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

The process of digestion is the form of breaking down big molecules into smaller ones to absorb into the body. The breakdown happens in two ways: mechanical and enzymatic. M

Mechanical breakdown is the chewing, mixing, and movement of food through intestines. 

Enzymatic breakdown is a chemical reaction where molecules are broken down to absorbable compounds. 

Both mechanical and enzymatic breakdown start in the mouth with chewing action, and the amylase enzyme released with saliva starts to break down starch chemically into sugars. 

In the Mouth: 

In the mouth, both mechanical and chemical breakdowns of food begin. Saliva is a lubricant that helps food carry itself. Dogs and cats are monogastric, and they start with good mechanical breakdown of the food they consume. 

Saliva is also an evaporative cooling method for dogs. 

Dogs have more premolars and molars than cats. This is needed when food needs to be crushed more. It also suggest dogs are more omnivorous type creatures than cats. 

Dogs and cats are very sensitive to amino acids, organic acids, and nucleotides. Therefore, dry pet food uses a palatant, which is usually sourced from yeast and its hydrolyzed versions. These yeast provide improve the acceptability of pet foods. They also like salty products. 

 

Dogs and cats also have several intriguing differences in taste receptors. Dogs especially like sweet things, and theobromine toxicity due to eating chocolate is a significant risk. Cats don't show much affinity towards sweet flavors. 

Cats are very sensitive and attracted to bitterness; quinine, tannic acid, and alkaloids are bitter to cats. Dogs are repelled by bitter flavors. 

Down the Hatch: Esophagus

Food passes from mouth to stomach through the esophagus. The mucus layer of the esophagus is also a lubricant. The food enters into the stomach with the relaxation of the cardiac sphincter. 

The Stomach: Reservoir, Mixer, Protein Starter

The stomach acts as a reservoir. Dogs want to eat fewer large meals than cats, which prefer small and frequent meals. The stomach also initiates chemical digestion, possibly of fats for dogs as well. 

With the entrance of food, gastric mucus, HCl, and the pepsinogen enzyme start to be secreted by glands from the mucosal lining. In dogs, there is also mucosal lipase, but it is not as effective as pancreatic lipase. The mucus protects stomach tissue from acid, and it is also a lubricant. The HCl in the stomach is also important for the enzyme pepsinogen to become pepsin. Pepsin hydrolyzes proteins into amino acids. 

Both neurological and hormonal stimulation are important for secretion of HCl and mucus. Neurological stimuli start with smell or anticipation of eating. Psychological stimuli such as fear and stress can also affect gastric secretion. The gastrin hormone is responsible for release HCl and mucus once food enters into the stomach. Enterogastrone is secreted when fat enters the duodenum. This enzyme counteracts gastrin. The pyloric sphincter is the muscle that relaxes, and food (chyme) enters the duodenum. This muscle controls the rate of food released into the small intestine. 

The Small Intestine: Where the Real Work Happens

Most of digestion up to the small intestine is mechanical. Pepsin activity in the stomach hydrolyzes some proteins, but most fats and carbs are intact. The major part of digestion and absorption occurs in the small intestine. Intestinal villi increase the surface area of the small intestine. Mucus is also secreted by Brunner's gland to lubricate the food mass. 

The pancreas and glands on the duodenum produce enzymes—intestinal lipase, amino peptidase, dipeptidase, nucleotidase, nucleosidase, and enterokinase—that break down macronutrients. 

The final digestion of carbohydrates happens at the end of the small intestine. The enzymes—maltase, lactase, and sucrase—are released to break down disaccharides into monosaccharides.  

Bile is also important for fat emulsification, which increases the surface area of fat. The bile also activates certain lipases. The secretin hormone is responsible for pancreatic bicarbonate secretion to adjust the pH so that enzymes can work properly. 

Microbes in the Small Intestine

There are healthy microbes in the small intestine that help digestion and production of certain vitamins. In the duodenum and jejunum, streptococci, lactobacilli, and bifidobacterium spp. exist. In the ileum, some anaerobic and E. coli bacteria exist. Cats have different bacteria in the duodenum, such as Pasteurella, Bacteroides, Eubacteria, and Fusobacteria. 

Short-chain fatty acids produced by bacteria is beneficial for the small intestine. FOS (fructooligosaccharide) is also beneficial for microbes to work. MOS is also beneficial to intestinal microflora. 

How Nutrients Cross the Wall

In the small intestine, macronutrients are broken down into their smaller molecules. These molecules are also absorbed in the small intestine and taken to cells throughout the body.  

Villi increase the inside surface area by 600 times compared to the outside area of the small intestine, and they are also responsible for the absorption of nutrients. on the surface of villi, there are microvilli and enterocytes that can absorb nutrients 

The passing through of nutrients can occur via passive diffusion and active pump in the cell membranes. 

The Large Intestine: Reclaiming Water, Housing Microbes

The major function of the large intestine is to recover water and certain electrolytes. There is also a microflora that can ferment dietary fibers. It is important to have favorable fermentable fibers and microflora in the large intestine because the microflora can take up short-chain fatty acids and ferment sulfur-containing amino acids to reduce the smell in the feces. Methane and sulfur dioxide can also be the end result of incomplete fermentation. Pet food may be supplemented by beta-glucans and MOS to reduce the bad smells and short-chain free fatty acids to reduce smell and improve gut health. 

What This Means for Everyday Feeding

Your pet can truly use a well-digestible formula. While most cats prefer multiple small feedings, many dogs can eat fewer, larger meals. Certain fibers, abrupt dietary changes, and extremely high fat loads can all cause gas or slow stomach emptying. Fermentable fibers can promote a more beneficial microbiome when used thoughtfully. Above all, keep in mind that the small intestine is the key to the breakdown and absorption of nutrients; therefore, ingredients and processing that improve digestibility frequently result in improved stool quality, more consistent energy, and general well-being.