Phytoestrogen content of purified, open- and closed-formula laboratory animal diets

Background and Purpose: Phytoestrogens exert estrogenic effects on the cent ral nervous system, induce estrus, and stimulate growth of the genital trac t of female animals. Over 300 plants and plant products, including some use d in laboratory animal diets, contain phytoestrogens. Therefore, the source and concentration of phytoestrogens in rodent diets were determined. Methods: Twelve rodent diets and six major dietary ingredients were assayed for phytoestrogens (daidzein, genistein, formononetin, biochanin A, and co umestrol), using high-performance liquid chromatography, Three rodent diets recently formulated to reduce phytoestrogen content also were assayed. Results: Formononetin, biochanin A, and coumestrol were not detected. Soybe an meal was the major source of daidzein and genistein; their concentration s were directly correlated to the percentage of soybean meal in each diet. Conclusions: High, variable concentrations of daidzein and genistein are pr esent in some rodent diets, and dietary phytoestrogens have the potential t o alter results of studies of estrogenicity, Careful attention should be gi ven to diet phytoestrogen content, and their concentration should be report ed. A standardized, open-formula diet in which estrogenic substances have b een reduced to levels that do not alter results of studies that are influen ced by exogenous estrogens is recommended.