Add on to the list above
Plant estrogens are scientifically called Phytoestrogens
Although they have similar actions as estrogen, they are not the same as the estrogen in our bodies. They can bind to receptor sites and have either pro-estrogenic effects or anti-estrogenic effects depending on how much estrogen you are already producing. If your estrogen level is low, empty receptor sites can be filled with Phytoestrogens. If your estrogen levels are high, the Phytoestrogens can compete with your own estrogens for binding to receptors. If the Phytoestrogens are successful they will decrease overall estrogenic activity.
You can do a search on yahoo for more info. I read about this a while back some article. Ican't remeber the site though