Nutrition and management of GI disease (Proceedings)

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

• 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.

Frequency of feeding

• 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.

Nutrient composition

Protein considerations

• The small intestinal mucosa receives the vast majority of its nutrients from the chyme present in the intestinal lumen, NOT from the blood supplied to the small intestine. In particular, glutamine present in protein is the preferred source of energy and nitrogen for the small intestinal mucosa. This is crucial because a lot of energy is required to produce new epithelial cells, which are completely renewed every 3-7 days, required to maintain the vital absorptive and secretory functions of the intestinal mucosa. Thus, in as few as 3 days of anorexia, the GI mucosa is being starved of nutrients vital to its survival.

• Of all the components of the diet, protein is the most antigenic, and in animals with dietary sensitivities, animal proteins are most likely to be the cause of the food allergy. No particular animal protein is less antigenic than others, it is merely a question of exposure and sensitization. For example, lamb is not "hypoallergenic" relative to chicken, beef or rabbit – it simply has been used in the past to treat dietary sensitivities because it was novel.

• In many GI diseases, especially inflammatory diseases like IBD (inflammatory bowel disease), the GI mucosa is compromised (e.g. has reduced digestive or absorptive capacity). In these situations, the diet should contain protein sources that are more easily digested (muscle, egg and milk protein, not fascia, connective tissue or some plant proteins) and of good biological value. This increases the probability that the protein will be digested and absorbed, so that the animal's energy and protein needs will be met. Further, it decreases the chances that intact proteins (which are more immunogenic) will not cross the damaged mucosa – causing more inflammation and possibly and an allergic reaction.

Fat considerations

• Dietary fat has several physiologic effects on the GI tract.

• It slows the rate of gastric emptying, so that foods higher in fat content are retained longer in the stomach.

• Diets high in fat decrease the tone of the lower esophageal sphincter. This may make gastroesophageal reflux more likely, especially in animals predisposed to it.

• Fat present in the duodenum is the strongest secretagogue for pancreatic exocrine secretions (e.g. lipase, colipase, etc), and bile. This is the premise behind feeding a high fat diets to dogs or cats when performing a bile acid assay – to achieve maximum gall bladder contraction. It is also the reason that diets for dogs or cats with pancreatitis or a tendency to develop pancreatitis are formulated with low levels of dietary fat.

• There are several reasons why fats are present in normal pet food diets: as an energy source, they increase palatability, and provide essential fatty acids. In general, dietary fat is highly digestible in normal animals, however, because fat digestion is a complex, multi-step process, it is easily disrupted in animals with some GI, pancreatic or liver diseases. This is important because undigested fats end up in the colon where bacteria use it to increase their numbers (overgrowth, bacterial colitis), and produce osmotic (free fatty acids) and secretory (hydroxy fatty acids) molecules from the fat. The end result of increased fat reaching the colon is diarrhea.

Steps that may be disrupted:

a) Pancreatic colipase and lipase are required to initiate digestion of luminal fats, and are decreased or absent in animals with exocrine pancreatic insufficiency.

b) Bile acids are required for emulsification (packaging) of fats into chylomicrons for absorption into the lymphatics. Dogs or cats with severe cholestatic liver disease (intrahepatic or extrahepatic) and reduced production, release, or re-uptake of bile acids, have greatly decreased fat digestion.

c) Severe intestinal diseases (e.g. loss of epithelial function in IBD, histoplasmosis, etc., or loss of lymphatic function, e.g. lymphangiectasia) also reduce the ability of animals to digest and absorb fats.

d) Lymphatic obstruction (lymphostasis) due to neoplasia, inflammation, or obstruction also prevents absorption of digested fats and results in steatorrhea (increased fat in the feces).

