The zoologist Jordi Casamitjana looks at the latest evidence regarding domestic cats to find out if they are really obligate carnivores who cannot survive with a diet of only vegan food.
Those who live with cats will recognize this: You bought some new food you discovered for them, you pour it into their bowls, they come over, they sniff it for a while, and move away leaving it untouched. One of the things that cat lovers like about cats is how “independent-minded” they are. They make their own decisions, they go wherever they want to, and they only eat what they like. They are “fussy” eaters. This is why many cats’ guardians stick to the food they know the cat likes and avoid experimenting with novelty.
However, what happens when people become vegan, and they are responsible for feeding non-human companion animals they live with? If they are ethical vegans who know animals have been used, abused and killed to produce the food they normally give to their companions, and the negative impact of this food on the environment, they will feel the need to do something about it.
If the companion animal is a dog, the solution is easy. Switch to a plant-based brand of dog food. Few people with sufficient knowledge of animals still contest the fact that dogs are omnivores capable to thrive with nutritionally balanced vegan dog food, and these days there are plenty of brands to choose from. I have written some blogs for one of them, in which I wrote the following: “Although they descend from one type of hunting wolves who lived about 30,000 years ago, dogs are no longer wolves and have evolved separately from them, not by changing ‘pray’ but by abandoning predation. Dogs became the modern dogs we know by joining human colonies in Eurasia about 15,000 years ago and gradually changing their dietary adaptations from carnivores to omnivores. They needed to be able to digest properly all the food scraps leftovers they got from humans. Their new adaptation obtained by domestication was ‘to eat what their human companions eat’, so they needed to be able to digest plant-based food, as this is what humans ate more.”
But what about cats? What vegans should feed to the companion cats they live with? In this case, the answer is not so straightforward. Many people say — including many vegans — that, contrary to dogs, domestic cats are obligate carnivores, and therefore they should not be fed with plant-based food. I decided to look at this issue closely, and see if I can find out whether this is true.
What Is an Obligate Carnivore?
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A carnivore animal is often defined as an animal whose food and energy requirements derive mainly from animal sources (muscle, fat and other soft tissues) whether through hunting or scavenging. An obligate carnivore (or a hypercarnivore) are often defined as a carnivore who depend solely on animal flesh for their nutrient requirements. However, the scientific definition of hypercarnivores is organisms that depend on animals for at least 70% of their diet.
If the percentage is lower, we have different terms. Mesocarnivores are animals that depend on animal flesh for between 50% and 69% of their diet (i.e. foxes). Hypocarnivores depend on animal meat for less than 30% of their diet (i.e. American black bear). The latter overlap with omnivore animals, who do not depend on flesh at all but may still consume it (I use the term “omnivorous” for the human equivalent, as I don’t believe that in humans it is a biological adaptation, but a cultural habit).
We should not be confusing any of these carnivores classification based on feeding adaptations with animals belonging to the order “Carnivora”, which is a different categorisation within the Class of mammals that includes the Families of canids (wolves, coyotes, dogs, etc.), felids (tigers, lions, cats, etc.), mustelids (skunks, weasels, etc.), ursids (bears, etc.) and others. Although many of the species of this order are indeed carnivores, not all are obligate carnivores, some are omnivores (bears, raccoons, etc.), and we find even herbivores (giant pandas).
We have also the difference between predators and scavengers. Although both are carnivores, the former hunt live prey, while the latter mostly eat flesh or bones from already dead animals. An obligate predator would be an animal who depends exclusively on predating live prey (i.e. many snakes).
It is often said that all felids are obligate carnivores. But, are they? An adaptation does not mean dependency. Being adapted to something means that you are especially good at doing this, but it does not mean that you cannot survive without doing it. It is clear that felids are adapted to eat flesh. They have senses that allow them to hunt efficiently, claws that help them to catch prey, long sharp tearing teeth that help them to tear flesh up, and short digestive tracks with high hydrochloric acid stomachs that allow them to avoid infections from rotten flesh (incidentally, none of these adaptations is seen in humans, so those who claim humans are carnivores don’t have a biological leg to stand on).
