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Why a Vegan Diet will Kill Your Cat (and Sicken Your Dog)

The rising popularity of vegan lifestyles has led some people to consider feeding vegan diets to their pets, but recent research proves that it is unwise, if not outright cruel, to feed them a truly vegan diet.


December 17, 2016 - Rachel Garner and Holland Doughtery

The rising popularity of vegan lifestyles has led some people to consider feeding vegan diets to their pets as a “cruelty-free” alternative to normal meat-based kibble. Because marketing has led people to believe that their dog or cat is able to thrive on a meatless diet, they feel comfortable imposing their ethical views about eating animal products or the agriculture industry onto their pet’s lifestyle as well. However, this is a situation in which human morality not only does not apply to animals but also ends up being dangerously inhumane when expected to be ethical. Canine and feline bodies are not built to subsist singularly on plant matter; both cats and dogs evolved to subsist on a meat-based diet - dogs as opportunistic omnivores and cats as obligate carnivores. Dogs on a vegan diet are very likely to suffer from malnourishment that will drastically affect their quality of life, while vegan-fed cats are guaranteed to be sick and risk death from malnutrition. Recent research into the evolutionary history and gastrointestinal tracts of both dogs and cats proves that it is unwise, if not outright cruel, to feed them a truly vegan diet.

Pets Evolved From Obligate Carnivores

Of the two pet species, dogs fare better than cats (but not optimally) on plant-based diets due to adaptations towards omnivory during the domestication process. The domestic dog evolved from ancestors of the gray wolf some 15,000 years ago (Driscoll et al. 2009). In the wild, the gray wolf is a carnivore, deriving most of its nutrients from hunted or scavenged prey, and any vegetable matter came from the guts of their prey, or from self-medication (like how dogs will eat grass to induce vomiting). During the domestication process, dogs adapted to utilize a more omnivorous diet while scavenging around human settlements, and therefore have more flexibility regarding nutrient sources (McNamara 2014). However, their gastrointestinal tract is still primarily geared towards digesting a higher protein diet and their bodies still respond best to nutrients from animal sources. In contrast, the digestive system of domestic cats is still very similar to that of their wild counterparts. The domestication of cats mostly modified their flight distance and tolerance of human proximity, rather than radically altering their physiology as occurred with domestic canines (Driscoll et al. 2009). Without significant physiological changes during the feline domestication process, it follows logically that cats still get the most nutrition from a high-protein, low-carbohydrate diet that maximizes available protein sources.

Visible Physical Attributes Indicate Meat Is A Primary Food Source

There are multiple places in which the physiological evidence for this evolutionary predisposition to carnivory is easily visible in domestic cats and dogs. Both species have molars that are shaped to best crush bone and grind viscera. Animals with a more herbivorous diet will have differently shaped dentition: cows have flat molars to pulverize plant matter and humans have a mix of grinding and tearing teeth to allow an omnivorous diet. Cats have far fewer grinding surfaces than dogs do - their dentition is entirely geared towards shearing meat and bone (Bowen, 2006). This is even evident in regard to jaw motion, as herbivores like cows have wide, flat molars and jaw muscles that allow them to chew side to side with repetitively with ease (Orr, 2016), while carnivores like cats and dogs have a large vertical range of motion in their jaw that allows for efficient removal of a single mouthful of food without the need for chewing.  

Digestive Systems Directly Indicate Dietary Specializations

The most obvious sign that an animal needs a meat-based diet is actually the setup of their digestive system - specifically, the ratio of their gut length to their body size. Every living thing will require the same six basic nutrients - protein, fat, carbohydrates, water, minerals, and vitamins - in species-specific amounts. Groups of animals differ in their “preferred” source of energy - while all animals need glucose, herbivorous animals are optimized to use carbohydrates (either directly or indirectly) to get it, while carnivores are optimized to utilize protein sources .

