Chapter 7 Anorexia

Pathophysiology and Mechanisms

Control of Food Intake

Regulation of initiation, maintenance, and termination of food intake is complex and involves integration of numerous internal and external stimuli by different areas of the central nervous system (CNS). In addition to signals for hunger and satiety, there are other factors influencing initiation and maintenance of food intake. Understanding these various stimuli will help clinicians assess which factors may be involved when evaluating anorexic patients.

Sensory Signals

Sensory signals affecting feeding behavior fall into two categories: orosensory and postingestive. When one thinks of orosensory stimuli, the concept of flavor immediately comes to mind; however, olfaction and perception of physical attributes of a food (e.g., temperature, texture, consistency) contribute to the relative appeal of that food. Sense of smell is more highly developed in dogs and cats than in humans; consequently, it is likely to have a greater role in influencing food intake in these species. Warming food often enhances its aroma, and this may explain why dogs and cats prefer foods that are warmed to approximately their body temperature. Food acceptance drops off sharply, however, when the food is perceptibly warmer than normal body temperature.1

Although olfaction is involved in initiation of food intake and contributes to what is perceived as the flavor of food, the way foods taste is important for maintaining food consumption. Dogs and cats are believed to be able to taste salty, bitter, acidic, and meaty (umami) flavors. Dogs, but not cats, can also taste sweet flavors.2 Increasing fat and protein content of foods generally improves palatability for dogs and cats, as will adding natural sweeteners for dogs. Cats, on the other hand, seem to have a preference for slightly acidic flavors. Texture and consistency of a food can be an important aspect of palatability for companion animals. Dogs and cats generally dislike sticky or powdery foods, while increasing moisture content correlates with enhanced appeal. However, individuals can develop strong preferences for a particulate texture or kibble shape and refuse foods of higher moisture content.

Postingestive sensory signals come from the gastrointestinal tract and prompt termination of a meal. Mechanoreceptors located in the stomach, for example, detect gastric distention and provide feedback signals to the brain via vagal stimulation.

Metabolic Signals

There are several orexigenic factors that are produced in the CNS; however, neuropeptide Y (NPY) plays the central role in regulating appetite in the hypothalamus. A variety of signals that influence NPY production will increase or decrease food intake.3 Decreased circulating insulin concentration can increase NPY levels while postprandial elevations in serum insulin concentration decreases NPY. Leptin, a peptide secreted by adipocytes, plays a key role in maintaining energy balance. Leptin levels increase as fat reserves increase, which reduces NPY production. The polyphagic effect of glucocorticoids is believed to be due to these substances increasing NPY while there is evidence that certain cytokines have the opposite effect on NPY production.

Digestion and absorption of nutrients from a meal also trigger release of factors that feedback to the CNS. Cholecystokinin release by the gut in response to arrival of fat and certain amino acids in the duodenum suppresses food consumption. Peptide YY is another factor believed to inhibit food intake; it is released by the gut after food ingestion.

Differential Diagnosis

Any condition or circumstance interfering with initiation of food intake or triggering negative stimuli during food consumption can lead to anorexia. Inciting causes can include pathologic conditions, drug therapy, and alterations in the patient’s diet, environment, and psychological status.

Reduced food intake is common in patients with organ failure, neoplasia, and conditions producing inflammation or hyperthermia. Underlying mechanism can be one or more circulating factors that act on appetite and satiety centers in the brain, including certain cytokines (e.g., tumor necrosis factor [TNF]-α, interleukin [IL]-1, interferon) or substances that would normally be metabolized by affected organs (e.g., uremic toxins). Chronic pain may directly cause negative stimuli that discourage food ingestion or may generate factors that trigger a reduction in appetite. Gastrointestinal tract diseases are often associated with anorexia. The cause of reduced food consumption may be pain or discomfort in conjunction with the act of eating (e.g., severe periodontal disease, stomatitis, dysphagia, pancreatitis, gastroenteritis). Another significant negative stimulus for food intake is nausea, a frequent clinical sign in gastrointestinal disorders including gastroenteritis, inflammatory bowel disease, pancreatitis, and hepatic diseases. Anorexia is a frequent consequence of conditions leading to hypokalemia and gastrointestinal ileus.

Anorexia may be iatrogenic in some patients. Factors that disrupt a patient’s normal routine (e.g., stress of hospitalization and treatment, change in diet or feeding management) can interfere with learned feeding behaviors. Many pharmaceutical agents affect appetite through the same mechanisms at work in gastrointestinal disorders (Box 7-1). Nausea and vomiting are common side effects of many classes of drugs, including antibiotics, cardiac glycosides, and chemotherapeutic agents. Nonsteroidal antiinflammatory drugs, corticosteroids, and chemotherapeutic agents can cause gastrointestinal tract pathology. Adynamic ileus is a side effect of some analgesic agents, particularly the narcotic analgesic agents.

One consequence of a patient experiencing noxious stimuli when it eats is that it may come to associate pain, nausea, or indisposition with the diet being fed or even with the act of eating. This phenomenon is called learned food aversion and can be a contributing factor in development and persistence of anorexia for many companion animals.

Evaluation of the Patient

Because anorexia is nonspecific and associated with a broad spectrum of conditions, the diagnostic plan is initially predicated on the medical history and physical examination. There are two specific aspects of the history and physical examination, however, that should receive special attention: the patient’s dietary history and the assessment of body condition.

Jul 10, 2016 | Posted by in INTERNAL MEDICINE | Comments Off on Anorexia
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