Despite its health and nutritional benefits, many women are hesitant to eat soy for fear of its supposed “estrogen” effects. An often-cited health concern is that consuming it could increase the likelihood of developing breast cancer or worsening outcomes of the disease.
This notion stems from the fact that soybeans contain isoflavones, a type of plant estrogen that can bind to estrogen receptors in the body. Since high levels of estrogen are associated with an increased risk of breast and endometrial cancers, it’s been thought that soy isoflavones could be too.
Now, a new Canadian meta-analysis has shown that soy isoflavones do not mimic estrogen’s effects related to female-specific cancers in postmenopausal women, adding to previous evidence that soy foods are safe to eat.
Here’s what to know about the new research, plus ways to include nutrient-rich soy foods in your regular diet.
Soy isoflavones differ from estrogen
Human estrogen is thought to play a role in breast cancer by stimulating the division and proliferation of breast tissue, thereby increasing the risk of cancer-causing DNA mutations.
High estrogen levels can also stimulate excessive growth, or thickening, of the inner lining of the uterus, called the endometrium. This can eventually cause cancerous cells to develop.
Soy isoflavones, however, are structurally different from human estrogen and do not convert to it once they’re consumed.
While isoflavones can bind to estrogen receptors, they are tissue-selective and don’t bind to all receptors equally. Studies suggest, for example, that soy isoflavones have estrogenic effects that benefit heart and bone health, and anti-estrogenic effects in breast tissue.
About the latest research
The new meta-analysis, published Nov. 27 in the journal Advances in Nutrition, combined data from previous studies and evaluated the effect of soy isoflavones on four markers related to estrogen-dependent female cancers. The markers were: endometrial thickness, the vaginal maturation index (a measure of estrogen status in the vaginal environment), circulating levels of estrogen and levels of follicle-stimulating hormone (which rise when estrogen is high).
Researchers from the University of Toronto looked at 40 randomized controlled trials published between 1995 and 2024 that involved a total of 3,285 healthy postmenopausal women. Trial participants, predominately middle aged, consumed either soy isoflavones from food or supplements, or a non-isoflavone control.
The researchers concluded that soy isoflavones had no effect on any of the four estrogen-related markers in postmenopausal women.
What past research says
The latest results are consistent with previously published findings. For example, a 2016 meta-analysis of 23 randomized controlled trials involving peri- and postmenopausal women found no effect of isoflavones from soy on endometrial thickness. And a 2009 analysis of 21 trials concluded that neither soy nor soy isoflavones affected estrogen or follicle-stimulating hormone levels in postmenopausal women.
Research also suggests that soy isoflavones can actually reduce the risk of breast cancer recurrence.
In 2023, a meta-analysis of observational studies found that soy isoflavone consumption was tied to a 26 per cent lower risk of breast cancer recurrence, especially in postmenopausal women. The greatest protection was seen with 60 milligrams of isoflavones a day, equivalent to two to three soy food servings (equivalent to one cup of soy milk, 85 to 100 grams of tofu or half a cup of cooked soybeans).
Soybean nutrition
Soybeans are nutrient dense, especially when it comes to protein: Soy is considered a “complete” protein, meaning it provides all nine essential amino acids the body can’t make on its own.
Whole soy foods are also good sources of fibre, omega-3 fats, brain-supportive choline and many vitamins and minerals, including folate, iron, magnesium, calcium and potassium. Incorporating whole soy foods into your diet is an excellent way of increasing your consumption of plant protein, as recommended by health eating guidelines.
Enjoy edamame beans steamed in their pods as a protein-rich snack, or add them shelled to stir-fries, soups, chili, green salads and whole-grain bowls. Diced firm or extra-firm tofu can be used in stir-fries, soups, tacos, burritos and stuffed bell peppers. Soft tofu can be scrambled and seasoned with turmeric, cumin, red pepper flakes and sea salt.
Try tempeh too. This fermented soy food can be marinated and grilled, added to stir-fries or crumbled into soups, tacos, burritos and chilis.
If you don’t drink dairy, consider soy milk. Unlike most milk substitutes, each serving supplies 8 to 9 grams of protein, similar to cow’s milk. Choose a product that’s unsweetened and fortified with calcium.
Leslie Beck, a Toronto-based private practice dietitian, is director of food and nutrition at Medcan. Follow her on Twitter @LeslieBeckRD