Soy is one of the most significant sources of isoflavones in our diet. Isoflavones are phytoestrogens (plant estrogens) that belong to a family of phytochemicals called polyphenols. Many of the health benefits of soy (other than its macro and micronutrient contributions), are due to these plant estrogens. The plant estrogens seem to have a positive effect on postmenopausal women by enhancing heart health and brain function. .
However, these plant estrogens have garnered much controversy. A commonly held belief is that isoflavones are beneficial for those who need estrogen (e.g. menopausal women), but detrimental for those who don’t (e.g. women with certain types of breast cancer and men who believe they can be “feminized” by estrogen).
This is not entirely true because isoflavones are not the same as human estrogen.
Isoflavones only have weak estrogenic effects in postmenopausal women. On the other hand, isoflavones appear to act as an anti-estrogen where breast cancer risk is concerned. That’s good news.
To sum up, while the effects of soy tend to be favorable for women, men are frequently concerned that soy may reduce testosterone levels, and generally cause a feminizing effect. Only two case studies have appeared in the literature regarding feminization of men ingesting soy.
In both cases, the men regularly consumed 14 to 20 servings of soy daily – one derived almost all of his calories from soy – and subsequently developed health problems, such as enlarged breast tissue and loss of libido. In both cases, when soy intake was reduced, their health and libido returned to normal.