Phytoestrogens mainly fall into the class of flavonoids: the coumestans, prenylated flavonoids and isoflavones are three of the most potent in this class. The best-researched are isoflavones, which are commonly found in soy and red clover. The uses for these isoflavones are just like that of soy, simply because isoflavones are found in soy. However, so are many other compounds so its not easy to say what exactly isoflavones account for.
Lignan has also been identified as a phytoestrogen, although it is not a flavonoid. The estrogenic properties of these biochemicals have been shown to be due to their structural similarities to the hormone estradiol. Mycoestrogens produced by fungi have similar structures and effects.
Some evidence suggests that phytoestrogens can help "balance" hormone levels throughout the body since oestrogen exists within them in trace amounts. Hormone balancing can improve such things as mood. Imbalanced hormone levels (for example those seen at puberty) can cause such things as acne.
According to a study by Canadian researchers, flax seed contains the highest total phytoestrogen content followed by soy bean and tofu. Isoflavones are found in high concentration in soy bean and soy bean products (eg. tofu) whereas lignans are mainly found in flax seed.