Protein is broken down in the stomach of an animal, but the cellulose found in plant matter requires much more time in the intestines to be broken down into absorbable nutrients. Therefore, the longer the animal’s gut is versus its body length, the more herbivorous the animal’s diet is. This rule has been shown to hold true across the animal kingdom, from mammals to birds to fish. Ruminants have the largest ratios: cows have a 30:1 ratio, horses 15:1. Humans, who evolved to eat a mixed diet, have a ratio of approximately 10:1. In comparison, dogs have a ratio of 6:1, depending on breed, and cats have a very short ratio of only 4:1 (Chiba, 2014).

The surface of of the intestine is also important in determining ideal diet: the human intestine, for example, has lots of twists and turns that create more surface area per unit of length, ideal for maximizing digestion and absorption of carbohydrates. In contrast, the intestine of carnivores like cats resembles a slightly wavy tube. Because they are getting most of their energy from protein, which is digested and absorbed higher up in the small intestine, they don’t need the extra length and surface area to digest fiber. We also know that gut capacity partially determines the digestive capacity of an animal, particularly herbivores - the smaller the gut capacity or ratio of gut length to body length, the faster the gut’s passage rate and the more nutrient-dense and easily digestible the animal’s diet must be in order for the animal to be able to obtain enough nutrients for survival (Demment & Van Soest, 1985). As we can see below, the coyote, a carnivore, has a shorter, far less complex gut than that of a deer, which consumes a high-fiber diet.

Cats and Dogs Are Internally Primed For Protein Digestion

Being carnivores, dogs and cats both have a short intestine length and are predisposed to do best on diets composed of quickly digestible foodstuffs like protein. Evidence for this can be found in their highly concentrated stomach acid, which has a high amount of enzymes that break down protein, and the lower levels of enzymes that break down carbohydrates (amylases) in their intestines. Cats and dogs also don’t have many of the specific enzymes that are required to digest many complex plant proteins. While not all carbohydrates are like this (corn and rice are easily digested by both species, if processed correctly), this can mean that less-digestible high-carbohydrate diets pass through the intestines of cats and dogs too quickly for the carbohydrates to be properly broken down and absorbed, and the end result is a lot of poop and not much nutrition. All animals need some amount of fiber to maintain a healthy intestinal lining and to prevent diarrhea, but too much fiber can impair protein absorption and cause digestive upset (Case et al. 2014). Based on the short, simple intestines of dogs and especially cats, we can see that they have fast passage rates, and therefore need easily-digestible, nutrient-dense food so that their physiology can make the most of what they do eat, and meat best fulfills all of those those requirements.

Obligate carnivores like cats and ferrets have a limited ability to digest carbohydrates and are instead extremely efficient at making new glucose from non-glucose materials (a process called gluconeogenesis). Humans are also capable of using the gluconeogenic cycle to get energy, but we can turn it on and off as needed because our liver is able to regulate it. For cats the gluconeogenic cycle is always on, and that causes big problems when their diet does not contain enough protein. Since the cycle is always running, a cat’s body preferentially burns protein for fuel over everything else - if they don’t get enough easily digestible protein, the body will start breaking down and digesting its own muscles for energy. A recent study found that cats fed high-carbohydrate diets were unable to stop their bodies from breaking down any available protein to create glucose, rather than shifting gears and utilizing the glucose from the carbohydrates in their food (Harmon, 2016). In extreme cases, this could mean that the cat would wind up digesting its own muscles for energy.

Plant-Based Diets Deprive Your Pets Of Crucial Nutrients

The inability to efficiently utilize carbohydrates as sources of energy is not the only problem with grain-heavy diets for dogs and cats: there’s also the issue of essential nutrients. The concept of essential nutrients is a common one - they are nutrients the body cannot create for itself that must be acquired regularly from the diet for an animal to survive. (For instance, vitamin C is a well-known essential nutrient for humans, guinea pigs, and some bats.) These can be amino acids, which build proteins; fatty acids, which build lipids; or vitamins. Imbalances or deficiencies in any of these can make a dog or cat very ill, and many of them are very hard if not biologically impossible for carnivores to synthesize from plant-based diets.

