Dog (and Cat) Digestion 101

Diet and exercise. We hear it so frequently that we become numb to it. The truth is, in its simplest form, this is the foundation of keeping our animals younger longer. To have healthy bones, muscles, nerves, brains, and organs, we must supply our animals with optimum nutrition. There is no way around it. However, for them to be able to utilize this nutrition properly, we must back up one step further.  


Crucial to the entire process of health and longevity is digestion. If there is one system that must be functioning at peak performance to improve longevity, it’s the digestive system. Any glitches in this system will ultimately lead to a cascade of chaos in other systems. To understand how to support digestion, we need to first understand how it works.


Digestion is similar, but different, in different species. The information that follows is true for “monogastric” animals which include dogs, cats, and horses (although horses use their hindgut a little differently for fermentation). The process is just a little bit different for “ruminants” such as cattle, sheep, and goats.

the mouth is where it all begins

The sight, smell, and anticipation of a meal triggers the production of saliva. Dogs and cats don’t chew their food like we do, and certainly not as much as herbivores like horses. Mucus in the saliva helps coat the esophagus for safe passage into the stomach. The esophagus is a muscular tube that transports the food via wave-like contractions.


The stomach has 4 primary purposes:


1. Storage

2. Protein digestion

3. Mixing the contents

4. Regulating the flow into the small intestine.


The stomach secretes hydrochloric acid, enzymes for protein digestion, and mucus to protect the stomach lining. These secretions are signaled by the composition of the food and are tightly regulated by nerves and hormones. Taste receptors in the mouth, and throughout the gastrointestinal tract, even influence digestion.


The mixed and partially digested stomach contents are called chyme. When sufficiently liquefied, the muscular outflow opening of the stomach (the pylorus) allows the chyme to pass into the duodenum, the first portion of the small intestine.

the intestines and beyond

Entering the small intestine, the chyme is highly acidic. Bicarbonate secreted from the pancreas raises the pH, so the chyme becomes alkaline. The pancreas also secretes enzymes for protein, carbohydrate, and fat digestion.


The liver produces bile that is stored in the gallbladder. (Interestingly, horses don’t have gallbladders.) When fat enters the small intestine, bile is released through the bile duct into the duodenum. It further alkalinizes the ingesta and facilitates fat digestion by breaking it down into tiny globules.


The intestines are home to complex communities of microbes, including bacteria. Many of these microbes are beneficial and, among a plethora of other crucial functions, aid in digestion and absorption of nutrients. They help keep the lining of intestines intact and healthy so only appropriate particles are absorbed into the bloodstream. This intestinal environment, and the resident microbes, are collectively called the microbiome.


Absorption of nutrients takes place along the length of the small intestine. The lining of the small intestine is folded into numerous fingerlike projections called villi, and each villus is covered in tiny hair-like microvilli. These villi and microvilli create more surface-area for nutrient absorption. The nutrients are passed into the bloodstream where they flow to the liver for sorting and processing before continuing throughout the body. Fat is absorbed first into the lymph, then it joins the bloodstream.

colon absorption followed by the back door exit

If everything is functioning properly, most of the nutrients are removed from the intestinal contents by the time they reach the large intestine, or colon. Here, dietary fiber is fermented by the microbes which produce some vitamins and fatty acids. These vitamins and fatty acids, water, and electrolytes, are absorbed across the colon wall.


After sufficient absorption in the colon, the contents pass through the rectum in the form of feces. Ideally, the feces end up outside rather than on the floor.


Any breakdown in this finely tuned system can lead to obvious problems such as diarrhea or constipation. There are also numerous problems that are less obvious and more insidious that can result from digestive disturbances including allergies, autoimmune diseases, an many manifestations of poor health.

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