One of the major ingredients in many pet foods is soybeans and soybean related products.  These soybean foods also contain phytoestrogens, which are non-steroidal compounds with estrogenic activity.  These phytoestrogens are reported to have both positive and negative effects on the health of humans and other animals.  A study was recently performed at the University of Pennsylvania to determine the amount of phytoestrogens available in 24 random commercial dog foods.  Some of the products listed soybean meal while other listed soybean oil and soybean fiber.  Of the 12 foods that did not contain any soy ingredients, only one had any phytoestrogens.  All of the foods that listed soy ingredients had some concentration of phytoestrogens.  The foods that listed soy fiber and soy oil had the lowest concentration of these chemicals.  The amount of these phytoestrogens was present in significant quantities so as to have a potential biological effect on the pet. 

The problem is we do not know to what extent these products are absorbed from the GI tract.  Also, if these phytoestrogens are absorbed, is that good or bad?  The answer is we don’t know.  These chemicals naturally produced in soybeans have been shown in some cases to inhibit the growth of cancer cells.  On the other hand, these chemicals have also been reported to inhibit normal sex hormone production.  An example is that sheep grazing on a type of clover containing these compounds were infertile.  Delayed puberty and immune deficiencies have also been reported in humans due to these natural compounds.  Whether or not these soybean products affect our pets and how will require further clinical studies.