Carbohydrate considerations

• Carbohydrates (CHO) are present in diets as simple sugars, disaccharides and polysaccharides whose primary purpose is to serve as a readily available source of energy. There is no requirement for CHO (e.g. there are no essential CHO) in either dog or cat diets, but they are inexpensive and readily available (notice I didn't say digestible) sources of energy for use in pet foods.

• Not all CHO are created equal....nor are starch digesting enzymes (e.g. amylase) able to breakdown all CHO. The digestible CHO present in most pet foods are α-linked CHO, such as rice, potatoes and other starches (e.g. corn, barley, wheat and tapioca). The most digestible of the many starch sources is rice, which is the CHO most often used in diets made for GI upset.

• Diets containing high amounts of simple sugars are the soft-moist foods.

• CHO intolerance can be observed in animals consuming diets containing more CHO than their previous diet. Diarrhea is a common problem in young kittens being switched from high protein/low CHO canned kitten food (or from milk onto dry) as there is a change in the microflora due to the new diet constituents, and any undigested CHO will result in osmotic diarrhea.

a) CHO intolerance is also observed in dogs or cats unable to digest certain types of sugar (e.g. lactose), thus, causing diarrhea and increased flatus (gas) production.

• β-linked CHO are found in the woody (e.g. cellulose) and meaty (e.g. guar gum) parts of plants, and make up a group of CHO called dietary fibers. The CHO present in dietary fiber are only made available when bacteria, with the enzymes necessary to break the β-linkage, digest them.

a) Dietary fibers are a complex group of CHO that are generally divided into two separate groups: insoluble (poorly fermentable) and soluble (highly fermentable) fibers.

b) Both soluble and insoluble fibers prolong digestion and absorption of other nutrients in the small intestine. For this reason, increased dietary fiber is recommended in dogs with diabetes mellitus, to slow absorption of dietary CHO and reduce post-prandial hyperglycemia.

c) In general, the amount or type of fiber that should be present in pet foods is widely debated. This is especially true in cats, whose natural carnivorous diet would contain little to no fiber, except that found in the digestive tract of prey or that consumed from chewing on grass. However, commercial therapeutic diets containing fiber may have 0-20% insoluble fiber, 0-7% soluble fiber, or a combination of soluble and insoluble fibers.

d) Other than its effects on digestion in the small bowel, fiber in the diet is primarily present to nourish colonic epithelial cells and ameliorate the signs of colonic disease. Colonocytes are like small intestinal epithelial cells in that they receive the majority of their energy from luminal sources. However, they are different in that they preferentially utilize butyrate (a four carbon, short chain fatty acid produced by bacterial fermentation of fiber, and other luminal CHO) for their metabolic energy needs. Because soluble fibers are more fermentable (digestible) than insoluble fibers, more butyrate is formed from these dietary fiber sources.

The effects of soluble and insoluble fibers

• The beneficial effects of fiber in colitis may be due to normalization of colonic segmental contractions, or its influence on colonic bacterial populations, or its ability to bind toxins or other luminal irritants (e.g. bile acids, fatty acids) and prevent further irritation.

• Finally, constipation is a common problem in older animals, and frequently diets high in insoluble fiber are recommended to increase fecal bulk and improve colonic contractions and motility. This is effective as long as the animal is adequately hydrated (dehydrated animals will pull even more fluid from the colon, making the feces even drier and harder), and as long as adequate colonic contractility remains (e.g. cats with obstipation or megacolon do not respond to high fiber diets). Thus, no blanket recommendations exist: if clinical response is not achieved with the diet that you believe most appropriate, then the alternate diet approach should be attempted.

