Should postmenopausal women eat soy? Researchers reveal why the answer is "yes."

Naturally occurring isoflavones in soy have no effect on key markers of estrogen-related cancers

ST. LOUIS, Jan. 7, 2025 /PRNewswire/ -- A University of Toronto study is providing reassuring evidence about the consumption of soy foods by postmenopausal women.* The analysis of 40 randomized controlled trials in over 3,000 women found that isoflavones in soy, naturally occurring plant compounds commonly classified as plant estrogens, had no effect on key markers of estrogen-related cancers, supporting soy's safety and potential contribution to health.

"Our findings support the notion that soy isoflavones behave differently from estrogen, particularly when it comes to cancers that depend on estrogen to develop," says Laura Chiavaroli, the study's senior author and an assistant professor of nutritional sciences at University of Toronto's Temerty Faculty of Medicine. "We hope our study will help women feel more comfortable including soy foods in their diet without being concerned that doing so is going to increase their risk of estrogen-related cancer."

Since soy isoflavones have previously demonstrated benefits on postmenopausal symptoms, such as hot flashes, yet do not have a harmful effect on estrogenic markers related to female-specific cancers as shown in the present study, they likely act as selective estrogen receptor modulators (SERMs). SERMs, like the breast cancer drug tamoxifen, have estrogenic effects in some tissues but either no effects or anti-estrogenic effects in others.

"Something we often hear is that people are confused about consuming soy because there are so many conflicting messages out there," says Gabrielle Viscardi, a registered dietitian and second-year PhD student in the department of nutritional sciences and the study's lead author. 

In their systematic review and meta-analysis, published in Advances in Nutrition, the researchers gathered results from trials that examined the effects of soy isoflavones on four different biological outcomes related to risk for endometrial and other female-related cancers. These included the thickness of the uterus lining, the vaginal maturation index — a measure of estrogen status within the vaginal environment — and levels of circulating estrogen and follicle-stimulating hormone. 

The trials from around the world included postmenopausal women who had consumed either soy isoflavones or a non-isoflavone control for at least three months. After assessing the risk of bias and the quality of the evidence, the researchers concluded that consumption of soy isoflavones did not affect these four estrogen-related markers.

Several health advocacy groups including the American Heart Association and the American Cancer Society already recommend soy foods as part of a healthy diet. They are sources of high-quality protein and are recognized by several governmental health agencies — including the US Food and Drug Administration and Health Canada — for their role in reducing the risk of heart disease.

*Although menopause is sex-specific, the term "women" reflects the original study and the published trials it references, with recognition that some persons experiencing menopause may identify differently than with this gender.

This study was funded by the United Soybean Board and the Canadian Institutes of Health Research through the Canada-wide Human Nutrition Trialists' Network.

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About Soy Nutrition Institute Global
Soy Nutrition Institute Global is a global scientific voice of soy for human health and nutrition. Soy Nutrition Institute Global discovers, illuminates, and advocates for soy's role in human health and nutrition using scientific evidence to promote consumption globally. For more information about the Soy Nutrition Institute Global, visit www.SNIGlobal.org.

About the Temerty Faculty of Medicine
The Temerty Faculty of Medicine at the University of Toronto is at the centre of one of North America's largest biomedical research, education and clinical care networks. With nine fully affiliated research-intensive hospitals — and dozens of community and clinical care sites — Temerty Medicine offers unparalleled learning opportunities in Canada's most dynamic city.

Powered by $995 million in annual funding, Temerty Medicine and our hospital partners are Canadian leaders in research and publish more academic papers than any other Canadian medical school. In 2024, U.S. News & World Report ranked U of T third globally in clinical medicine, driven in part by highly cited papers from Temerty Medicine researchers in top-tier journals.

About the University of Toronto (U of T)
Founded in 1827, the University of Toronto is Canada's leading institution of learning, discovery and knowledge. U of T is one of the world's leading research-intensive universities and ranked Canada's most innovative university. We're a global leader in transforming innovative ideas into products, services, companies and jobs.

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SOURCE Soy Nutrition Institute Global