Amylase activity in a cat pancreas runs at roughly 5 to 10 percent of a dog's on the same weight of tissue, and an adult cat holds condition on 26 to 30 percent protein on a dry matter basis. Those 2 numbers explain most of the soft stool seen in cats eating grocery kibble with 35 to 45 percent starch.
Carbohydrate is not poison. It is a load the hindgut was not built to carry at that volume, and the litter box reports the overflow within 24 to 48 hours.
5 to 10 Percent Amylase, and What It Costs
Cats have little salivary amylase, so starch digestion starts in the small intestine and depends on pancreatic amylase plus brush border enzymes. Measured per gram of pancreas, feline amylase activity is commonly cited at 5 to 10 percent of canine activity, and sucrase and glucoamylase are correspondingly modest.
Cooked starch is still digested, because extrusion gelatinizes 80 to 90 percent of it and that is what holds a kibble together, but the margin is thin. A diet at 30 to 40 percent starch leaves 5 to 15 percent of that starch reaching the colon, and the answer from the colon is fermentation, gas and water.
This is the same species trade-off as the slit pupil and the 48 Hz hearing floor: the animal is specialized, and specialization shows up as a narrow tolerance rather than a hard limit. The hearing range piece covers the sensory version of the same argument.
Protein: 26 to 30 Percent, 5.5 to 6.5 g per kg
Adult maintenance diets are usually formulated at 26 percent protein dry matter, growth and reproduction at 30 percent, and renal diets drop to 26 to 28 percent with restricted phosphorus. For a 4 kg cat that is 22 to 26 g of protein a day, or 5.5 to 6.5 g per kg of body weight.
Two amino acids cannot be left to chance: taurine at 0.10 to 0.20 percent of dry matter, which is 1000 to 2000 mg per kg of diet, and arginine, because the cat urea cycle has little reserve. Obligatory nitrogen loss runs 2 to 3 g of protein a day even at rest, so a 4 kg cat on a 15 percent protein diet is running a deficit it will pay for with muscle.
Why High-Carbohydrate Diets Give Soft Stool
Three mechanisms, in order of how often they matter. First, osmotic: unabsorbed starch and its split products pull water into the colon. Second, fermentative: the 5 to 15 percent that arrives feeds a bacterial population tuned to protein and fat, and the gas and short chain acids change stool consistency. Third, mechanical: stools move faster, so transit falls from 24 to 36 hours toward 12 to 18.
On a 7 point stool scale the result is usually a 5 or a 6, first log-shaped but soft, then a pile that holds shape only on carpet. Onset is 24 to 48 hours after the switch, which is why owners blame the new food for a problem the old food started.
Numbers to Read on the Label
| Nutrient | Adult, % DM | Growth, % DM | Typical dry, as fed |
|---|---|---|---|
| Protein | 26 to 30 | 30 to 34 | 30 to 36 |
| Fat | 9 to 15 | 15 to 20 | 12 to 20 |
| Carbohydrate, by difference | under 10 | under 10 | 35 to 45 |
| Taurine | 0.10 to 0.20 | 0.20 | 0.15 to 0.25 |
| Crude fiber | 1 to 3 | 1 to 3 | 2 to 4 |
| Moisture | 75 to 82 | 75 to 82 | 8 to 12 |
Guaranteed analysis is as fed, so convert before comparing: divide by 100 minus moisture and multiply by 100. A can at 10 percent protein and 78 percent moisture is 45 percent protein dry matter, a different food from the 30 percent kibble beside it on the shelf.
Fiber, Transit and the 7 to 10 Day Switch
Fiber at 1 to 3 percent supports the colon without overfeeding it, and psyllium at 1 to 2 g a day is the usual first step for a cat with soft stool and no other signs. Change foods over 7 to 10 days in 4 steps of 25 percent, every 2 to 3 days, and feed 3 to 4 small meals rather than 1 large one while the gut adapts.
Water matters more than most owners expect: a cat on dry food drinks 150 to 250 mL a day, while a cat on food at 75 percent moisture takes 180 to 220 mL from the food itself. If intake falls under 100 mL a day for 2 days, the stool will say so within 1 week, and a cat on kibble at 8 to 12 percent moisture is already the driest case in the room.
Enzyme Supplements: What They Help
Powders sold for cats usually combine protease, lipase and amylase, dosed at 500 to 2000 amylase units per meal and mixed into food, not given on an empty stomach. They are worth the money in exocrine pancreatic insufficiency, where a cat loses weight despite a good appetite and passes 2 to 3 bulky stools a day; that diagnosis needs a TLI blood test, not a supplement trial.
In a cat with a normal pancreas, extra amylase does not turn a 40 percent starch diet into a low-starch one. The cheaper fix is 10 percent less starch and 5 percent more protein in the bowl, held for 3 to 4 weeks before judging.
When Soft Stool Is Not the Diet
See a vet for diarrhea lasting 3 days, blood or mucus in the stool, weight loss of 5 percent in 30 days, vomiting 2 or more times a week, or no food for 12 hours. Parasites, pancreatitis, inflammatory bowel disease and hyperthyroidism in cats over 10 all present as the same soft stool, and none of them respond to a food change.
Keep the change simple while you wait: same diet, smaller meals, no treats, and a photo of the stool each day. Age moves the baseline, which is what the cat age tool is for, and the slit pupil in cat eye pupil is the reminder that feline physiology rarely generalizes from dogs.
Terms mentioned in this article
Frequently asked
Do cats really digest starch badly?
They digest cooked starch, just with less margin. Feline pancreatic amylase is commonly cited at 5 to 10 percent of canine activity per gram of tissue, and sucrase and glucoamylase are also low, so a 30 to 40 percent starch diet leaves 5 to 15 percent of that starch reaching the colon.
How much protein does an adult cat need per day?
Most adult maintenance diets are built at 26 to 30 percent protein dry matter, which for a 4 kg cat is 22 to 26 g a day or 5.5 to 6.5 g per kg. Growth and reproduction go to 30 percent, and the taurine minimum sits at 0.10 to 0.20 percent of dry matter.
How fast should I switch to a lower-carbohydrate food?
Over 7 to 10 days in 4 steps of 25 percent, moving every 2 to 3 days, and splitting the day into 3 to 4 small meals while the gut adapts. Soft stool that persists past 3 days, or that brings blood, weight loss of 5 percent or vomiting twice a week, needs a vet rather than another food.