Essential Amino Acids Build Organs and Filter Urine:

Cats have an absolute requirement for an amino acid only found in animal tissues for proper heart, eye, and digestive function: taurine. While cats can synthesize some of it themselves from other nutrients in their food, they are physically incapable of making enough on their own to keep up with the massive amount their bodies need daily (McNamara, 2014). Cats need taurine to build the pigments in their eyes and to help them digest fat. Chronic taurine deficiency causes degeneration of tissue in the eyes and can lead to structural heart issues, preventing a cat’s heart from pumping blood correctly. Low taurine levels were found to be a major cause of heart disease for pet cats until commercial feline diets were reformulated to provide the correct amount in the 1980’s (Case et al. 2011). Cats are unable to survive without sufficient taurine on a regular basis, as a deficient animal will begin to break down its own tissues to obtain it. By the time a taurine deficiency is symptomatic in a cat, the damage is not always reversible and it will kill the cat if left untreated (McNamara 2014).

Cats also have a uniquely high requirement for arginine, an amino acid necessary for converting  ammonia into the far less toxic urea before it can be urinated out. Ammonia is a byproduct of breaking down protein for energy - gluconeogenesis - which is the cycle that cats are specialized to do and cannot stop their bodies from undertaking. Such a high level of gluconeogenesis means that without sufficient arginine the cat will develop ammonia toxicity from high levels in the bloodstream, another condition which is fatal if untreated (McNamara 2014). Having enough arginine - which is found in the highest concentrations in meat - is vital, as cats get severely ill or even die after just one arginine-free meal (Morris 2002). *

Essential Fatty Acids Regulate Immune Response and Digestive Tract:

Cats also need very high amounts of arachidonic acid, a crucial fatty acid that can only be obtained through animal fats. Cats require arachidonic acid for the production of an immune response, for blood to be able to clot properly, for regulating skin, for fur growth, and for supporting digestive and reproductive function. Without enough arachidonic acid, cats will suffer from painful dermal and gastrointestinal problems and a decreased inflammatory response. While cats are not totally incapable of synthesizing arachidonic acid (McNamara 2014, Bauer 2006), plants do not produce arachidonic acid and there does not appear to be any scientific evidence that cats are able to make enough from precursors found in the plant matter in their diets to keep them healthy. Pharmaceutical developments have produced arachidonic acid from fungi, but it may be unlikely to be used in pet food due to the competition from the human health and infant formula markets (Ji et al. 2014).

Vitamins Help Pets See And Digest Their Food:

Most mammals (including dogs) are able to process beta-carotene from plant tissue in order to get enough Vitamin A in their diet - cats, however, lack this pathway and require a dietary source of pre-processed vitamin A such as animal fat or organ meat (McNamara, 2014). Vitamin A is crucial for proper skin, fur, muscle, and nerve function, and chronic deficiency will lead to night blindness and muscle weakness.

All animals need vitamin B in their diet, but cats have a unique requirement for high levels of the vitamins B3 and B6. Dogs also need B vitamins to stay healthy, but they are able to break down the amino acid tryptophan (McNamara 2014) to create a small amount if their diet doesn’t provide it. Because a cat’s digestive system burns through protein at such a high rate, their bodies don’t convert tryptophan into B3, which means they are obligated to get all their B vitamins from their diet. While B vitamins do occur naturally in many foods (Higdon et al. 2013, 2014), it is most efficiently acquired through a meat-based diet: in the wild, the high concentrations of B6 and B3 in organ meats would fulfill a cat’s daily requirements. B vitamins are water-soluble, which means cats cannot store them for later and must always have a steady supply, as the excess amounts each day are excreted in their urine. Without proper levels of B3 and B6, cats are unable to use protein to provide energy. (McNamara 2014).

