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Feline nutrition fundamentals

Cats are not small dogs and they are certainly not small people. Ten million years of eating nothing but other animals left them with metabolic constraints that determine everything else in this guide.

1. What “obligate carnivore” actually means

The phrase gets used loosely. Precisely, it means that over roughly ten million years of eating nothing but other animals, the cat lineage lost metabolic machinery it no longer needed. Those losses are permanent and they are the reason feline nutrition is not simply canine nutrition scaled down.

A dog is a facultative carnivore: it thrives on meat but retains the enzymes to make do with a mixed diet. A cat did not keep those options. Five specific adaptations do most of the work:

The five metabolic adaptations that define feline nutrition.
Adaptation Consequence
No dietary need for carbohydrate Cats have no minimum carbohydrate requirement at all. They have low intestinal amylase, low disaccharidase activity, and lack hepatic glucokinase, relying instead on hexokinase. They handle a large starch load poorly and slowly.
Constitutively high protein turnover The hepatic transaminase and urea-cycle enzymes that break protein down for energy run at a high, largely fixed rate. A cat cannot dial them down when protein is scarce, so it keeps degrading body protein for glucose even while starving. This is why a low-protein diet costs a cat muscle much faster than it would a dog.
Loss of specific synthetic pathways No meaningful synthesis of taurine, arachidonic acid, or retinol from beta-carotene; too little delta-6-desaturase; too little vitamin D from skin; tryptophan-to-niacin conversion largely blocked. Each becomes an absolute dietary requirement.
Blunted thirst response Descended from the African wildcat, a desert animal that took its water from prey. A cat does not compensate fully for a dry diet by drinking more, so total water intake falls when the food is dry.
Sensory wiring for meat Cats cannot taste sweetness: the Tas1r2 sweet-receptor gene is a non-functional pseudogene. They are instead strongly responsive to amino acids and to the nucleotides and free amino acids released by animal tissue, which is why palatants in commercial food are animal digest.

Note

None of this makes a cat unable to digest cooked starch. A well-processed, gelatinised starch in a commercial diet is digested reasonably efficiently. The argument is not that carbohydrate is poison; it is that carbohydrate is not required, that it displaces protein and fat from a fixed calorie budget, and that a cat's glucose handling is not built for a large, sustained starch load.

2. Protein

Protein is the nutrient a cat is least able to economise on. AAFCO sets the adult minimum at 26 % of dry matter and the growth minimum at 30 %, but those are floors designed to prevent deficiency, not targets. Most good wet foods land between 40 and 55 % protein on a dry-matter basis, and the evidence suggests cats do better in that range than at the floor.

Quantity is not the whole story

“Crude protein” on a label is not measured by finding protein. It is measured by burning the sample, measuring nitrogen, and multiplying by 6.25. Anything nitrogenous counts. That has two practical consequences:

Biological value

What actually matters is whether the protein supplies all eleven essential amino acids plus taurine in usable proportions, and whether the cat can digest it. Roughly ranked for cats:

Tier Sources Notes
Excellent Named muscle meat and organ: chicken, turkey, rabbit, salmon, beef, liver, heart Complete amino acid profile, high digestibility, naturally taurine-rich. Heart is the single richest practical taurine source.
Good Named meals: “chicken meal”, “salmon meal”; whole egg A meal is rendered and water-removed, so it is protein-dense by weight. A named meal is a legitimate, concentrated ingredient. Egg is the reference protein for biological value.
Watch Unnamed “meat meal”, “poultry by-product meal”, “animal fat” By-products are not inherently bad (organ meat is a by-product and is excellent) but an unnamed source means you cannot verify species or consistency between batches.
Discount Pea protein, corn gluten meal, soy protein isolate, potato protein, wheat gluten Raise crude protein on the label without matching the amino acid needs of an obligate carnivore. Fine as a minor component; a problem when they sit high in the ingredient list.

Common mistake

Reading crude protein straight off a wet food label and concluding it is protein-poor. A can showing 10 % protein and 78 % moisture is 45 % protein on a dry-matter basis: considerably richer than a kibble showing 32 %. You must convert to dry matter before comparing anything. How to do the conversion →

3. Fat

Fat is the cat's preferred energy substrate, the carrier for vitamins A, D, E, and K, and the source of two fatty acids they cannot make. It also drives palatability more than any other component. AAFCO's minimum is 9 % of dry matter; typical good diets run 15–25 %.

