Soy intake in Asian countries is about five times higher than in the U.S.
Part of the reason for this is because of culture.
Japanese monks don’t sit in their temples chomping down hamburgers; and cowboys in Texas don’t sit around the campfire eating tofu.
But a primary reason for the low consumption in the U.S. is public concern about the “estrogenicity” of soy foods.
Estrogenicity refers to the estrogen-like effects that soy is purported to have on the human body.
Soy contains high concentration of isoflavones, the polyphenolic compounds found in all legumes. Because some studies have demonstrated that these isoflavones can mimic estrogen, they have been classified as phytoestrogens.
This is where the controversy starts—for decades there has been disagreement on whether the evidence of adverse effects is substantiated or exaggerated.
The adverse effects, if true, would be of far greater consequence than the old playground teasing about men turning into “soy boys.” The effect would extend to hormone-dependent cancers, thyroid function, cognitive health, or even the entire endocrine system. Forget the old joke of men growing breasts—most experts agree the implications, if true, would be worse for women than men.
Because of this estrogen concern consumption of soy foods is extremely low in Canada and the U.S. Only 3%–4% of North American populations report consumption of a soy-containing product on any given day.
However, according to the authors of a new study, although isoflavones are commonly classified as phytoestrogens, they differ from the hormone estrogen at both the molecular and clinical levels.
The researchers noted that much of the evidence of adverse effects results from animal studies, which have limited implications for human comparisons. While rodent studies are usually helpful for preliminary research, in this case there are substantial differences in how mice and humans metabolize soy.
Conversely, in almost all the human trials soy is shown to provide a positive effect in the prevention of cardiovascular disease, some cancers, bone density deterioration and menopause symptoms.
The study was published in Advances in Nutrition in January 2025.
Study details
To conduct the study, researchers conducted a systematic review and meta-analysis of 40 previous trials. In totality the trials involved 3,285 women.
To gauge how soy affected the women, the researchers tracked four measures of estrogenicity: Endometrial thickness, vaginal maturation index, follicle-stimulating hormone and estradiol. The researchers chose these particular measures because they are known to be affected by the hormone estrogen and were evaluated in many trials involving isoflavones.
The included trials assessed the effect of a median reported dose of 75 mg/d of soy isoflavones compared to a placebo over a median intervention period of 13 to 24 weeks.
Following data synthesis and analysis, the study concluded: “Soy isoflavones had no statistically significant effect on any measure of estrogenicity.”
Receptor binding
One primary key to understanding the findings is that soy isoflavones bind to different receptors than estrogen—causing substantially different physiological effects in the body.
For example, isoflavones preferentially bind mostly to estrogen receptor (ER)ß in comparison to ERa; meanwhile estrogen binds equally to both receptors.
According to the researchers, while this may seem like a small and inconsequential action of biology these receptors actually have different tissue distributions—and when bound by ligands result in different and sometimes opposite physiological effects.
In general, activation of ERa is seen as exerting proliferative effects (producing new tissue, cells, etc.); meanwhile ERß produces antiproliferative effects.
The researchers also noted that the results of the current study were consistent with an earlier 2016 systematic review and meta-analysis. In the earlier study researchers also found no change in endometrial thickness after supplementing soy isoflavones to both premenopausal and postmenopausal women.
While the authors of the study spent some detail on how the findings are important for consumers “seeking high-quality plant protein foods with low environmental impact and cost,” the key conclusion is not really a plant-based versus carnivore discussion.
Anyone who has visited a health food store knows soy-based supplements are purchased by both meat-eaters and vegetarians alike. Many demographics—from body builders at the gym to elderly women trying to maintain bone density—benefit from soy protein. And this study helps prove they can receive the soy health benefits without the estrogenicity concern.
Final word
“Despite their common classification as phytoestrogens, the results of this analysis provide a strong rationale for not assuming that soy isoflavones will exert health effects similar to the hormone estrogen,” the researchers wrote in the study summary.
Soy protein is a key ingredient in the following Optimal Health Systems products:
• Optimal Complete Performance – Exotic Chocolate
• Optimal Complete Performance – Vanilla
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Sources: Advances in Nutrition, JustAPedia (phytoestrogen).