But this only means they are efficient meat-eaters in the wild, not that they can only eat meat, and need it to survive. If they could obtain the nutrients they need from sources other than animals, they could probably survive if the food has changed in such a way that their essential nutrients are now bioavailable to them (meaning able to be absorbed and used by the body). Therefore, their dependence on flesh may be simply circumstantial. In the wild, flesh from hunting or scavenging may be the better source of food for them in particular habitats and environments, but this may change if they move to new ones. If they migrate — or there has been a big change in the environment — perhaps they could gradually change their dependency on meat until it goes below 69% (becoming now Mesocarnivores). This could happen to individuals forced to survive in exceptional circumstances, or to species forced to evolve due to environmental change.
One could argue that the dependency of animal flesh in obligate carnivores is behavioural, not metabolic. They may say these animals are hardwired to predate, so their instinct would always lead them to chase and kill prey. Hunting behaviour is indeed very instinctive (although modern ethology sees most behaviours as a mix of instinctive and learnt patterns), but many predatory animals double as scavengers when the opportunity arises, so they are far more flexible and adaptable than people think. If they really depended on hunting, big cats would not be able to survive in zoos, where they still eat meat but do not hunt anymore.
There may be felids that eat more animals than others, though. Felids, where they depend on flesh, is slightly lower than 70%. Felids who have evolved to be more flexible than others as their habitat has now changed and the sources of nutrients are more varied. Are domestic cats one of these?
The answer may well be found in their common name: “domestic”. The cats that live with humans are no lions or tigers. They are not cheetahs or pumas. They are domestic cats of the species Felis catus, which is a species created by humans via artificial selection, as domestic dogs (Canis familiaris) or domestic cows (Bos taurus) also are. They have, therefore, undergone genetic changes that made them different from their wild counterparts. Changes in morphology, anatomy, and behaviour. Even changes in metabolism (dogs can digest carbohydrates easier than modern wolves can).
Have cats changed enough through the domestication process so they are no longer obligate carnivores (as happened to dogs)? Perhaps not. We know the process of domestication in cats is more recent (the earliest known indication for the taming of an African wildcat is from about 7500–7200 BCE). Scientists also believe that African wildcats from which domestic cats evolved were attracted to early human settlements in the Fertile Crescent by the rodents eating human plant-based food, not this food itself. We also know that cats are more independent and, especially if they are allowed to roam free, mate with whom they choose (rather than whom their companion human choose). All these factors suggest that domestic cats are more similar to the original wild species they evolved from than domestic dogs are from the wolves they evolved from. But this does not mean they are identical. The difference could be sufficient so they are no longer 70% dependent on animal flesh. If it is now 69% or lower, it would mean that, technically, they could no longer be labelled as obligate carnivores. This is getting interesting.
The Experts Must Know
I am a zoologist who specialised in animal behaviour, but I am not a vet. I thought that I would need to complement my zoological knowledge with the expertise of vets who have studied this issue for a long time and have conducted research to assess whether any myth needs to be debunked. And no better expert on this subject than Prof Andrew Knight. He is a vet originally from Australia but now he lives in the UK.
Prof Knight is a Professor of Animal Welfare and Ethics at Winchester University, in the UK. He is also the Founding Director of the University’s Centre for Animal Welfare, an Adjunct Professor in the School of Environment and Science at Griffith University (Queensland), an EBVS European and RCVS Veterinary Specialist in Animal Welfare Science, Ethics and Law, an American and New Zealand Veterinary Specialist in Animal Welfare, and a Fellow of the Royal College of Veterinary Surgeons. He is a well-recognised expert in the veterinary field of domestic companion animals with around 150 academic publications. He has been honoured with 14 awards and 15 research grants. And he is also a long-term vegan whom I have known for years. So, I asked him about the issue of feeding domestic cats plant-based diets. He replied giving me the following quote from a 2016 paper he co-authored titled “Vegetarian versus Meat-Based Diets for Companion Animals:”
“In order for dogs and cats to thrive within modern, domesticated environments — for the duration of their artificially increased lifespans — they must be provided with diets that are sufficiently palatable, bioavailable, and that are nutritionally complete and reasonably balanced. Each species and life stage (e.g., juvenile, adult, pregnant, lactating, geriatric) requires a particular nutrient profile; the provision of which can prevent malnutrition and can assist in the management of other diseases.
