Dog Health and Nutrition  

Dog digestive system

Food gets broken down into a simple form that can be absorbed and used by the body in a process called “digestion.” In mammals, this process takes place in the digestive or alimentary tract--often simply called the “gut.” This is a hollow tube the food passes through and is acted upon by secretions from organs that discharge into the tube. These secretions contain digestive enzymes that speed up the process of hydrolysis, by which food is broken down.

The three major classes of nutrients that need to be digested are carbohydrates, protein and fat. Other nutrients (minerals, vitamins and water) are absorbed in more or less the same form as they are found in food. But they may need to be released from proteins, fats or carbohydrates before they can be absorbed.

Digestion begins in the mouth

Digestion begins in the mouth, where food is mechanically broken down and mixed with saliva before it’s swallowed. Although dogs aren’t strictly carnivores, their teeth are particularly suited to meat eating, and can cut, chew and crush food. Still, many dogs have a tendency to bolt down their food, often chewing only the toughest of foods before swallowing.

The sight and smell of food stimulates the flow of saliva, causing the dribbling and “lip smacking” often seen at mealtimes! Once the food arrives in the mouth, its taste and physical presence help increase saliva production. Saliva contains mucus, a very effective lubricant that coats the food to help with swallowing.

What the stomach does

When food is swallowed, it passes down the esophagus, whose muscles contract with a “wave” motion called peristalsis, and arrives at the stomach within a few seconds. The stomach has several functions. It’s a storage organ; it’s a mixing bag, where more digestive enzymes are added to the food; and it’s a regulator valve that controls the rate of flow into the small intestine. Protein digestion begins in the stomach.

The stomach secretions contain protein-digesting enzymes (proteases), hydrochloric acid, and mucus. The major enzyme, pepsin, is secreted in an inactive form, pepsinogen, to stop it from digesting the cells that produce it. Pepsinogen is activated in the stomach in the presence of hydrochloric acid, which also creates the correct acid environment for the enzymes to function at their optimum rate. Mucus lubricates the food, and protects the lining of the stomach wall (which is largely protein) from being digested by its own enzymes. The secretion of acid, mucus and enzymes depends on the composition and quantity of food eaten, and is regulated by hormones and nerves.

The wall of the stomach is muscular, particularly in the pyloric region. The stomach contents are mixed thoroughly, and push towards the pyloric sphincter--a muscular ring that acts as a regulator valve. By this time, the mixture is a thick milky liquid called chyme, and several factors control its passage into the small intestine. Strong waves in the stomach cause the pyloric sphincter to relax, and allow food to pass into the duodenum (the first part of the small intestine). Chyme passes through more easily when it’s very fluid.

On the other hand, the rate of emptying is reduced by the presence of chyme, acids, fats or irritants in the duodenum, which inhibit movements in the stomach. This ensures that the stomach contents are well mixed and sufficiently well digested before they leave the stomach. It also ensures that the small intestine doesn’t receive more chyme than it can cope with efficiently.

The work of the small intestine

The duodenum is the main site for digestion in the small intestine. Here, more enzymes are added to the chyme, some of which come from the intestinal wall and others from the pancreas. The pancreas is one of the major glands of the body, and has two functions: releasing digestive enzymes into the gut, and releasing hormones into the blood. Pancreatic juice also contains sodium bicarbonate, which neutralizes the acid chyme arriving in the duodenum, and provides an alkaline environment for optimum functioning of pancreatic and intestinal enzymes. These enzymes include proteases to continue protein digestion, amylase for carbohydrate digestion, and lipase for fat digestion. Enzymes in the intestinal juice generally start off the later stages of digestion.

The regulation of pancreatic juice release is largely controlled by two hormones--secretin and pancreozymin. These are secreted from cells in the wall of the small intestine. Another important function of the pancreas is to secrete the hormone insulin into the bloodstream to control blood sugar levels.

The liver is the other major organ associated with the small intestine. Bile is produced continuously in the liver, stored in the gall bladder, and passed into the gut through the bile duct when it’s needed. Bile contains bile salts that act like detergents by turning fat into tiny globules that can then be processed by the lipase enzymes in pancreatic juice. It’s the pigments in bile that give feces their characteristic color.

The digestion of food is completed in the small intestine, and once the food has been broken down to its simplest form, it can be absorbed across the wall of the intestine and into the blood. The end products of digestion are carried to the liver, where they are metabolized. Fat is absorbed into the lymph vessels, and is later transferred to the bloodstream.

The small intestines are very long, and absorption takes place along its entire length. Folds and finger-like projections, villi, in the lining of intestinal wall dramatically increase the surface area for absorption. In some dogs, the absorptive area of the small intestine may be as large as the floor of a small room!

The role of the large intestine

By the time the food that’s been eaten reaches the large intestine, most of the nutrients have been digested and absorbed. In this part of the gut, water is absorbed, and some fermentation of dietary fiber by bacteria takes place. This process is responsible for the production of gas, often associated with flatulence!

Feces are around 60-70% water, and the rest is made up of undigested food, dead bacteria and some inorganic material. The feces are stored in the rectum and evacuated through the anal sphincter. Although defecation is voluntary, problems may occur in old age or during bouts of diarrhea or other illness.

Measuring the digestibility of food

For any given food, we can discover the amount of each nutrient present, using chemical analysis. But this doesn’t give a true picture of the actual nutritional value of the food, since only nutrients absorbed from the digestive system are of use to the animal. A proportion of each nutrient eaten will inevitably be lost in the feces.

Digestibility is a better measure, because it shows the availability of the nutrient content of the food. We can calculate digestibility from the difference between the nutrient intake in food and that voided in feces.

Since feces consist not only of undigested, unabsorbed material but also cell debris and material excreted into the digestive tract, the difference between intake and output measured in this way is called “apparent digestibility.” To measure true digestibility, it’s necessary to use control diets free of the nutrient being studied, to establish the output when the intake is zero. For most practical purposes, apparent digestibility is used, as it measures the net amount of digestion.

Within the same species, digestibility is more a characteristic of the food than the individual animal. But the digestibility of a particular food will be different if it’s fed to two different species of animal—dog and cat, for example--because of differences in their digestive systems.

One way of illustrating these differences is to compare the length of the gut with body length. Herbivores such as the horse have a high ratio, since vegetative foods generally require more prolonged digestion than animal-derived materials. In omnivores such as dogs and people, the ratio is lower. And carnivores such as cats have the lowest ratio of all.

So, diets with a high vegetable content tend to have lower digestibility in dogs because of their indigestible fiber content, whereas the digestibility of meat-based diets is usually very high.

Digestibility values provide an index that can be used to estimate how much of the food must be fed to a normal, healthy individual in order to supply the correct amount of nutrients and energy. Where the digestibility value is low, a larger quantity of the food must be eaten to meet the requirements of the animal. Similarly, a diet of low digestibility will result in the production of a greater volume of feces.