Digestion & Absorption in Dogs and Cats: A Clear, Vet-Friendly Guide

Good nutrition isn’t just about what goes into the bowl—it’s about what the body can break down, absorb, and use. In dogs and cats, digestion transforms big, complex nutrients into small, soluble pieces that can cross the gut wall and fuel every cell. This journey starts the moment food enters the mouth and ends when undigested residues leave the body as feces.

From First Bite: Mouth & Taste

The mouth brings food in, grinds it (a little or a lot, depending on the eater), and mixes it with saliva so chewing and swallowing are easier. Dogs and cats both have incisors and canines for grasping and tearing, but dogs carry more premolars and molars than cats, a hint that dogs evolved with a more omnivorous menu while cats stayed closer to strict carnivory. Taste plays a surprisingly important role: both species detect amino acids, organic acids, and nucleotides commonly found in meat. Dogs typically enjoy sweet flavors and tend to dislike bitterness, whereas cats ignore sweet tastes and are tuned to detect bitter compounds—an evolutionary nudge toward meat-heavy choices rather than fruit and plant sugars.

Down the Hatch: Esophagus

Swallowed food slides down the esophagus with help from mucus and coordinated muscular waves. At the stomach entrance, a ring of muscle called the cardiac sphincter relaxes to let food in, then tightens again to prevent reflux.

The Stomach: Reservoir, Mixer, Protein Starter

The stomach stores meals (especially handy for dogs that prefer bigger, less frequent feedings), bathes them in acid and enzymes, and meters out small amounts of chyme into the small intestine. Hydrochloric acid keeps pH low and activates pepsin from pepsinogen to begin protein breakdown into smaller chains. Dogs secrete gastric lipase, but most meaningful fat digestion still happens later in the small intestine. Hormones and nerves orchestrate this phase: gastrin boosts acid and motility when the stomach stretches with food, while enterogastrone released from the duodenum taps the brakes when fatty chyme arrives. Gastric emptying slows with larger meals, thicker or more viscous foods, higher fat content, and certain fiber types; in cats, dry kibbles can leave the stomach more slowly than canned foods, and even kibble shape can make a difference.

The Small Intestine: Where the Real Work Happens

Chemical digestion and absorption peak here. The pancreas sends in amylase for starch, lipase for fat, and a suite of proteases—trypsin, chymotrypsin, carboxypeptidase, and nuclease—many released as inactive precursors and switched on in the intestinal lumen. The intestinal lining adds its own enzymes: brush-border maltase, lactase, and sucrase split disaccharides into glucose, galactose, and fructose; aminopeptidases and dipeptidases finish protein fragments into absorbable amino acids; nucleotidase and nucleosidase process nucleic acids. The pancreas also delivers bicarbonate to neutralize acid, creating the ideal pH for enzymes to work. Meanwhile, bile from the liver (stored in the gallbladder) emulsifies dietary fat and activates lipases so tiny, water-friendly micelles can form and carry fatty components to the cell surface for uptake. Hormones keep things synchronized: secretin prompts bicarbonate release and bile flow when acidic chyme arrives, and cholecystokinin squeezes the gallbladder and stimulates pancreatic enzyme release in response to fat.

Microbes in the Small Intestine

Healthy dogs and cats carry relatively modest bacterial populations in the small intestine compared with the colon, but those microbes still help maintain a balanced environment and can produce beneficial short-chain fatty acids. Species and segment differences exist—dogs often host streptococci, lactobacilli, and bifidobacteria in upper segments with more anaerobes distally, while cats may show higher overall counts and a different mix. Certain fermentable fibers such as fructooligosaccharides and mannanoligosaccharides can encourage “good” bacteria and discourage less friendly strains.

How Nutrients Cross the Wall

The inner surface of the small intestine is built for absorption, with folds, villi, and brush-border microvilli multiplying surface area hundreds of times. Enterocytes—the absorptive cells—live fast, turn over in just a few days, and move nutrients into circulation using several strategies. Water and electrolytes can move by passive diffusion according to osmotic gradients. Many sugars and amino acids hitch rides on carrier proteins that use sodium-linked transport, allowing movement even against concentration differences. Small peptides that enter cells are usually split into single amino acids before being released to the bloodstream. Monosaccharides and amino acids drain into villus capillaries and then the portal vein to the liver, where glucose may become glycogen and amino acids are routed to tissues or transformed as needed. Fats follow a different route: micelles deliver fatty acids and monoglycerides to the brush border, enterocytes reassemble them into triglycerides, package them as chylomicrons or VLDL with cholesterol and phospholipids, and send them into the lymphatic lacteals to join the bloodstream near the heart. Fat-soluble vitamins ride along with normal fat absorption, most minerals are absorbed in ionized forms, water-soluble vitamins diffuse or use carriers at low intakes, and vitamin B12 uniquely requires intrinsic factor.

The Large Intestine: Reclaiming Water, Housing Microbes

Chyme passes through the ileocecal valve into the large intestine. Compared with herbivores and omnivores like pigs or horses, dogs and cats have a smaller cecum and shorter colon, reflecting their carnivorous heritage. Even so, colonic bacteria ferment some undigested fiber to short-chain fatty acids that nourish colon cells and support gut health. The colon’s main job is to absorb water and sodium efficiently, even without the villi seen in the small intestine. Fecal odor and gas largely reflect what escaped digestion: proteins degraded by bacteria produce indole, skatole, and hydrogen sulfide, while resistant carbohydrates and fermentable fibers generate hydrogen, carbon dioxide, methane, and various short-chain fatty acids. The specific mix depends on diet composition and each animal’s microbial community.

Enzyme End-Products—In Plain Language

By the time digestion is done, starch and other carbohydrates have been reduced by amylase and the brush-border enzymes to glucose, galactose, and fructose; proteins have been cleaved by pepsin in the stomach and pancreatic and intestinal proteases to single amino acids and a few tiny peptides that are promptly finished in the enterocyte; and dietary fats, with help from bile, intestinal and pancreatic lipases, and cholesterol esterase, have become glycerol, free fatty acids, and small glycerides that the body can rebuild and ship where needed.

What This Means for Everyday Feeding

A formula that digests well is one your pet can actually use. For many dogs, fewer, larger meals are fine, while most cats prefer several small feedings. Sudden diet changes, very high fat loads, and certain fibers can slow gastric emptying or increase gas. Thoughtful use of fermentable fibers can support a healthier microbiome. Above all, remember that the small intestine is the star of nutrient breakdown and uptake, so ingredients and processing that enhance digestibility often translate to better stool quality, steadier energy, and overall well-being.