At a glance, the digestive system and all of its extremities is pretty similar when comparing dogs and cats. Both have sharp pointy teeth for ripping flesh, they lack flat molars for grinding plant matter, their jaws are hinged and don’t move from side to side, neither produces salivary amylase (amylase is the enzyme in our saliva that begins the digestion of starches in the mouth), both have highly acidic stomachs to facilitate the digestion of meat, bones and fat, and theirs is but a brief intestinal tract, which prevents pathogenic bacteria from taking hold and causing problems. All in all, it’s pretty clear that nature has carefully designed these creatures as finely tuned hunting and killing machines. Look out.
But there are a few things that set dogs and cats apart, and they make the current state of conventional cat food all the more concerning. They all come back to the fact that, while dogs definitely should eat a prey-based diet (or a fresh diet of at least 80% animal protein), it would not technically be incorrect to classify them as omnivores. They are not capable of, nor have I ever met a dog inclined to eat an indiscriminately omnivorous diet like humans do, but they do have the digestive capabilities to tolerate some plant matter and obtain nutrients from it, and in the wild they will select to consume some from time-to-time. When done correctly, I actually recommend that you feed your dog some plant matter for nutritional reasons.
I personally classify dogs as carnivores with omnivorous tendencies, and I make this distinction because I think classifying them as omnivores is genuinely misleading. I also find that it is often done by people who are about to try and justify feeding their dog loads of plants for reasons that have nothing to do with the health of said dog.
But we’re here to talk about cats. Cats are what we call obligate carnivores. They are physiologically, biologically, metabolically obligated to eat other animals. They simply cannot survive otherwise. Fortunately, this is widely accepted and even vegan pet food websites will acknowledge that cats are true carnivores. Which makes it all the more bewildering that vegan cat food exists at all, or that around half of the ingredients in most commercial cat foods are primarily sources of carbohydrates.
I mentioned that dogs and cats don’t produce salivary amylase, which I think is very telling of the fact that they simply have no evolutionary need for it. They don’t eat the starch that amylase is required to break down. Dogs do, however, produce a small amount of amylase in their pancreas, which makes sense because they are known to scavenge for berries, occasionally eat grass or consume the stomach contents of prey. But it is not enough to digest food that is ~50% plants every day for the rest of their lives and doing so puts enormous strain in the pancreas.
I thought we were supposed to be talking about cats?! Yeah yeah, I’m getting to them.
Cats produce only FIVE PERCENT of the amylase in their pancreas that dogs do . Five percent! Of an already tiny amount. This is not the marker of an animal that should be eating any starchy foods, let alone half of their diet.
Supporting this is the fact that cats have very high protein requirements – around double that of dogs. Protein requirements are high in part because they cannot synthesise the essential amino acids arginine and taurine (or convert it from methionine and cystine), which must all be obtained from animal protein. A single meal without arginine can lead to death. This is because after feeding, the liver naturally produces ammonia . Without sufficient arginine, cats are unable to convert this ammonia to urea so that it can be expelled in the urine, and the result can be ammonia toxicity (admittedly this is very unlikely, but not technically impossible).
Other serious nutrient deficiencies that will occur in cats if they’re not fed animal protein include vitamin A, B3, D3 and arachidonic acid.
Arachidonic acid is a fatty acid for which cats and other obligate carnivores have particular requirements for. Arachidonic acid is present only in animal tissue, not plants, and, unlike dogs, cats are unable to synthesise it from linoleic acid and will thus become deficient without a dietary source . Liver is a particularly abundant organ.
Similarly, vitamin A occurs naturally only in animal tissue and, while omnivorous(ish) animals like humans and dogs can convert b-carotene in plants to vitamin A, cats cannot.
Cats also require more than twice as much Niacin (B3) than dogs for health, but cannot synthesise enough to maintain sufficient levels . They can, however, easily obtain their high metabolic requirements from an animal protein-based diet.
Unlike humans, neither cats nor dogs are able to produce sufficient vitamin D3 through photosynthesis. Human produce the natural oil 7-dehydrocholesterol in our skin, which reacts with UV light and is converted to vitamin D3, before being reabsorbed back into the body. Dogs and cats, however, produce insufficient 7-dehydrocholesterol in the skin to meet the metabolic need for vitamin D3 photosynthesis  so it must be obtained from animal proteins (plant based vitamin D2 is not an appropriate substitute). Liver from pasture raised animals is an excellent source of vitamin D3.
Really, it’s no wonder we hear so many cases of beloved family cats falling ill to urinary tract infections, pancreatitis, periodontal disease, diabetes and cancer, to name but a few. Commercial cat food is nothing short of poison. You can read more about the issues with the pet food industry in Australia in my post here, and you can take the overwhelm out of getting your cat started on fresh, species appropriate diet using my Essential Blend freeze-dried balancing base.
As an interesting postscript, all of the information in this post is referenced below and came from a text book produced by Hill’s Pet Nutrition, effectively proving that they don’t even believe their own bullshit.
 Hand, M.S. (Ed), Thatcher, C.D. (Ed), Remillard R.L. (Ed), Roudebush P (Ed), Novotny B.J.(Ed), 2010. Small Animal Clinical Nutrition.Mark Morris Institute: 5th edition.