Although special attention must be paid to ensure adequate levels of certain nutrients such as proteins, amino acids (e.g., taurine, carnitine, methionine, lysine, and tryptophan), vitamins (e.g., Vitamins A, B3, B9, and B12), minerals (e.g., calcium, iron, zinc, and copper), and certain fats, it is important to remember that dogs, cats — and indeed all species — require specific nutrients, rather than specific ingredients. There is — at least in theory — no reason why diets comprised entirely of plants, minerals, and synthetically-based ingredients (i.e., vegan diets) cannot meet the necessary palatability, bioavailability, and nutritional requirements of cats and dogs. Indeed, a growing number of commercially-available diets aim to do so.”
The key concept to take from this is that animals require specific essential nutrients, rather than specific ingredients. Expanding on this thought, this means that animals do not require specific sources of ingredients, such as animal, plant, fungi, bacterial or algal sources. They need proteins, fats, hydrocarbons, minerals, vitamins, etc. They don’t need leaves, legs, stems, ears, fruits, hearts, seeds or bones. They need the nutrients contained in them. So, their classification as herbivores, omnivores or carnivores is purely circumstantial, more based on what they normally eat in the wild than what they are obligated to eat because of their biology.
How Digestion Works
This makes absolute sense once you know how digestion works. The purpose of digestion is to break down food into the building blogs of matter animals need to grow and replace lost cells and tissues, and to operate their metabolism effectively. Different digestive systems are better at doing this for different types of foods, but the end is the same. No matter if you are a herbivore better adapted to digest plants or a carnivore better adapted to digest flesh, the product of the digestion will be the same: the building blogs of proteins (amino acids), the fuel for energy (simple carbohydrates and fats), and the elements and simple chemicals for skeletons and an efficient metabolism (vitamins and minerals broke into bioavailable forms).
Digestion breaks down organic matter in its constituents so they can be used to rebuild the biological matter the animals need. And this digestion takes place in different locations. In the mouth, in the stomachs (it could be more than one), and in the intestines (also, more than one type). But also, outside the body. Part of the digestion may start outside when bacteria and fungi begin decomposing dead biological matter. And this is key because it means that “food” can undergo a transformation process to aid digestion before being eaten.
This is what we humans do with cooking. By applying heat, adding bacteria or yeast (in fermentation methods), breaking up the food with tools (cutting, blending, etc.), and adding chemicals that alter the biological structures (salt, acid, spices, etc.), we begin the breaking down of biological matter and start the digestion process “outside” the body. And this allows us to use a wider range of food sources. And if we can do it to feed ourselves, we could do it to feed others. We can create food for our companion animals in which we have already broken-down matter and we have made nutrients available to them, so they do not need to do it with their digestive system if it is somehow too specialised to digest particular types of food. If cooking works for us, it should work for other species too. We just need to be sure it is the right cooking and produces the right essential nutrients for them.
This is what companies that market food for companion animals do — or should do. They look at the nutritional requirements of the animals they are catering for, find sources for those nutrients and process them (with cooking, for instance) so they become better digestible and bioavailable, and make them palatable so the animals like them. And they can do this now for domestic cats without having to source any of their essential nutrients from animal products.
I asked Prof Knight if the amino acids like taurine (which is the one often quoted as being a nutritional requirement for cats that can only be obtained from meat) added in vegan cat food come from meat or are synthesised in labs without the need for animal flesh. He replied: “Yes, vegan diets use plant, mineral and synthetic sources to supply nutritional needs. They’re created in animal feed manufacturing plants.”
So, as modern humans following a vegan diet add vitamin B12 in their food (either as a fortified food or in supplements) to ensure they do not lack any nutrients, vegan cat food from reputable companies has all the nutrients needed by cats (including taurine, long-chain polyunsaturated fatty acids, and vitamin A) for the same reason. Arguing that a human can thrive with a balanced human vegan diet with all required nutrients, but a cat cannot thrive with a balanced cat vegan diet with all required nutrients, makes no sense. If all the essential nutrients are there in a bioavailable form, and the food is palatable for whoever is intended to, it should work for both.
