The phytoestrogen content of rodent diets.

The commentary by Boettger-Tong et al. (1) described the authors' experience with a commercially available, unidentified rodent diet with high estrogenic activity and the impact of this diet on the normal in vivo uterotropic response of ovariec-tomized 30-day-old Sprague-Dawley rats to exogenously administered estradiol. The purpose of this letter is to corroborate the authors' findings; to report the phytoestro-gen content of the rodent diets most frequently used in estrogenicity, toxicity, and carcinogenicity studies; and to recommend that careful consideration be given to the selection of a standardized open-formula diet, ideally containing minimal or nonde-tectable levels of estrogenic substances, to be used in studies that are influenced by exogenous estrogenic substances. The authors reported that the lot of feed in question contained very high amounts of two well-known phytoestrogens, genistein (210 mg/kg diet) and daidzein (140 mg/kg), which induced a near maximal uterotropic response in control rats prior to hormone treatment. Boettger-Tong et al. (1) stated that the exact source of the phytoestrogen contamination in the rodent diet remains an open question. In our initial study, we reported that soybean meal was the major source of daidzein and genistein in all six natural ingredient rodent diets assayed and that their concentration was directly correlated with the soybean meal content in the diets (2). Three of the six natural ingredient diets assayed contained high total daidzein and genistein concentrations. Rodent Laboratory Chow #5001