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Soy hormone fears came from extremes
Estrogen-like molecules
Soy contains molecules with a structure similar to estrogen, which raised worries about feminizing boys and causing problems with hormone levels in young women. As scientists looked deeper, soy only seemed to have that effect when someone consumed truly unusually large amounts of soy milk or tofu.
The extreme cases
One case study involved a person who consumed about eight times the average in Asia, which is already high, drinking multiple bottles of soy milk and eating multiple servings of tofu per day. Cases like that are very rare and extreme, and the evidence that soy affects hormone levels is pretty weak.
Weaker than human estrogen
Soy does have plant estrogens, but they are a lot weaker than human estrogen. They may do enough to help with some menopause symptoms.
Why does soy resemble estrogen?
Soy isoflavones such as genistein and daidzein have a ring structure close to that of the hormone estradiol. That shape lets them attach to estrogen receptors, especially the beta type. They bind far more weakly than estradiol does, and they switch the receptor on only weakly. Binding studies show that genistein attaches to estrogen receptor beta with a small fraction of estradiol's affinity.
What do studies on soy and men's hormones show?
Researchers pool the results of many small trials in meta-analyses to see if a pattern holds. A 2010 meta-analysis by Hamilton-Reeves and colleagues, published in Fertility and Sterility, covered dozens of studies of soy foods and isoflavones. It found no effect on testosterone or estrogen levels in men.
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