The essential fatty acids

Omega-3s: not strictly required, genuinely useful

EPA and DHA are not on the AAFCO required list for adult cats, but the evidence for them is among the better evidence in feline nutrition. DHA supports neural and retinal development in kittens and is required in the diet of queens during gestation and lactation for that reason. In adults, marine-source omega-3s have reasonable support for reducing osteoarthritis signs and are used adjunctively in chronic kidney disease. Note the source: cats convert plant-source ALA (flaxseed) to EPA and DHA very poorly, so flax is close to useless here. Fish oil, krill oil, or green-lipped mussel are the practical sources.

Note

Fat is 9 kcal per gram against 4 for protein and carbohydrate, so a high-fat food is calorie-dense and a small measuring error becomes a large calorie error. This is a common route into obesity when someone switches a cat to a richer food and keeps the portion size the same.

4. Carbohydrate

The genuinely contested area. Here is what is well supported and what is not.

Reasonably well established

  • Cats have no dietary carbohydrate requirement, the NRC sets no minimum.
  • Cats digest and absorb large starch loads less efficiently than dogs, and excess reaches the colon.
  • Reduced-carbohydrate, higher-protein diets improve glycaemic control in diabetic cats and increase the rate of diabetic remission.
  • Carbohydrate displaces protein and fat within a fixed calorie budget; extruded dry food generally cannot be made below roughly 25 % carbohydrate because starch is needed for the kibble to hold together.

Not established, despite frequent claims

  • That dietary carbohydrate causes feline diabetes in otherwise healthy cats. The dominant risk factors are obesity, inactivity, and being a neutered male; total calories matter more than macronutrient split.
  • That grain-free is nutritionally superior. Grain-free products routinely replace grain with potato, tapioca, or pea starch at a similar or higher carbohydrate level. “Grain-free” is a marketing category, not a nutritional one.
  • That carbohydrate causes urinary crystals. Urine pH, mineral load, and above all urine concentration drive that, not starch.
  • That any specific carbohydrate percentage is a threshold for harm. There is no validated cut-off.

The honest summary: carbohydrate is not the villain it is sometimes made out to be, but there is no argument for wanting more of it. Where two products are otherwise comparable, the lower-carbohydrate one gives the cat more of what it actually uses.

Finding the carbohydrate figure

Manufacturers are not required to declare it, and almost none do. Estimate it from the guaranteed analysis by difference:

Nitrogen-free extract (carbohydrate by difference)

Carbohydrate % = 100 − protein − fat − moisture − ash − crude fibre

All figures as-fed, from the guaranteed analysis. If ash is not declared, assume about 2 % for wet food and 6–8 % for dry. Convert the result to a dry-matter basis to compare across formats. The answer is an estimate, not an analysis, because guaranteed-analysis figures are minimums and maximums rather than actual values.

5. Fibre

Not an essential nutrient for cats, but functionally useful. Fermentable fibres such as beet pulp, psyllium, and inulin feed colonic bacteria and produce short-chain fatty acids that nourish the colonic lining; insoluble fibres add bulk and speed transit. In practice fibre earns its place in three situations: constipation and megacolon, hairball management, and weight-loss diets where it adds satiety without calories. In a healthy cat on a good diet it is unremarkable, neither a benefit to chase nor a problem to avoid.

6. The nutrients cats cannot make themselves

These are the hard constraints. Each is a documented deficiency syndrome, not a theoretical concern, and each is the reason a cat cannot simply eat what a dog or a person eats.

TaurineFatal if deficient

A sulfonic amino acid found almost exclusively in animal tissue. Cats have very low activity of cysteine dioxygenase and cysteine sulfinic acid decarboxylase, the enzymes that build taurine from cysteine, and they obligately conjugate bile acids with taurine, so they lose it continuously in bile whether or not they can spare it. There is no glycine fallback as there is in dogs.

