Soy is a common, but controversial, component of many commercial animal foods. Advocates of the legume argue that it offers health benefits, while detractors claim that soy has been linked to health problems. Unfortunately, the long-term effects of soy have not been studied for most animals, and most claims have been extrapolated from human studies. While it is difficult to make broad generalizations that apply to all animals, soy is unlikely to cause acute illness in animals, unless an animal is allergic to it.
Soy for Poultry
According to Oregon State University, soy has been a significant component of poultry feeds for many years. Because soy is such a valuable component of feeds, and few, better alternatives exist, it is likely that soy will continue to be a component of poultry feeds until and unless a better protein source is devised. While alternatives to soy exist, such as legumes and cereals, these may cause other health problems.
Soy for Rodents
Rodents have been the subject of numerous studies of soy-based diets, and in some cases, soy appears to be harmful. In laboratory studies, some mice injected with estrogen-dependent, tumor-causing cells experienced increased tumor growth when fed soy products. Other studies have shown that female mice suffer reduced fertility when fed a soy-rich diet, while male rodents often develop breasts.
Soy for Dogs
Soy is a common component of dog foods. The VCA Animal Hospital states that soy has been shown to have a protective effect on the renal system. Some research suggests that soy may help altered dogs to avoid gaining weight. While rigorous studies have yet to be conducted, the VCA Animal Hospital states that anecdotal evidence shows that soy products also may mitigate the symptoms caused by an enlarged prostate.
Soy for Cats
Bolton Veterinary Hospital encourages feeding cats a biologically appropriate diet, based on animal proteins. While not stating that soy is harmful to cats, the hospital recommends avoiding cat foods that contain these and other plant-based proteins. If present, soy should be one of the last ingredients listed on the food package. For cats, the less soy in the food, the better.
Regardless of the potential health problems or benefits that arise from soy consumption, many animals -- especially dogs -- are allergic to soy. If your dog develops itchy skin, gastrointestinal upset or breathing difficulties, consult your veterinarian, as these may be signs of an allergy. Your veterinarian likely will recommend placing the dog on an elimination diet to determine which item is causing the allergy. It is important to note that dogs can develop food allergies at any time -- just because your dog has never been allergic to soy does not mean he will not develop an allergy to it at some point.