Phytoestrogens are substances found in plants (phyto-) that have estrogen-like properties. Phytoesterogens latch onto the receptor sites on cells meant for estrogen. This results in a partial stimulation of those receptors, less intense than if true estrogen had attached, but stronger than if nothing had stimulated those receptors at all. This leads to a dual result. In women who have considerable estrogen, high consumption of phytoestrogens tends to occupy the receptor sites and keep real estrogen away. The net effect is to reduce the overall action of estrogen. Since estrogen promotes a number of forms of cancer (such as breast cancer), this may reduce cancer risk. However, in women who have little estrogen (after menopause, for example), consumption of large amounts of phytoestrogens will tend to mimic the effect of the missing estrogen, reducing symptoms such as hot flashes.
To see that this is not just an academic concern, consider the mistakes made regarding estrogen. Numerous observational studies indicated that treatment with estrogen as part of hormone replacement therapy could dramatically reduce risk of heart disease. However, when double-blind studies were performed, it became clear that hormone replacement therapy actually increases heart disease risk.
Thus, all conclusions about phytoestrogens based on observational studies are suspect. Unfortunately, it is not possible to perform double-blind studies on food. For this reason, the only solid information we have regards specific phytoestrogens that can be controlled in a double-blind format. To learn more, see the following articles: