The gastrointestinal tract acquires and digests food, absorbs nutrients and water, and expels unabsorbed material in the form of feces. Diet and nutrient composition have multiple effects on the function of the gastrointestinal (GI) tract, thus diet may be included as a cause, or a component of treatment in many GI diseases. Understanding how diet and nutrients may interact to influence GI physiology and pathology will allow the clinician to formulate or select diets to effectively treat specific diseases. The dietary characteristics that can be altered to affect GI function and ameliorate signs of disease include the amount of food, its form, the frequency of feeding, and the nutrient composition of the diet. These will all be reviewed below, followed by a discussion of the major types of commercial diets available to manage different GI diseases.
Amount of diet
Frequency of feeding
- The more food that is put into the GI tract at any given time, the less that gets digested and absorbed.
- The more food that is consumed, the longer it takes for the stomach to empty (e.g. may increase the chance it will be vomited up)
- Diets with increased nutrient density (e.g. recovery formulas, puppy or kitten foods) will also empty from the stomach more slowly. This is important, as while foods with increased nutrient density allow smaller meals to be fed, they will still affect gastric outflow.
- Gastric volume in the normal dog or cat is approximately 40-60 ml/kg body weight.
- In general, when the GI tract is dysfunctional, small, frequent meals are best (e.g. 3-6 meals per day).
- Feeding small, frequent meals results in less gastric retention of food and increases the amount of food that is digested and absorbed.
- Cats in particular are physiologically and anatomically designed to eat small, frequent meals in the wild (e.g. mice, etc), and will tolerate this approach much more readily.
Form of the diet
- In general, the more liquid the diet, the faster it will empty from the stomach. This can be problematic if too much liquid is introduced into the stomach at one time, as the small intestine is not used to dealing with large boluses of food at one time. When too much liquid diet is introduced into the small intestine at one time, it can cause vomiting, diarrhea and cramping – collectively a condition called "dumping" syndrome.
- Liquid diets come in two basic forms: elemental or polymeric. Elemental diets are generally powdered diets containing large amounts of carbohydrate, and small amounts of fat and protein present in the form of glucose, amino acids and fatty acids. These diets are not balanced, but can be used to provide readily available nutrients to the GI tract. There are no complete and balanced elemental diets available for the veterinarian, but some of these diets can be used short term in dogs- they do not contain enough arginine, taurine, carnitine or other AA for cats.
- Polymeric liquid diets are made of intact nutrients, and usually contain balanced (like normal canned or dry pet foods) and complete nutrition. An example of a veterinary polymeric diet is Clinicare.
- In most cases, the clinical problem dictates the form of diet and frequency of feeding that is chosen. For example, if a naso-esophageal feeding tube is employed to feed a cat with hepatic lipidosis, the size of the tube dictates that only liquid diets can be fed through the tube. Also, because liquid diets must be used, and a large volume is required to meet the cat's energy requirements, the food must be administered at a constant rate via an infusion pump. Alternatively, an animal with severe oral disease (e.g. stomatitis, fractured jaw, etc) that makes chewing or eating painful, may be able to lap liquid diets or gruels.
- The best rule of thumb is to feed a diet using a form and frequency that is most appropriate for the region of the GI tract that is functioning. In other words, use a diet that is most likely to be able to pass. For example, if the animal has megaesophagus (dilation of the esophagus secondary to lack of smooth muscle function, e.g. myasthenia gravis, or due to an obstruction, e.g. stricture secondary to esophagitis or neoplasia), the type of food administered is very important in achieving success. In cats or dogs with esophageal strictures, food fed in liquid or gruel form is often able to successfully pass, while other food types will be stuck and regurgitated back up. Alternatively, in animals with megaesophagus due to reduced muscle function, foods fed in meatball form (canned food made up into a ball) or dry may work better than liquids, as they may stimulate esophageal contractions more effectively or be heavy enough to pass down into the stomach with elevated feeding.
- When introducing food into the duodenum or jejunum directly, both form and frequency are extremely important, as the small intestine is used to receiving small boluses of liquid food that is introduced over several hours. Thus, liquid polymeric or elemental diets are administered to animals like IV fluids – with a pump, so the infusion is slow and continuous, to avoid causing further discomfort or diarrhea.