Critics of soy often vilify its estrogen content. At the ridiculous end of the spectrum, some mainstream men’s fitness magazines consider it a “feminizing” food, while—at the more evidence-based side of the spectrum—women with a history of breast cancer are advised to limit their intake.
Strangely, we hear very little about high levels of estrogen in commercial dairy products. Unlike plant-based soy estrogens, the estrogen in cows’ milk is estrone sulfate, a hormone that has been linked to increased risks of breast cancer.
Some research suggests one possible factor in the rise of hormone-related cancers may be that “modern dairy cows are usually pregnant and continue to lactate during the latter half of pregnancy, when the concentration of estrogens in blood, and hence in milk, increases.”
That same study—which correlated incidence rates for breast, ovarian, and corpus uteri cancers with food intake in 40 countries—concludes that:
Milk was most closely correlated with incidence[s] of ovarian cancer”, likely due to elevated levels of estrogen and progesterone. Meat was most closely correlated with breast cancer incidence, followed by milk and cheese.
Men are also affected. As this study points out, “estrogen levels in prostate fluid are also correlated very well with…prostate cancer.”
While data that encompasses all soy estrogens and prostate cancer risk appears inconclusive, some research theorizes that a specific estrogen called deidzein may offer a degree of protection from prostate cancer.
If estrogen in foods is a concern, commercial dairy products merit greater concern than soy.