Both dogs and cats have an absolute vitamin D requirement because they cannot synthesize it in their skin from sun exposure the way humans do. Of the two types of D vitamin they can utilize, D3 - the form that’s most available for the animal to absorb and use in the body - only comes from animal sources. Cats and dogs can utilize the alternate D2 if D3 isn’t available (Case et al. 2011), but it is much less bioavailable compared to the plant-based D3 (Morris 2002), which puts animals at risk of chronic deficiency without simultaneous D3 supplementation. Chronic vitamin D deficiency causes joint and muscle pain and makes animals prone to bone fractures.

Malnourishment From Feeding Cats and Dogs A Vegan Diet

Could Kill Them

A dog on a vegan diet will survive, but will rarely thrive, as their bodies are constantly on the edge of nutrient deprivation. Whereas a meat-based diet easily provides all the essential nutrients they need to survive, a plant-only diet provides insufficient quantities of these nutrients, and often in forms that are particularly difficult for dogs to process. Vegan dogs will be prone to B and D vitamin deficiencies and will be malnourished due to their physical inability digest large quantities of plant matter.

A cat on a “truly vegan” diet will be, at minimum, severely ill and at risk for permanent physical damage. Cat metabolisms are similar to racecar engines: they require high-quality fuel that burns quickly, and only operate well within a very narrow range of conditions. Cats must ingest sufficient amounts of high-quality meat protein on a regular basis or their bodies will start digesting their own tissues in order to continue functioning. Plant-based diets lack the necessary concentrations of every essential nutrient a cat needs in large quantities, and the results of this shortage on a cat’s health are horrific: on top of Vitamin A, B, and D deficiencies, arginine shortage will become fatal after only a few meals, and chronic taurine and arachidonic acid deficiencies will lead to irreversible degeneration of teeth and vital organs. In combination, any of these deficiencies can and will kill a domestic cat.

Vegan pet diets are simply unable to reliably provide the correct balance of bioavailable nutrients and protein to carbohydrate ratio needed to fulfill the unique and complicated requirements of animals optimized for a carnivorous lifestyle. Humans can survive on a vegan diet because our evolutionary history shaped us into omnivores. None of our essential amino acids require animal tissue. In contrast, most of the most critical essential nutrients for both dogs and cats are found in their proper concentrations in meat - an easily digestible complete protein that fulfills their evolutionary needs. These animals, especially cats, did not evolve to subsist on carbohydrates and no amount of wishful thinking will cause an animal’s digestive system to change from the gluconeogenic cycle it has been optimized for over millennia.

The most ethical thing to do is to feed a species the diet they have evolved to thrive on. Cats and dogs should be fed a high-protein, low-carbohydrate diet that has been optimized for the nutrient needs of the particular species of pet. There are good, high-protein, low-carbohydrate pet foods out there that have enough fiber to promote gut health while maximizing nutrient availability. Most pets don’t get to make much of a choice about what they eat, so it is up to their human owners to make sure they are getting a healthy, optimized diet that will allow them to live long, happy lives. Attempting to enforce human-centric standards of ethics on an animal that will suffer from it has no place in pet ownership.

* Because cats produce so much ammonia from protein metabolism, it is important to not feed excessive protein, either. Excess ammonia raises the pH of urine and when combined with magnesium and phosphorous imbalance, can cause painful urinary tract disorders.  The best way to prevent this is through proper hydration and feeding the cat several small meals instead of a few large ones, and is why it’s extremely important that cats are fed high-quality protein as part of a well-balanced diet.

About the Guest Author:

H.C. Dougherty is a 4th year PhD student at the University of California, Davis, studying modeling of sustainable livestock production systems. Dougherty has a B.S in Animal Science and a minor in Ag Business from Iowa State University.

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Rachel is an educator and animal science writer. With prior professional experience in zookeeping, visitor education, shelter behavior management, and more, she works to translate pertinent field-specific knowledge into comprehensive explanations about current animal related topics.