Many people claim that soy is estrogenic and feminizing because it contains phytoestrogens known as isoflavones. These claims primarily stem from some rodent studies and a couple rare human cases where individuals consumed in the ballpark of 15 to 20 servings of soy per day and reported gynecomastia (breast growth in men).[1] That’s hardly sufficient evidence to support the idea that soy, in reasonable amounts, is feminizing.
Fortunately, researchers have performed multiple meta-analyses where they looked at all the research on the topic of soy and isoflavones and found no link between soy consumption and feminization, low testosterone, or high estrogen levels in men.[2][3] Similarly, a 2009 meta-analysis found that soy had no effect on estrogen levels in women.[4]
This is because the phytoestrogens in soy are 1,000 times weaker and preferentially bind to different receptors in our bodies than true human estrogens do.[5][6] This allows soy to actually have anti-estrogenic effects in tissues where we don’t want excess estrogen (e.g., breast tissue), which may also be why soy products have been shown to improve menopausal symptoms.[7]
Unlike soy, however, dairy contains mammalian estrogens (even if no hormones are added), which has the same effect as the estrogen produced by our own bodies, and is therefore pro-estrogenic.[8] So if you’re worried about the hormonal effects of your food choices, it may be worth ditching dairy for a healthy plant-milk such as soy.