A consistent drive for high production rates has led to increases in animal stocking densities, genetic selection for maximal productivity, and invasive husbandry. This has morphed commercial farm animal practices into an industry often criticized for having serious animal welfare and environmental issues surrounding it. Nowadays, such facilities provide animals and their by-products as food items for humans and companion animals alike. Surprisingly, with 95 million pet cats and 83 million pet dogs in the U.S. alone, cats and dogs combined outnumber children almost by a factor of two.

More and more people living with a companion animal show an interest in vegetarian diets for their animals. This can be due to increasing concerns about the health, animal welfare, and the environment related consequences of animal farming. In this study, researchers from the U.K. reviewed evidence published from four other studies that have examined the nutritional adequacy of vegetarian diets for cats and dogs. The researchers also surveyed 12 “pet food” companies. Finally, they reviewed evidence concerning the health of long-term vegetarian, carnivorous, and omnivorous companion animals.

While reviewing the health status of companion animals on meat-based foods, the researchers found several worrying problems with easily available commercial foods. For example, they found potentially pathogenic microorganisms to have contaminated such foods. And some of these microorganisms may also lead to disease among the human guardians. Such contamination might originate in the widespread use of slaughterhouse products condemned as unfit for human consumption. This includes “4-D” meat (from animals that are disabled, diseased, dying, or dead on arrival at slaughterhouses). Such ingredients are often labelled as “meat derivatives” or “by-products.” This may shock loving companion animal guardians.

Previous studies describe how—due to expensive labor costs—staff do not always remove plastic ear tags from farmed animals. And companion food manufacturers may use old or spoiled supermarket meat without removing its foam packaging. Finally, similar to humans, companion animals have not evolved to excrete modern oceanic pollutants present in fish and other aquatic animals. These can include mercury and plastics, which accumulate in tissues. And with time, they can reach high enough level to impair health. Given these potentially hazardous ingredients present in commercial meat-based foods, it is not surprising that many controlled studies demonstrate increased risks of several diseases for cats and dogs who are fed on such diets long term.

In 1987, P.D. Pion and colleagues showed that supplementing food with taurine reversed heart muscle failures in cats. And since then, most meat-based and vegetarian companion animal food manufacturers alike supplement their companion food products with synthetic taurine. This review in fact reveals that a growing body of population studies suggest that both cats and dogs may thrive on vegetarian diets, if they are nutritionally complete and well balanced. In the case of cats, the researchers advise to regularly monitor urinary acidity. They point out that in some cases correcting urinary alkalinization through appropriate dietary additives might be necessary.

That being said, the authors do not give veg companion animal diets a full recommendation. Instead, they note cautiously that “those interested in vegetarian companion animal diets should be aware of concerns about the nutritional adequacy of some such diets demonstrated by a number of studies over a significant number of years.” In addition to this point, however, they note that consumers “should also be aware that similar concerns exist about commercial meat-based diets.”

Selective Naturalism

A common concern about vegetarian companion animal diets is that they are “unnatural.” Despite the vagueness of the definition, many people often presume that domesticated cats and dogs should eat meat, because their wild ancestors would have consumed prey animals. Meanwhile, when wild cats, dogs, or wolves kill prey, they eat as much as possible at once due to intense competition. This is usually followed by times of hunger.

In contrast, this study highlights that we are feeding domesticated cats and dogs assorted body parts. These body parts are typically from animals their ancestors would never have eaten. And we conserve the food in tins or packets. Then we serve it to our companion animals at predictable times daily. Sometimes, we make dry food available all the time. This bears little resemblance to the natural feeding behavior of their ancestors. Plus, we also commonly give cows’ milk to cats, even though some are lactose intolerant as adults— not unlike some humans.

To encourage domesticated cats and dogs to eat such diets, dry food may be sprayed with a combination of refined animal fat, lard, restaurant grease, and other oils that we sometimes consider too rancid for human consumption. Alternatively, producers sometimes choose to add “digest.” This is a processed soup of sorts made of partially-dissolved intestines, livers, lungs, and miscellaneous viscera of chickens and other animals.

Clearly, modern human guardians often create conditions far from what’s “natural” to protect the health and welfare of their companion animals. Accordingly, the researchers suggest that the resistance to the concept of vegetarian companion animal diets likely stems from lacking knowledge about the hazardous ingredients found within meat-based diets instead of linking health risks with vegetarian complete foods. The researchers urge readers to remember that dogs and cats are just like us; we all require specific nutrients, rather than specific ingredients, regardless of whether we are omnivorous or vegetarian.

Regardless of whether guardians choose veg or meat-based diets, the researchers encourage companion animal guardians to check the labeling claims of nutritional adequacy of any commercial diet. They also encourage us to ask manufacturers what steps they take, and what evidence they can provide, to ensure both nutritional adequacy and consistency in their foods. Animal advocates may use this review as a potential green light for promoting vegetarian companion animal diets. And in this way, it will help to improve the welfare of both farm animals and companion animals.