweats. Yet, many people are hesitant to eat them because they contain isoflavones, according to Laura Chiavaroli, the study’s senior author and an assistant professor of nutritional sciences at U of T’s Temerty Faculty of Medicine.

Early studies linked large doses of these naturally occurring, estrogen-like plant compounds to an increased risk of cancer in mice. Not only was the level of isoflavone much higher than it would be in humans eating soy foods, but “scientists now know that rodents break down isoflavones differently than humans,” according to the American Institute for Cancer Research, which wasn’t involved in the U of T study.

“Something we hear very often is that people have a lot of concern about consuming soy because there are so many conflicting messages out there,” Gabrielle Viscardi, a registered dietitian, PhD student in U of T’s department of nutritional sciences and the study’s lead author, said in a statement.

Though similar to estrogen in structure, the U of T research supports the understanding that soy isoflavones function differently in the human body.

“We have estrogen receptors throughout our bodies, but, contrary to the hormone estrogen, isoflavones from soy don’t bind to all the estrogen receptors equally,” said Viscardi. “That’s why we see a beneficial effect on the cardiovascular system and no effect on the female reproductive system.”

According to Sunnybrook Hospital, which also isn’t affiliated with the U of T study, soy foods are safe to eat “even for people with hormone-sensitive cancers like breast and prostate cancer.” Anna Van Osch, a registered dietitian with the Odette Cancer Centre, added, “Furthermore, studies suggest that up to three servings of soy foods — like one cup of soy milk or one-third cup of tofu — per day may be protective against colorectal, prostate and breast cancer.”

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