A number of compounds shown in Figure 14.12 that occur in plant foods as glycosides and other conjugates have weak estrogenic/antiestrogenic actions, and are collectively known as phytoestrogens. They all have two hydroxyl groups that are the same distance apart as the hydroxyl groups of estradiol and can bind to estrogen receptors. The amounts of phytoestrogens produced increase in response to microbial and insect attack, suggesting that they have antibacterial or antifungal actions in the plant. They produce typical estrogen responses in animals, with a biological activity 1/500 to 1/1,000 of that of estradiol.
High consumption of legumes, especially soya beans, which are rich sources of phytoestrogens, is associated with lower incidence of breast and uterine cancer, as well as a lower incidence of osteoporosis. The estrogenic action is probably responsible for the effects on the development of osteoporosis, whereas three factors may be involved in the effect on hormone-dependent cancer:
1. The isoflavones are mainly antiestrogenic, because they compete with estradiol for receptor binding, but the phytoestrogen-receptor complex does not undergo normal activation, thus it has only a weak effect on hormone response elements on DNA. Even those phytoestrogens that have a mainly estrogenic action will reduce responsiveness to estradiol because they compete for receptor binding but have lower biological activity.
2. The phytoestrogens increase the synthesis of sex hormone binding globulin in the liver by stabilizing mRNA, leading to a lower circulating concentration of free estradiol.
3. Some of the phytoestrogens inhibit aromatase and therefore reduce the endogenous synthesis of estradiol, especially the unregulated synthesis that occurs in adipose tissue.
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