Another synthetic compound, bisphenol A, shares some structural features of DES, in that it has two phenolic groups, but in this case only a single dimethylated carbon separates the two rings, rather than the substituted double bond in DES. As might be expected, the reduced distance between the phenolic hydroxyls and the greater flexibility in the structure lead to an affinity for the ERs several orders of magnitude less than estradiol or DES.40 Although bisphenol A is only weakly estrogenic, it is still considered to be an "environmental estrogen" and is present in small quantities in a wide range of plastic products. The low-level exposure of humans to bisphenol A, although currently considered to have minimal health risks, continues to be the focus of numerous studies.41
Phytoestrogens. Several natural plant substances that have general structural features similar to those of DES and estradiol also have estrogenic effects and have been termed phytoestrogens.42 These include genistein, from soybeans and a species of clover; daidzein, from soybeans; and coumestrol, found in certain legumes. Genistein and daidzein are examples of isoflavones. These and others have antifertility activity in animals.43 Many claims have been made about the beneficial effects of consuming products containing these phytoestrogens, including preventing and treating cardiovascular disease, reducing post-menopausal symptoms, and preventing osteoporosis. Because of the numerous other components present in many of the commercial products as well as a lack of well-
designed studies to test the effects of the phytoestrogens themselves, however, positive health effects specifically because of direct hormonal action of the phytoestrogens are uncertain. Questions have also been raised about a possible contribution of phytoestrogens to an increased incidence of breast cancer. These concerns, however, are contradicted by studies that suggest a chemoprotective role for soy products containing phytoestrogens (this could well be due to other components of the mixture).44 The long history of use of soy products in the world and no global correlations with increased breast cancer risks suggest that any connection between phytoestrogens in general and breast cancer is quite small.45
Recent studies have demonstrated that genistein and other phytoestrogens binds preferentially to ER^ over ERa.46,47 The clinical relevance of this difference in binding is unclear, but further research is being vigorously pursued to understand the distinctions between the phytoestrogens and classical estrogens.44
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