Q. Is it safe for children to drink soy milk?
A. Soy and soy milk do contain molecules that interact with estrogen receptors and therefore have weak, estrogen-like effects. However, because these effects are weak, the molecules may actually act like anti-estrogens by competing with the body's natural estrogens when estrogen levels are high. For this reason, soy products have been hypothesized to reduce the risk of breast cancer. The evidence isn't conclusive, but there's some suggestion that soy consumption during childhood may reduce risk of breast cancer later in life. Soy milk or other soy products may also reduce risk of prostate cancer, but again nothing conclusive — and we don't know about the effects of consumption during childhood on prostate cancer risk.
Regular cow's milk contains many hormones, including estrogens, and we really don't understand their long-term effects.
So there's a lot to be learned. But there's also the reality that for centuries in some civilizations, people have been consuming large amounts of soy products regularly throughout life without apparent adverse effects. I don't think children need to avoid soy milk. Still, where there's uncertainty, moderation is a good policy, so limiting children to drinking one or two glasses of soy milk a day makes sense.
— Walter C. Willett, M.D., Dr.P.H.
Harvard School of Public Health