Physiology of Food Intake
Food intake is controlled by a complex interaction of internal and external influences, including gastrointestinal, environmental, and CNS phenomena. Knowledge of the factors that influence normal feeding activity in cats enables the selection of feeding regimens and foods that will help maximize intake in clinically ill patients. The gastrointestinal aspects of food intake include gastric distension, rate of gastric emptying, the release of gastrointestinal hormones such as cholecystokinin and bombesin, and the absorption of nutrients such as amino acids, fatty acids, and glucose. The ingestion of a protein meal results in the absorption of amino acids that cross the blood-brain barrier and directly alter the concentration of neurotransmitters in the CNS. Neurotransmitters of importance in the control of food intake include the catecholamines operating through alpha adrenergic receptors (stimulatory) and beta adrenergic receptors (inhibitory); serotonin (inhibitory); dopamine (inhibitory); and opioids (stimulatory).(1) Aberrations in any of the internal control systems for appetite, hunger, and satiety can result in pathological changes in food intake. External influences of food intake include diet palatability, food composition and texture, and the timing and environment of meals.
Feeding Behavior of the Cat
The introduction of a new therapeutic diet or an unfamiliar environment can contribute to a cat’s unwillingness to eat. Palatability, a concept which includes aroma, taste, texture, and consistency, comprises several unique features in the cat.(2) Cats prefer diets containing high fat and moderate levels of protein.(3) When added to certain cat foods, nucleotides and monosodium glutamate improve palatability synergistically.(4) Cats display a strong preference to substances that taste acid to humans. Phosphoric acid sprayed on dry cat food is a potent palatability enhancer. Acidic foods with a high moisture, protein, and fat content that are most likely to be accepted by cats, include meats,(5) fish, dairy products (especially yogurt), eggs, and poultry. Rancidity negatively affects palatability in cats. It is important to recognize that cats, more than other domestic animals, are strongly influenced by habit in their selection of foods and are less likely to switch easily from one type of food to another. Cats rely heavily upon their sense of smell in the selection of foods and prefer strong meat, fish, or cheese odors. Cats are more likely to accept a novel diet unless they are put in a new environment or stressful situation.(6,7) It is often assumed that, as hunger increases, poor palatability becomes less important in limiting food intake. In cats, however, in the laboratory and in the home environment, a nutritionally complete but relatively unpalatable diet will be refused for long periods.
Food aversion appears to be an important component of the anorexia of cats, especially when the cat is offered food immediately before or after the administration of a drug or treatment (methotrexate, doxorubicin, erythromycin, tetracycline) known to cause nausea or vomiting. Food aversion also appears to be an important component of the anorexia of cats with hepatic lipidosis. Cats that refuse to eat a diet that they associate with nausea may continue to avoid that diet even after full recovery due to the association with the unpleasant sensation. One should therefore tube-feed these cats as soon as the diagnosis of hepatic lipidosis has been made, rather than force-feed several commercial diets that the cat can develop an aversion to. Cats should not be offered any food by mouth for approximately seven days following placement of a feeding tube. Cats showing an interest to eat can then be presented with food.
Anorexia in Cats
One of the most common manifestations of illness in companion animals is a lack of appetite. Cats are especially likely to develop anorexia when they are ill, necessitating a thorough search for the primary cause if identifiable. Inflammatory, infectious, or neoplastic diseases can cause inappetance probably due to release of cytokines such as interleukin-1 and tumor necrosis factor. Painful oropharyngeal lesions such as lymphocytic/plasmacytic stomatitis can cause reluctance to eat or dysphagia. Nausea secondary to chemotherapy administration or due to delayed gastric emptying can result in inappetance, probably via stimulation of the chemoreceptor trigger zone and via gastric distension, respectively. Anorexia is commonly seen in diseases of the gastrointestinal tract and liver. Feline pancreatitis is associated with vague and nonspecific signs, including lethargy and anorexia. The historical lack of sensitive and specific markers for feline pancreatic disease, as well as the low index of suspicion for pancreatic disorders in the cat, has contributed to the relatively infrequent antemortal diagnosis of pancreatitis in cats. Speculation is also increasing between the association of feline liver disease (hepatic lipidosis and cholangiohepatitis), inflammatory bowel disease, and pancreatitis, all disorders that are commonly associated with anorexia in the cat. Anorexia may also be a prominent feature of extra intestinal diseases. Chronic pain from any source may lead to a loss of appetite. Anorexia may be profound in severe congestive heart failure and may be a major symptom in cats with uremia, respiratory failure, and various endocrinopathies (diabetic ketoacidosis). In cats with cancer, anorexia may result from a decreased sense of taste and smell secondary to chemotherapy, pain, the effect of the tumor on the gastrointestinal tract (e.g., partial intestinal obstruction) or liver (metastases), and possibly, the release of an anorectic substance by the tumor (tumor necrosis factor). In addition, cancer chemotherapeutic agents are associated with nausea and anorexia secondary to stimulation of the CRTZ and toxicity to the enterocytes. Medications such as antihypertensives, diuretics, antibiotics, and narcotic analgesics may also cause anorexia.