Deficiency causes
Dilated cardiomyopathy (a dilated, failing heart), central retinal degeneration progressing to irreversible blindness, reproductive failure, and poor kitten growth. The cardiac and early retinal changes are reversible if caught and supplemented in time; advanced retinal loss is not.
Why canned food needs twice as much
AAFCO sets 0.20 % DM for canned against 0.10 % for extruded, because heat processing and the greater gut microbial degradation associated with wet diets increase losses.
Best food sources
Heart, dark poultry meat, shellfish, and mice. Cooking leaches taurine into the cooking water, which is one reason home-prepared diets so often come up short.
ArginineFatal within hours

Cats cannot synthesise enough ornithine or citrulline to keep the urea cycle running without dietary arginine. A single arginine-free meal can produce hyperammonaemia, salivation, vomiting, ataxia, tetany, and death within hours. This is the fastest-acting nutritional deficiency described in any domestic species, and it is why feeding a cat a plant-based diet is dangerous rather than merely suboptimal. Arginine is abundant in all animal protein, so a cat eating meat never encounters this.

Arachidonic acidEssential fatty acid

Cats have negligible delta-6-desaturase activity and cannot convert linoleic acid into arachidonic acid. It must come pre-formed from animal fat. Deficiency impairs skin barrier function, platelet aggregation, wound healing, and reproduction.

Pre-formed vitamin A (retinol)Animal sources only

Cats lack the intestinal dioxygenase that cleaves beta-carotene into retinol. Carrots do nothing for a cat. Vitamin A must arrive as retinyl esters from liver, fish oil, or supplementation. It is also one of the few nutrients where oversupply is a genuine clinical problem: hypervitaminosis A from a liver-heavy diet causes painful bony proliferation along the neck and forelimbs.

Niacin (vitamin B3)Unusually high requirement

Most mammals make niacin from tryptophan. In cats a competing enzyme, picolinic carboxylase, runs at high activity and diverts the pathway, so conversion is far too slow to meet requirements. The dietary requirement is roughly four times a dog's, which is why the profile figure of 60 mg/kg looks so high next to other B vitamins.

Vitamin D3Dietary only

Cats have very little 7-dehydrocholesterol in their skin and cannot make useful vitamin D from sunlight. A cat sunbathing on a windowsill is thermoregulating, not synthesising. All of it must be eaten: and it has a defined AAFCO maximum, because excess causes hypercalcaemia and soft-tissue mineralisation including of the kidneys.

Vitamin B12 (cobalamin)Animal sources only

Available in usable form only from animal tissue. Cats also have an unusually short cobalamin half-life (around 13 days, against weeks to years in other species) so intestinal disease that impairs absorption produces clinical deficiency quickly. Low cobalamin is a common, easily missed, and readily treatable finding in cats with chronic diarrhoea, weight loss, or chronic enteropathy.

Thiamine (vitamin B1)Easily destroyed

Cats require roughly three to four times as much thiamine as dogs, and it is the least stable vitamin in pet food: degraded by heat processing, by prolonged storage, and by thiaminase enzymes present in raw fish and raw shellfish. Deficiency causes anorexia, then a characteristic ventroflexion of the neck, ataxia, dilated pupils, seizures, and death. Historic thiamine-deficiency recalls of canned cat food are the reason this vitamin is watched closely, and the reason an all-raw-fish diet is a known way to injure a cat.

Vegan and vegetarian diets

A cat cannot be safely fed a diet free of animal-source nutrients without synthetic supplementation of taurine, arginine, arachidonic acid, retinol, vitamin D3, and B12 at minimum. Commercial products exist that attempt this; the published evidence on long-term outcomes is thin, and case reports of taurine-deficient cardiomyopathy on home-made plant diets exist. If this matters to you, it is a conversation to have with a board-certified veterinary nutritionist, not a decision to make from a label.

7. What good looks like

Pulling the above together, a species-appropriate diet for a healthy adult cat looks roughly like this on a dry-matter basis. These are not regulatory requirements; they are what the biology above points at.

Target ranges for a healthy adult cat, dry-matter basis. Compare with the AAFCO minimums, which are lower by design.
Component Reasonable target AAFCO adult minimum
Protein 40–55 % 26 %
Fat 15–25 % 9 %
Carbohydrate under 15 % where practical no requirement
Moisture (as fed) 70 %+ for most of the calories not specified
Taurine 0.2 %+ 0.10 % dry / 0.20 % canned
Phosphorus as low as the life stage allows 0.5 %

Rule of thumb

A food that hits those numbers, carries an AAFCO complete-and-balanced statement for the right life stage, names its animal proteins, and comes from a manufacturer that can answer the WSAVA questions is a good food. Almost everything else on the packaging is marketing.

Sources