As the health of dog and cats depends on the optimal levels of each nutrients in commercially prepared foods are formulated. Soy is added to commercial pet foods as a protein source. Antioxidant, anticarcinogenic, estrogenic/antiestrogenic, antilipidemic effects of phytoestrogens in soybean have been investigated with in vitro and in vivo animal and human studies. However, there are naturally occuring phtoestrogens, such as genistein in soy bean exert estrogen-like activity, exposure to high levels for long periods of time may result in significant endocrine disruption.