Abstract
The classical phytoestrogens, so far known, constitute a group of plant-derived compounds which include mainly isoflavones, lignans, coumestanes, stilbenes and the flavonoids quercetin and kaempherol. The discovery of many more novel estrogen-like compounds in the plant kingdom demonstrates that the spectrum of phytoestrogens in nature is expanding. The classical as well as the novel phytoestrogens show a complex mode of action via interaction with the nuclear estrogen receptor isoforms ERα and ERβ, exhibiting either estrogen-agonist or estrogen-antagonist effects. Their final biological activity, assessed by cell culture assay systems, animal studies and clinical trials, depends on multiple factors such as the chemical structure of the phytoestrogen, the kind of tissue and cell type, the intrinsic estrogenic status, the route of administration, the metabolism as well as the time and the level of exposure. They are characterized by high tissue specificity and dose-dependent activity. However, although phytoestrogen intake as food or dietary supplements, in particular soya products and the iso-flavones genistein and daidzein, has been associated with "health promoting effects", some data indicate increased disease risk. Evidently, phytoestrogen supplementation should be viewed with caution until further studies satisfactorily delineate the effects of individual phytoestrogens on human health and disease.
Estrogens are steroid hormones with a complex mode of action, characterized by high tissue specificity and dose-dependent activity. They exert pleiotropic effects on a diverse range of tissues, such as ovary, testis, prostate, breast, uterus, bone, liver, immune system, cardiovascular and central nervous system.1 Estrogens promote breast and endometrial cancer in women and exacerbate autoimmune diseases, whereas the loss of estrogens during menopause has been correlated with osteoporosis, coronary heart disease, depression and neurodegeneration. Compounds which antagonize the estrogenic effects (antagonists) in some tissues, such as breast and uterus, while mimicking the estrogens effects (agonists) in other tissues, such as bone, brain and cardiovascular cells, are known as selective estrogen receptor modulators (SERMs).2