Dietary intervention in GI disease

Highly digestible diets

This term is not defined in a regulatory sense, but has generally been reserved for products with protein digestibility of > 87% (typical diets are 78-81%), and fat/CHO digestibilities > 90% (typical diets are 77-85% and 69-79%, respectively). In general, the commercially available diets in this category are formulated with highly digestible protein and carbohydrate sources, have moderate to low levels of fat, and some soluble fiber, but generally very low concentrations of insoluble fiber. Some pet food manufacturers add increased amounts of omega 3 fatty acids (fish oil), fructooligosaccharides or mannosoligosaccharides (prebiotics), and added anti-oxidant vitamins and minerals. The products from each company have different formulations and additives, and thus, individual animals may respond differently to each of these different diets. Just because an animal does not respond to one highly digestible diet, does not mean that all highly digestible diets will be ineffective as their compositions are quite variable.

Novel antigen or elimination diets: adverse reactions to food

Adverse reactions to food are generally classified by dividing them into immunological and non-immunological categories. The immunological (or immune-mediated) causes of adverse reactions to food are the true food allergies or food sensitivities, and can be IgE or non-IgE mediated events. These types of events are associated with both dermatologic and/or gastrointestinal disease in affected dogs and cats. One would guess that the condition is both readily diagnosed and common, based on the number of exclusion diets available on the pet food market; however, in reality neither is true. Other causes of diarrhea and pruritus are more common, and many causes of diarrhea will respond to dietary manipulation. Thus, these other differentials should be ruled out before an elimination trial is performed (as it is both expensive and difficult to perform in households which have multiple pets).

Non-immunological causes of adverse reactions to food are in two categories: food intolerance and dietary indiscretion. Food intolerance primarily causes signs of GI upset, but occasionally dermatologic signs such as pruritus can be observed, and will confuse the issue. Food intolerance may be caused by metabolic (e.g. lactose intolerance), pharmacologic (intolerance of food additives, etc), or idiosyncratic (any food substance, additive, preservative, etc that is results in an adverse reaction in the GI tract). Food poisoning can also be a form of food intolerance. Most animals will food intolerance will respond well (in a period of 2-3 weeks) to a diet change to one that does not contain the offending agent. That may or may not require feeding an exclusion/novel antigen diet, but often animals respond well to those diets because they are devoid of many of the offending agents.

Ideally diets used for adverse reactions to food should contain: 1) a reduced number of novel, highly digestible protein sources or a protein hydrolysate (no intact proteins, molecular weights < 10,000 daltons), 2) assure protein is highly digestible, 3) avoid additives and vasoactive substances (e.g. preservatives, antimicrobials, humectants, coloring agents, flavors, flavor enhancers, emulsifying agents, stabilizers, and thickeners, and 4) be nutritionally adequate for the animal's lifestage. The above recommendations can be achieved with a commercial novel antigen diet or by preparing a home-made elimination diet. For the purposes of a dietary trial, a home-made elimination diet (using a single, novel protein source and a single, highly digestible CHO source) may be reasonable and very effective. However, for long term feeding, home-made diets must be balanced, or nutritional deficiencies will result. This is particularly important in cats because of their specific and unique nutritional requirements.

Dietary trials for dietary sensitivity should last a minimum of 8-12 weeks. Diet trials for dietary intolerance are much shorter, and in general, a positive response will be since within 2 weeks if the diet is acceptable.

Increased dietary fiber

Many, if not most, commercial pet foods contain small amounts of soluble or insoluble fiber or both. However, the "high" fiber diets are generally those products containing increased amounts of insoluble fibers. Fiber amounts have generally been designated as normal (< 5% of dry matter), mildly increased (5-10% of dry matter), moderately increased (10-15% of dry matter), and greatly increased or high fiber (15-30% of dry matter). For many weight control or weight loss diets, an increased amounts of dietary fiber is added to the food to dilute calories, induce satiety and limit food consumption. While there is a great deal of difficulty in assessing satiety in dogs and cats, the use of increased amounts of dietary fiber in dog foods continues to be advocated. The second major use for high fiber diets is for dogs and cats with colitis – for the reasons discussed above. The key point to remember is this: not all dogs or cats will respond appropriately to diets high in fiber, or to certain types of dietary fiber. Thus, a one diet fits all approach is to be discouraged.