Two bags both say 38 percent protein. One is chicken meal and egg; the other is corn gluten meal, pea protein and wheat. The cat absorbs roughly 32 g of protein per 100 g of dry matter from the first and 23 g from the second, because digestibility, not the number on the front, decides how much of that 38 percent the cat can use. The rest leaves in the litter box.
38 percent says nothing about usable protein
Crude protein is measured as nitrogen, and nitrogen shows up in feathers, hair, urea and indigestible fiber alike. A formula at 38 percent DM with 90 percent digestibility delivers 34 g of usable protein per 100 g; at 70 percent digestibility the same 38 percent delivers 27 g. That 7 g gap is about a third of a 4 kg cat's daily protein need, which is 12 to 16 g on a dry matter basis in most home calculations.
Labels rarely print digestibility, so you estimate it from ingredients and processing. Extrusion runs 90 to 150°C for 10 to 20 seconds, and the Maillard reaction between sugars and lysine can cost 10 to 20 percent of available lysine in a heavily cereal-based recipe. Gentle drying at 60 to 80°C costs less.
Animal protein digests at 85 to 95 percent
Muscle meat, egg and fish land in the 85 to 95 percent band for apparent digestibility in cats. Whole egg is at the top, near 92 to 95 percent, and its amino acid pattern is close enough to the cat's requirement that it is used as the reference protein. Poultry meal sits at 88 to 92 percent, fish meal at 85 to 92 percent, and organ meat lower, at 80 to 88 percent, because connective tissue and fat dilute the protein.
Two caveats. Apparent digestibility is not corrected for protein the gut secretes, so true digestibility runs 3 to 8 points higher. And the number belongs to the ingredient, not the bag: 60 percent chicken meal with 25 percent cereal is not an 88 percent diet.
Plant protein sits at 60 to 80 percent
Whole grains and legumes run 60 to 80 percent in cats, below the animal band for 2 reasons: a higher fraction of the protein is bound in fiber, and the amino acid pattern is wrong for a carnivore. Corn sits near 60 to 70 percent, wheat at 70 to 78, soybean meal at 75 to 82. Isolated pea or soy protein can reach 85 percent, but that is a purified isolate, not the whole seed on the label.
Plant protein is a dilution, not a poison. A recipe drawing 40 percent of its protein from plants needs 4 to 6 points more crude protein for the same usable amount, plus added taurine, methionine and arachidonic acid.
The AAFCO amino acid profile is a floor
Cats need 11 amino acids from food, and the profile sets a minimum for each one rather than a target. Taurine is the one with the clearest number: 0.10 percent DM for adult maintenance and 0.20 percent for growth and reproduction. Arginine has no stored reserve worth mentioning, and a single arginine-free meal can raise blood ammonia within hours. Methionine is the precursor for cysteine and taurine, so a formula can pass the taurine line and still be short on methyl donors.
Meeting the floor is not the same as meeting it in a form the cat can absorb. A diet can hit 0.10 percent taurine by analysis and lose 20 to 40 percent of it in processing, which is why taurine is added after extrusion in most canned and dry recipes. Look for a complete and balanced statement for the life stage, and for taurine on the label.
Ash under 8 percent: a screen, not a verdict
Ash is what remains after burning the food at 550°C, and it is mostly calcium, phosphorus, magnesium, potassium and sodium. No AAFCO rule caps it. The 8 percent line is an industry convention: a dry cat food at 6 to 8 percent DM usually reflects a moderate bone and mineral load, while 9 to 12 percent suggests a formula built on bone-rich meals or a lot of fish bone.
High ash matters for 2 reasons. It drags magnesium and phosphorus up, and struvite risk rises when urine pH drifts above 6.4 with magnesium over 0.08 percent DM. It also displaces protein: at 12 percent ash, 12 g of every 100 g of dry matter is mineral, not food. But low ash is not automatically better, because calcium and phosphorus still have to hit 1.0 and 0.8 percent for growth, and a 4 percent ash food may simply be undersupplied.
Six sources side by side
| Source | Protein, % DM | Apparent digestibility, % | First limiting amino acid | Ash, % DM |
|---|---|---|---|---|
| Whole egg | 45 to 50 | 92 to 95 | none in practice | 4 to 6 |
| Fresh chicken muscle | 78 to 82 | 92 to 95 | methionine | 4 to 6 |
| Chicken meal | 62 to 68 | 88 to 92 | methionine | 14 to 18 |
| Fish meal | 65 to 72 | 85 to 92 | methionine, lysine | 15 to 20 |
| Corn gluten meal | 60 to 65 | 60 to 70 | lysine, tryptophan | 2 to 4 |
| Pea protein isolate | 80 to 85 | 75 to 80 | methionine | 3 to 5 |
Read the table as a budget. A 70 percent chicken meal, 20 percent corn formula lands near 34 percent protein DM with an average digestibility around 80 percent, which is 27 g usable. An all-animal formula at 36 percent with 90 percent digestibility gives 32 g.
Reading the panel past the first 3 slots
Ingredients are listed by weight before processing, so fresh chicken at 70 percent water can outrank chicken meal that contains 3 times the protein per gram. Split a named meat into 3 fractions, chicken, chicken meal and chicken fat, and no single line has to appear first. Add up the animal fractions and compare them with the sum of the plant fractions; that ratio, not the first word, tells you what the protein is made of.
Watch 3 more patterns: poultry by-product meal, which is a legitimate concentrated protein but names no species; hydrolyzed protein, useful for allergy trials but not a quality upgrade; and pea fiber, chickpeas and potato protein used to push crude protein up without animal protein. Format changes the arithmetic too, since wet food at 78 percent water carries 10 to 12 percent protein as fed but 45 to 55 percent DM (wet versus dry food).
Five checks before the next bag
- Convert every protein number to dry matter: divide by (100 minus moisture) and multiply by 100.
- Add up named animal fractions and plant fractions separately; aim for animal at 70 percent of the protein or more.
- Confirm taurine at 0.10 percent DM for adults or 0.20 percent for kittens, on the guaranteed analysis or the nutritional adequacy statement.
- Treat ash above 9 percent DM as a reason to check phosphorus and magnesium, not an automatic rejection.
- Change one variable at a time over 7 to 10 days, and keep the water target at 40 to 60 mL/kg in view (wet food ratio).
Bones, raw fish and supplements are a separate safety list; the common toxic items are in the toxicity checker.
Terms mentioned in this article
Frequently asked
Is 38 percent protein enough for a cat?
It depends on digestibility, not the number. At 90 percent digestibility, 38 percent DM gives about 34 g of usable protein per 100 g, comfortably above a 4 kg cat's 12 to 16 g daily need. At 70 percent digestibility the same label delivers 27 g, and a cat eating less than its ration can fall short.
Does high ash mean bad cat food?
Not by itself. Ash reflects the mineral load, so 9 to 12 percent DM usually means bone-rich meals, which also raises magnesium and phosphorus. Check those two numbers and the urine pH target. A food at 4 percent ash can be just as wrong, because growing kittens need 1.0 percent calcium and 0.8 percent phosphorus.
Can plant protein replace meat in cat food?
Only partly. Plant sources digest at 60 to 80 percent against 85 to 95 percent for animal sources, and they are short on taurine, methionine and arachidonic acid, which have to be added back. A formula can meet the AAFCO amino acid floor with plants, but it usually needs more crude protein to deliver the same usable amount.