Diagnostic Approach to the Anorectic Cat
Anorexia is a prominent symptom in a wide variety of intestinal and extra intestinal disorders but as an isolated symptom is of little specific diagnostic value. An accurate medical and dietary history is integral to defining the potential causes for anorexia and weight loss. Diets of poor palatability result in poor acceptability of the food and an inability to satisfy caloric requirements. The cat is not anorectic per se, because a more palatable diet would be readily consumed. Calculation of the cat’s metabolizable energy requirement (MER) (Table 1) and energy density of the diet will help determine whether an animal with weight loss is actually receiving sufficient calories to maintain its MER. The results of the history and physical examination will help determine the extent and order of laboratory diagnostic testing. A complete blood count and serum biochemical profile, including renal and hepatic evaluation and electrolytes should be obtained. A urinalysis completes the assessment of renal insufficiency and protein losing nephropathy, and helps confirm diabetes mellitus. A complete fecal examination (smear, float) should be performed in patients with gastrointestinal signs such as diarrhea and weight loss. Tests for infectious diseases, such as FeLV, FIV, and toxoplasmosis are useful in cats. Radiographic evaluation of the chest and ultrasound evaluation of the abdomen should be performed if laboratory findings fail to uncover a specific cause for the anorexia.
Management of Anorexia
The primary goal of therapy in managing the anorectic cat with or without weight loss is to support the patient while correcting the underlying cause. Careful attention should be paid to alleviation of pain. Symptomatic therapy includes correction of fluid and electrolyte imbalances, reduction of environmental stressors, and dietary modification to improve palatability. Palatability of the diet can be improved by adding flavored toppings such as chicken or beef broths, seasoning with garlic powder, increasing the fat or protein content, varying the texture or appearance of the food, and heating the food to improve its aroma and temperature. Strained chicken, lamb, veal, and baby foods are frequently accepted when foods that are more solid are rejected. The abovementioned diets are not complete and balanced and should not be fed as the sole source of nutrition for more than two to three weeks. The manner in which the food is presented is also important for influencing feeding. Foods with the texture of baby food are useful because they can be easily force-fed or placed on the cat’s paws. Putting a small amount of food on the cat’s lips or paws usually stimulates a licking response. If palatable food is then placed directly in front of the mouth, the cat may continue to lick and ingest the food possibly due to dopaminergic stimulation. Gerber® baby foods should be avoided because of the onion powder present in their meat-based products. Heinz body anemia has been well documented in cats secondary to onion powder ingestion.
Feeding bowls used should be wide and shallow, so the sides do not touch the cat’s whiskers as it is eating. Many cats do not tolerate having their whiskers touched and, therefore, lift food out of a bowl with their paws. Offering several smaller fresh meals frequently is associated with greater food intake than if a single bowl of food were offered once a day. Mild stimulation of the cat by petting and stroking its back frequently stimulates eating if a palatable food is present. If the environment outside the cage is noisy or the cat appears to be disturbed by other cats or dogs, the food can be placed in a cardboard box or a towel can be draped over the front of the cage to reduce the anorexigenic stimuli.
