Overview of the Meetings

In this report we will discuss implications for researchers of variability in estrogenic activity in animal feed and provide examples of a number of end points that are sensitive to estrogenic components in rodent diets. Importantly, we will report how research is impacted by variability in estrogenic components in soy-based and casein-based diets. We also describe control procedures to address the issue of batch-to-batch variability in animal diets. A primary concern expressed during these two workshops sponsored by the National Institutes of Health (NIH) was a need to increase awareness within the scientific and laboratory animal medicine community about the variability in estrogenic activity in diets associated with different components used in diets and the potential impact on a wide range of end points. The primary objective of the first workshop was to review the literature to assess whether dietary estrogens could significantly affect hormonal and other end points in experimental animals. Based on the consensus that there were significant effects, a second workshop was held to develop strategies to reduce the problems caused by batch-to-batch variation in estrogenic compounds in animal diets. Meeting participants agreed that it was critical to inform scientists, laboratory animal medicine personnel, animal caretakers, scientific societies, and funding agencies about these issues. Participants also developed a plan that provides a solution for confirming the level of estrogenic activity in each diet in order to address the problem of undefined batch-tobatch variability in estrogenic activity in diets. The following critical issues were discussed at the two workshops: • Even though the protein content of soycontaining animal diets is constant, the phytoestrogen content can vary 2to 5-fold between different batches of the same commercial rodent diet. Other estrogenic contaminants also vary from batch to batch. • Phytoestrogens in soy-based animal diets are biologically active and affect experimental results, including masking the effect of positive control estrogens. There are species and strain differences in the response to variation in dietary phytoestrogens. • Research findings support the need for laboratory animal diet manufacturers/vendors to confirm content and activity of estrogenic compounds in at least some of their rodent diets and make this information available to investigators. These assays should ideally be done on a batch-to-batch basis for both soycontaining and for soy-free diets so that researchers will be able to use batches with similar estrogenic content. • These findings also emphasize the importance of the diet in rodent studies, because uncontrolled variability in estrogenic activity in feed can interfere with replication of results within and between laboratories. There is thus a need to educate investigators about these issues. Findings presented at the workshops provided evidence that some batches of diets with no measurable isoflavones (and thus assumed by many scientists to be estrogen-free) contained other substances with estrogenic activity, such as zearalenone, a nonsteroidal estrogen produced by mold (Katzenellenbogen et al. 1979), and other unidentified estrogenic compounds. These findings provide a rationale for the recommendation to use bioassays to complement chemical analysis in assessing the total estrogenic activity of rodent diets.