But are the right nutrients in vegan cat food palatable and bioavailable to cats? The only way to answer this is by conducting scientific research to check if that is the case.
The Evidence Speaks for Itself
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Let’s begin with bioavailability (the ability to be absorbed and used by the body).
First of all, I must say that just giving cats your vegan food leftovers will not do. When I talk about nutritionally balanced cat food, I mean food scientifically formulated specifically for cats, adding all the nutrients that cats need. Food produced by reputable pet food companies or carefully made at home with the right information and ingredients (Jed Gillen has authored a useful book titled Obligate Carnivore that can help on that front). Prof Knight writes the following about plant-based diets for companion animals:
“Such diets are supplied completely, or as supplements to be added to (sometimes cheaper) homemade diets, for which recipes may be supplied. Such recipes are also available in books (e.g., Vegetarian Cats and Dogs). However, the homemade option appears to be less popular. In a study of European users of these diets, Semp found that 39% of participating dog and cat owners used only complete diets; 9% used only homemade diets; while the remaining 52% primarily used complete diets, with regular addition of some homemade ingredients.”
However, if the right essential nutrients are there in vegan food given to cats, but they are not in a form that cats can absorb and process them according to their metabolic requirements, we would find that domestic cats fed only vegan cat food would develop many health problems you don’t find in cats fed traditional cat food. There have been recent studies looking at this.
Although the veterinary profession has generally held the view that cats cannot thrive on a vegan diet, new research has challenged this traditional view. A 2021 study conducted at Ontario Veterinary College at the University of Guelph in Canada, and published in BMC Veterinary Research, concluded that cats fed a plant‑based diet are less likely to suffer from a range of adverse health consequences, including gastrointestinal and hepatic disease, and had more ideal body condition scores than those who were fed meat. The researchers (Sarah A. S. Dodd, Cate Dewey, Deep Khosa & Adronie Verbrugghe) asked more than 1,000 cat guardians about the health issues of their companion cats, and which sort of diet they gave them. They concluded the following:
“A total of 1325 questionnaires were complete enough for inclusion. The only exclusion criterion was the failure to answer all questions. Most cats, 65% (667/1026), represented in the survey were fed a meat-based diet and 18.2% (187/1026) were fed a plant-based diet, with the rest fed either a combination of plant-based with meat-based (69/1026, 6.7%) or indeterminable (103/1026, 10%). Cat age ranged from 4 months to 23 years, with a median of 7 years, and was not associated with diet type. No differences in reported lifespan were detected between diet types. Fewer cats fed plant-based diets were reported to have gastrointestinal and hepatic disorders. Cats fed plant-based diets were reported to have more ideal body condition scores than cats fed a meat-based diet. More owners of cats fed plant-based diets reported their cat to be in very good health.”
The interesting thing about this is that this study looked at the final effects on the cats themselves giving a diet (longevity and general long-term health), which is the gold standard in veterinary intervention, without checking if the vegan cat food supplied was nutritionally complete. And yet, even accounting for “poor” plant-based diets, it found no significant adverse effects.
In the past, it has been suggested that feeding vegan diets to cats may predispose them to lower urinary tract diseases since a cat’s urinary system is tuned for high acidity, but plant-based food tends to be alkaline. I remember that Prof Knight used to advise cat guardians feeding fully vegan diets to their companion cats to regularly test the cat’s urine in case they were particularly susceptible individuals who could develop such problems. I have now asked him if the new research has made him change his advice. He replied: “By late 2021, Dodd et al. study published earlier this year was the only published study examining this in a large sample of cats. And it did not find an increased risk of lower urinary tract disease in vegan cats, with respect to meat-based cats. Hence, it appears that our worry about this may have been unfounded. My forthcoming study on the health of vegan vs meat-based cats will help provide the answers.”
