“The findings of this important study “support soy isoflavones as an alternative therapy” and “the consumption of soy foods,” since they provide all the benefits for menopause without the risks of estrogen and while being “associated with a reduction in the risk of estrogenic-related cancers.”

Despite all the evidence, women have been cautioned about getting too excited about soy because of the risk that soy isoflavones, being phytoestrogens, could have an unwanted effect on breast cancer. Women are constantly warned away from soy despite all the research that shows soy is safe, and even beneficial, for women with breast cancer. A 2023 systematic review and meta-analysis found that women who have the most isoflavones in their diet have a 29% lower risk of breast cancer. Every 10 mg of isoflavones consumed a day reduced the risk of breast cancer by 11.7% (Nutrients 2023,15(10),2402).

The misinformation stems from assumptions about soy isoflavones acting like estrogen and from over reliance on animal studies that can produce different results from human studies because rodents metabolize soy isoflavones differently. Though rich in isoflavones that really do help menopause, soy isoflavones behave differently than estrogen. Estrogen binds to both ERα and ERβ estrogen receptors; soy isoflavones bind preferentially to ERβ receptors. That difference is important because ERα exerts proliferative effects, while ERβ exerts antiproliferative effects. That means that soy isoflavones are what is known as selective estrogen receptor modulators (SERMs), explaining their benefit on menopause without the negative impact of estrogen and hormone replacement therapy.

A new landmark systematic review and meta-analysis has established the safety of soy by looking at 52 controlled studies of 3,285 postmenopausal women. The median duration of the studies was from 13-24 weeks. The median dose of isoflavones was 66-77mg a day. The results showed that soy isoflavones had no statistically significant effect on any measure of estrogenicity, including endometrial thickness, vaginal maturation index, FSH and estradiol. All of these measures are affected by estrogen, demonstrating that soy isoflavones exert their benefits without estrogenic effect.

All of this suggests that soy isoflavones act as SERMs that have tissue specific effects that, remarkably, allow it to have all of the positive estrogen-like effects without the negative ones. Soy isoflavones, then, really do have benefit for menopause, osteoporosis, cardiovascular disease and memory in postmenopausal women, and really don’t introduce risk for breast, or endometrial, cancer. The findings of this important study “support soy isoflavones as an alternative therapy” and “the consumption of soy foods,” since they provide all the benefits for menopause without the risks of estrogen and while being “associated with a reduction in the risk of estrogenic-related cancers.”

Advances in Nutrition. January 2025;16(1):100327.