A cat pushes a meal through 1.2 to 1.7 meters of small intestine in 3 to 5 hours, and by the end of that run almost every usable calorie is gone. The work is not spread evenly. Three segments split it: the duodenum stages enzymes and bile, the jejunum absorbs the bulk, and the ileum salvages the two things that cannot be replaced elsewhere, cobalamin and bile salts. Knowing which segment owns which job explains why one cat loses weight on a normal diet while another passes pale, bulky stool on the same food.
One villus, roughly 20 times the area
Absorption happens on a folded surface, not a smooth tube. Each villus in the small intestine stands 0.5 to 1.0 mm tall, packed at 20 to 40 per square millimeter, and every cell on it carries microvilli about 1 to 2 micrometers long. Those two folds multiply the flat tube area by 15 to 20 times, which brings a cat's small intestine to roughly 3 to 4 square meters of working surface inside a body that weighs 4 kg. A villus is also a small pump: a central lacteal takes chylomicrons into lymph, and a capillary loop takes sugars and amino acids into the portal vein. Damage the top third of the villi and absorption drops before any blood test moves.
Duodenum: the 15 cm that sets up everything
The duodenum is short, 10 to 15 cm in a typical adult cat, and mostly a control point. Chyme leaves the stomach at pH 1.5 to 2.0 and must be neutralized to 6.0 to 7.0 within the first 10 to 20 cm, or pancreatic enzymes stay inactive. Bicarbonate from the pancreas does that work, while bile salts emulsify fat into droplets small enough for lipase to reach. Most of the fat in a meal is broken down in this first stretch, and the duodenum also releases the signals that slow stomach emptying when fat or protein arrives too fast. This is why a cat with duodenal inflammation vomits within 30 to 60 minutes of eating rather than hours later.
Jejunum: about a meter of quiet work
The jejunum makes up close to 1 meter, more than half the total length, with the tallest villi and the densest brush border. Transit here runs 40 to 90 minutes, which is the window where 80 to 90 percent of amino acids and sugars cross the lining. Glucose goes through the SGLT1 transporter with sodium, fructose through GLUT5, and dipeptides through PepT1, which is why hydrolyzed protein diets are absorbed well even when whole protein digestion is poor. Fatty acids longer than 12 carbons are packaged into chylomicrons and leave by lymph, bypassing the liver on the first pass. A cat with jejunal disease loses weight while eating normally, because the food still moves but the surface that takes it in is gone.
Ileum: the 20 cm that pays for itself
The ileum is only 15 to 25 cm long and absorbs little bulk, but it owns 2 jobs nothing else can do. It takes up cobalamin (vitamin B12) through a receptor that needs intrinsic factor, and it reclaims 90 to 95 percent of the bile salts the liver sent out, cycling that pool 6 to 10 times a day. Remove or inflame the ileum and both fail quietly: bile salts spill into the colon and pull water with them, producing diarrhea, while cobalamin stores, which last a few months, drain until a deficiency shows as weight loss, dull coat, and poor appetite. A serum cobalamin below the reference range is one of the most useful signals that the problem sits in the small intestine rather than the stomach.
Colon: 25 to 40 cm of water management
The colon handles what is left after 3 segments have taken their share: water, sodium, chloride, and the short-chain fatty acids that colon bacteria make from fiber. Transit here is slow, 12 to 36 hours, and 90 percent of the water in incoming material is reclaimed. Stool leaves at 65 to 75 percent water, which is the only reason a cat on dry food can stay hydrated at all. Hydration also shows in urine concentration, which has its own reading rules. When colonic transit speeds up, water recovery falls and stool turns loose; when it slows, water recovery rises and stool turns into small hard pieces.
All 3 segments on one timeline
| Segment | Length | Transit | Main job | Failure shows as |
|---|---|---|---|---|
| Duodenum | 10 to 15 cm | 5 to 15 min | Neutralize chyme, start fat digestion | Vomiting 30 to 60 min after eating |
| Jejunum | 80 to 100 cm | 40 to 90 min | Amino acids, sugars, fat uptake | Weight loss with a normal appetite |
| Ileum | 15 to 25 cm | 10 to 30 min | Cobalamin, bile salt recycling | Low B12, watery diarrhea |
| Colon | 25 to 40 cm | 12 to 36 h | Water and electrolyte recovery | Loose stool or hard small stool |
What poor absorption looks like in the box
Fat that is not absorbed leaves pale, greasy, bulky stool, often 2 to 3 times normal volume, that sticks to the litter and smells sharper than usual. Protein that escapes absorption feeds colonic bacteria and produces gas, borborygmus 2 to 3 hours after a meal, and stool with visible mucus. Rapid transit shows up as undigested muscle fibers in stool, which look like small dark threads, and as 3 or more stools a day. Weight is the summary number: a 5 percent loss over 2 months in a cat that eats well points at the small intestine, not at pickiness.
Feeding to match the machinery
- Split the daily ration into 3 to 4 meals; a single 60 g meal loads more chyme than 1 meter of jejunum handles comfortably.
- Keep fat at 15 to 20 percent of dry matter for most cats, and drop to 10 to 12 percent only under veterinary direction.
- Raise digestibility rather than volume: a diet at 85 to 90 percent protein digestibility leaves less substrate for the colon.
- Fast 8 to 12 hours before an abdominal ultrasound; a full stomach hides the pancreas and pushes the intestine aside.
- Recheck weight every 2 weeks and cobalamin every 3 to 6 months in a cat with known small intestine disease.
Blood in stool, vomiting more than 2 times a week, or weight loss of 5 percent in 2 months needs a vet visit with a fecal exam and bloodwork, not a food change at home.
Terms mentioned in this article
Frequently asked
How long does food take to pass through a cat?
A normal meal moves through the small intestine in 3 to 5 hours and the colon in 12 to 36 hours, so total transit runs roughly 15 to 40 hours. Wet food and small meals move faster than a large dry meal. Stool seen 8 hours after eating is usually from an earlier meal, not the one just finished.
Why does my cat lose weight but still eat well?
Weight loss with a good appetite points at the small intestine, where surface area rather than food volume limits absorption. Ask for a serum cobalamin, a folate, and a fecal exam, and weigh the cat every 2 weeks: a 5 percent drop in 2 months is enough to justify imaging.
Do cats absorb nutrients in the colon?
Only water, sodium, chloride, and short-chain fatty acids made by bacteria. Protein, fat, and most sugars must be absorbed in the small intestine. That is why colonic disease produces water problems and stool changes, while small intestine disease produces weight loss and nutrient deficits.