Phytoestrogens represent polyphenolic and non-steroid compounds that have a similar structure and biological activity as human estrogens. They are the plant secondary metabolites and widespread in over 300 various plant species. The phytoestrogens are divided into two main subgroups, isoflavonoids and lignans. The isoflavonoids are divided into isoflavones and coumestans. Isoflavones are most present in soybean and red clover. A great number of isoflavones has been identified in the plants in the form of non-active glycosides (daidzein and genistein) and in the form of 4'-methylated derivatives (formononetin and biochanin A). Coumestans can also be found in red clover and legumes, while lignans are most present in the flax seed. Coumestans have the most pronounced estrogenic effect of all phytoestrogens. The most famous representative of this subgroup of compounds is coumestrol. Lignans include matairesinol, secoisolariciresinol, lariciresinol, pinoresinol and their metabolites, enterodiol, enterolactone and equol. Numerous studies in animals and humans have shown that phytoestrogens can have protective effects in the estrogen-dependent conditions and estrogen-dependent diseases. It was also shown that the use of phytoestrogens can have a positive effect on insomnia and cognitive functions, i.e. neural disorders, such as Alzheimer's disease. They can express the antioxidant activity in two ways, as the scavengers of free radicals or by the formation of chelating complexes with metal ions. Free metal ions increase the amount of reactive oxygen species by reduction of hydrogen peroxide resulting in a very reactive hydroxyl radical.
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