Let’s now look at palatability. Again, there is recent research on this, this one conducted by Prof Knight himself and Dr Liam Satchell from Winchester University, and also published in 2021. They surveyed 4,060 dog or cat guardians to determine the importance to them of pet food palatability, and the degree to which their animals displayed specific behavioural indicators of whether they were happy eating the food offered to them. The study compared three major dietary groups reported: conventional meat (1,178 cats), raw meat (64 cats) and vegan (127 cats).
The cats’ guardians were asked to report the extent to which they thought their cat displayed ten indicator behaviours (including eating quickly, salivating, guarding the food, or sniffing the food). The authors concluded that the evidence suggests that the diets made little difference to the food-orientated behaviour of the cats in this sample, except perhaps that cats on vegan diets lick their food less often, and cats on conventional diets leave more food. The study overall conclusion is the behavioural indicators studied did not generally vary when dogs or cats were fed conventional meat, raw meat and vegan diets, suggesting these animals find vegan food as tasty and desirable as the alternatives.
Therefore, the most up to date evidence indicate that the average vegan diet given to cats is not less bioavailable or palatable to cats than a conventional meat-based diet. In other words, it shows that vegan food is as healthy and tasty for cats as it is for humans — once you adapt it to each species. And to be extra sure, testing the cat’s urine regularly, and slightly modifying the diet if any problem is detected, can allow many cats to thrive in a vegan diet even if they are particularly sensitive to alkaline food.
Can We Say with Certainty Domestic Cats Are Obligate Carnivores?
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No, I don’t think we can. Although they may be in theory, they could easily be Misocarnivores instead. In practice, there is now enough evidence against the broad statement that they must be hypercarnivores (70% or more dependent on meat). If they were, they would not survive with fully vegan diets (0% food from animal origin), let alone show better health outcomes than those in meat-based diets (as reported in recent studies). If they were, they would not even try to eat plant-based cat food, cats being very “fussy” eaters and still having a good set of instincts telling them what is good for them and what is not. If they were obligate predators, they would not eat in bowls with dry food coming from a bag or wet food coming from a tin.
If domestic cats do all this, what is more likely, that the domestication process they have gone under has shifted them towards the omnivore adaptation (or at the very least becoming mesocarnivores rather than staying as hypercarnivores) as happened already with domestic dogs, or that they remained as obligate carnivores as their wild counterparts? After all, we have evidence that domestication does change feeding abilities, and that most domestic cats have regularly eaten, for centuries, things they would have never eaten before in the wild (i.e. milk from cows, bull’s flesh from pouches, or tuna’s flesh from tins).
But even if we may not be sure if they have changed biological feeding adaptations with domestication, we know they have changed feeding behaviour. Remember, the meat-based diet cat guardians normally give them does not come from mice, rats, or small birds (the natural prey of the original cats, which probably was an obligate predator) but from totally unnatural animal sources for them. If they were able to adapt to such radical change (because it doesn’t matter what the sources are but what are the nutrients), why do some people think they cannot adapt to a nutritionally balanced plant-based diet (considering it doesn’t matter what the sources are but what are the nutrients)?
You cannot have it both ways. You can either claim that domestication has not caused any change in cats, so they are still obligate African bird-rodent predators, or if you accept that they can now survive with human-made food from cows, pigs or oceanic fish, they could also survive with human-made food from non-animal sources supplemented with cat-specific essential nutrients (which can be synthesised in a lab now). Let me repeat it: animals require specific nutrients, rather than specific ingredients or sources of food.
Healthy humans are living — and reproducing — long lives on nutritionally balanced plant-based diets with the right nutrient supplementation. Their existence proves humans are not obligate carnivores, as some claim. Also, healthy domestic cats are living — and reproducing — long lives on nutritionally balanced plant-based diets with the right nutrient supplementation. Their existence proves that domestic cats may not be obligate carnivores after all (in the sense used by those who oppose giving them a vegan diet).
Domestic cats may be more carnivores than dogs are, but with the invention of nutritionally balanced plant-based cat food (with added taurine, arginine, vitamin A, arachidonic acid, and the rest of essential nutrients), they may no longer be the obligate carnivores once were. Especially now that eco-friendly and vegan pet food is on the rise, so there is a higher chance that commercial vegan cat food will be nutritionally complete.
It’s all based on the available evidence.
Jordi Casamitjana