If dietary manipulation fails, and the cat is capable of ingesting food, drug therapy can be attempted prior to offering the food (Table 2). Administration of benzodiazepine derivatives (diazepam or oxazepam) or cyproheptadine increases appetite momentarily, but are unreliable for ensuring adequate caloric intake. The benzodiazepine derivatives are contraindicated in patients with severely reduced hepatic function, particularly if they are showing signs of hepatic encephalopathy. In addition, the appetite-stimulating properties of these agents appear to wane with time when used in sick animals. Although evidence for the efficacy of B vitamins is difficult to find, a deficiency of B vitamins is associated with anorexia. The addition of B vitamin complex to intravenous fluids (1cc/L) is a simple way of ensuring an adequate source of these vitamins to anorexic cats. Enteral feeding techniques should be considered if these efforts fail or if prolonged nutritional support (greater than several days) is required. The preferred route of nutrient administration is by oral or enteral feeding. Enteral feeding is the safest, simplest, least expensive, and most physiologic route, and should be used whenever possible. In cats that are totally or partially anorexic, enteral feeding can be accomplished by one of several techniques: appetite stimulation, oral force feeding, and tube feeding.
Oral force-feeding is of limited benefit in cats that have been anorectic for prolonged periods (> 2 weeks) and can be stressful when performed. It should therefore only be attempted for a short time (two to three days) and abandoned in favor of tube-feeding techniques if unsuccessful to induce voluntary food intake. Force-feeding via nasogastric intubation or gastrostomy tube is the most reliable and efficient method for ensuring adequate alimentation. Nasogastric tubes are preferable for the short-term support of the anorexic cat (< 7 days) and necessitate the alimentation of commercial liquefied feline diets (CliniCare Maintenance; Pet Ag Inc., Hampshire, IL, or Hill’s Prescription Diet a/d., Hill’s Pet Nutrition Inc., Topeka, KS, or Iams Eukanuba Recovery Formula; Dayton, OH). Gastrostomy tubes placement is preferred for long-term alimentation and permits feeding of a variety of commercial canned diets.
TABLE 1: Calculation of Energy Requirements of Adult Cats at Maintenance
Formula 1:
Convert body weight from lbs to kg by dividing by 2.2
Calculate resting energy requirement (RER)
RER = 70(BWkg)0.75 or 30(BWkg) + 70
Maintenance energy requirement (MER) = 1.2-1.4(RER)
Formula 2:
Convert body weight from lbs to kg by dividing by 2.2
MER = BWkg x 60-80
TABLE 2: Appetite Stimulants
Benzodiazepine derivatives:
Diazepam 0.2 mg/kg IV
Oxazepam 0.5 mg/kg PO q 12-24 hours
Analgesics:
Butorphenol 0.2 mg/kg SC
Antiserotonergic:
Cyproheptadine 0.2-0.5 mg/kg PO q 12 hours
Antiemetic:
Metoclopramide 0.2-0.4 mg/kg SC or PO
References
1. Leibowitz SF. Hypothalamic neurotransmitters in relation to normal and disturbed eating patterns. In: Human Obesity, eds. RJ Wurtman and JJ Wurtman, pp. 137-143. New York Academy of Sciences, New York, 1987.
2. Houpt KA. Ingestive behavior problems of dogs and cats. Vet Clin North Am Small Anim Pract 12:683-690, 1982.
3. Kane E, Morris JG, Rogers QR. Acceptability and digestibility by adult cats of diets made with various sources and levels of fat. J Anim Sci 53:1516-1523, 1981.
4. Allen TA. Food preference and palatability. Proc 9th ACVIM Forum, 239-242, 1991.
5. Adamec RE. The interaction of hunger and preying in domestic cat (Felis catus): An adaptive hierarchy? Behavioral Biology 18:263-272, 1976.
6. Mugford RA. External influences on the feeding of carnivores. In: The Chemical and Nutrition, ed. M. Kare and O. Maller, pp 25-50. New York Academic Press, 1977.
7. Mugford RA and Thorne CJ. Comparative studies of meal patterns in pet and laboratory housed dogs and cats. In: Nutrition of the Dog and Cat, ed. RS Anderson, pp. 3-14. Oxford: Pergamon Press, 1980.