Vegan animal diets: facts and myths

Friday, 20 March, 2015

Thousands of vegans have companion animals, or care for rescued cats or dogs. What a vegan should feed them, however, remains a contentious point of debate. Andrew Knight, a European veterinary specialist in animal welfare science, ethics and law, and a professor of Animal Welfare and Ethics at the University of Winchester, tells us more. 

When taken to their logical conclusions, common misperceptions about companion animal diets can lead us to some remarkably interesting places. We can, for example, extrapolate from such ‘common knowledge’ that a fearsome predator must stalk both the shallows and the ocean depths. Incredibly strong, this monster apparently swims 10-20 miles out to sea where it engages Bluefin tuna — who can weigh up to half a ton — in underwater battles to the death. Although it sometimes preys on smaller species such as salmon, whitebait and prawns, it seems to have a predilection for tuna, and has never been known to lose a battle. Disturbingly, it’s also incredibly ugly. Entirely covered with hair, which appears ghastly when wet, the water transforms this oddly hydrophobic beast into a hissing, spitting mass of claws. This terrifying apparition is, of course, a house cat.

Terrestrial predators

And in Africa, not even those on land are safe. Based on ‘common knowledge’ of feline diets it would also seem that burgeoning feral cat colonies there have reverted to their traditional ways, stalking and hunting large game, notably cows, sheep and pigs, and threatening endangered buffalo herds. These fearsome feline predators apparently like to wash down their kills with cows’ milk. Kittens apparently acquire the taste naturally because mother cats leave them with certain chosen, lactating cows for safe-keeping, whilst out hunting other game…

“Utterly ridiculous!” did I hear you scoff? Think again! Millions of cat and dog guardians worldwide consider it entirely natural to feed their feline and canine companions canned fish and prawns, body parts from cows, sheep, pigs, turkeys, ducks and chickens. It’s ‘common knowledge’ that cats and dogs naturally eat such animals, and that cats drink cows’ milk. Surely these millions of experienced pet guardians can’t all be wrong?

In a fascinating display of illogic, many of those same people consider vegan companion animal diets to be uniquely ‘unnatural’. Some do care about the plight of so-called ‘food’ animals, and the heavy environmental burdens inflicted by the livestock sector. But they worry that vegan diets might deprive their animal companions of essential nutrients. 

As a companion animal veterinarian with a particular interest in the health and nutritional issues surrounding vegan companion animal diets, I’ve trawled through the scientific literature for studies describing their adverse effects. Oddly perhaps, given the strength of ‘urban wisdom’ on this issue, I’ve struggled to locate any scientific evidence demonstrating that cats and dogs fed well-planned and nutritional vegan diets are less healthy than the norm. Yet, I have found evidence of one kind. I’ve accidentally located more than ten published studies documenting hazardous ingredients in commercial meat-based diets, or adverse health effects in cats and dogs maintained on them. Interested readers will find them summarized here. 

Meat-based diets: some ugly truths

Diseases demonstrated to be more likely following long-term maintenance of cats and dogs on some commercial meat-based diets include kidney, liver, heart, thyroid, neurologic, neuromuscular, skin, and infectious diseases, and bleeding disorders.

Additionally, after examining and treating many thousands of animals for around a decade, I’ve become convinced that rates of diseases such as cancer, kidney and liver disease are far higher than would occur naturally. These have been particularly common in my elderly patients, when they may eventually result in severe illness, and sadly, euthanasia.

If these animals were exposed over many years to toxins not severe enough to cause acute reactions, but sufficient to cause cellular damage, these are exactly the sort of effects one might expect to see. And particularly, if such toxins were included within the diet. The first major organ exposed to such toxins after their ingestion and intestinal absorption would be the liver, which has the responsibility for converting them into molecules suitable for excretion via the bloodstream. And the kidneys are most responsible for such excretion. Hence both liver and kidney cells would be highly exposed to any such toxins, and at risk of damage. But toxins carried by the bloodstream could also damage cells throughout the body, and long-term cellular damage can predispose to cancer. 

But are there toxins in common animal diets? My research revealed that, particularly when imported from regions such as the US, with weaker regulations, commercial pet foods constitute a vast industrial dumping ground for slaughterhouse waste products, ‘4-D’ meat (from dead, dying, diseased or disabled animals), old or spoiled supermarket meat, large numbers of rendered dogs and cats from animal shelters, old restaurant grease, complete with high concentrations of hazardous free radicals and trans fatty acids, and damaged or spoiled fish, complete with potentially dangerous levels of mercury, PCBs and other toxins. The combined results are rendered irresistible to many cats and dogs by the addition of ‘digest’ — a soup of partially dissolved intestines, livers, lungs and miscellaneous viscera of chickens and other animals.

Vegan companion animal diets

Unsurprisingly, therefore, numerous cases indicate that transitioning animals to healthy vegan diets can result in increased overall health and vitality, decreased incidences of cancer, infections, hypothyroidism (a hormonal disease), ectoparasites (fleas, ticks, lice and mites), improved coat condition, allergy control, weight control, decreased arthritis, diabetes regression and even cataract resolution.

Additionally, there are a limited number of more rigorous studies examining the health of populations of cats and dogs maintained on vegetarian or vegan diets, long-term. Wakefield and colleagues (2006) compared the health of 34 cats maintained on vegetarian diets for at least a year, with that of 52 cats maintained on meat-based diets for at least a year. There were no significant differences in age, sex, body condition, housing, or perceived health status, with most cats described as healthy or generally healthy.

Brown and colleagues (2009) studied 12 sprint-racing Siberian huskies who were fed either a commercial diet recommended for active dogs (n=6), or a meat-free diet formulated to the same nutrient specifications (n=6). These diets comprised their sole nutrient intake for 16 weeks, which included 10 weeks of competitive racing. Regular veterinary checks and blood tests were performed. All dogs remained in excellent physical condition with normal blood results throughout.

These results are hardly surprising, when we consider that animals need specific nutrients, not ingredients. There is no scientific reason why a diet comprised only of plant, mineral and synthetically-based ingredients cannot be formulated to meet all of the palatability, nutritional and bioavailability needs of the species for which it is intended. In fact, several commercially-available vegan diets for cats and dogs aim to do so, and have jointly supported thousands of healthy vegan cats, dogs and ferrets (who are also naturally carnivorous) for many years. Suppliers of such diets are listed here. 

However, use of a nutritionally complete and reasonably balanced commercial diet, or a nutritional supplement added to a home-made diet, is essential to avoid nutritional deficiency, and eventually, subsequent disease. Dietary transitions should occur gradually, and I also advise regularly checking urine acidity using pH test strips (from veterinarians, or easy to locate online), or even more accurate pH meters. Vegan diets can result in more alkaline urine, which can result in urinary stones and serious blockages in a small proportion of animals, especially male cats. Advice about urinary monitoring, and dietary additives that can correct urinary alkalinisation if necessary, should all be taken seriously, and is provided here.

For more information on transitioning your companion animals to a vegan-friendly diet, see here. Remember, cats, dogs and ferrets need specific nutrients – not specific ingredients.

By Dr. Andrew Knight


Brown WY et al. An experimental meat-free diet maintained haematological characteristics in sprint-racing sled dogs. Brit J Nutr 2009; 102: 1318–1323.

Wakefield LA, Shofer FS, MIchel KE. Evaluation of cats fed vegetarian diets and attitudes of their caregivers. J Amer Vet Med Assoc 2006; 229(1): 70-73.


With grateful thanks to Lifescape magazine for portions excerpted from the author’s article in the May 2008 edition.


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