Phytoestrogens act as estrogen agonists by occupying estrogen receptor sites when natural estrogens are unavailable. For example, the body's natural estrogens inevitably decline with the onset of menopause and phytoestrogens may help to offset this decline if they can fill receptor sites instead. Once docked on the sites they exert estrogen-like activity and can initiate the required effect as natural estrogen would have done.