Disturbances of Hunger and Appetite
In addition to the broad sociologic and economic influences on access to and intake of nutrients, food intake is influenced by (1) volition, learned behaviors, and psychiatric disorders; (2) gastrointestinal and systemic hormones; (3) the microbiome; (4) CNS regulatory mechanisms; (5) the presence of gastrointestinal or systemic diseases; and (6) functional or mechanical digestive tract disorders that retard the normal flow of intraluminal contents. Some individuals have an amazing ability to lose weight by reducing calorie intake, but diets in general have had a disappointingly limited long-term impact on the management of obesity. Brain-gut communications with ghrelin, leptin, cholecystokinin, (CCK), GLP-1, neuropeptide Y, and orexin are potent mediators of appetite (see Plates 1-46 and 1-56). In the CNS, serotonin and neuropeptide Y pathways are particularly active. Learned behavior and visual, olfactory, and auditory stimuli initiate reflexes via cortical connections to the hypothalamus and limbic system. These complex mediators interact in ways that are incompletely understood to lead to a variety of specific eating disorders.
Polyphagia, overeating, binge eating, and obesity are among the most important issues influencing our society. Hyperorexia, or food intake in excess of body requirements even when it poses a severe risk to health, is a formidable medical challenge. It is the most common preventable cause of a host of malignant, endocrine, cardiovascular, musculoskeletal, and respiratory disorders, as well as of cirrhosis and hepatocellular carcinoma associated with a fatty liver. In some persons, such behaviors may be a reaction to stress, obsession, or depression, but it is clear that learned, inherited, and acquired factors influence overeating, as do genetic influences. The fecal microbiota, acquired during child- hood and altered by antibiotics, influence the risk for obesity. The hyperorexia of diabetes and hyperthyroid- ism does not result in obesity, because nutrient stores have been depleted by concomitant nutritional wastage or energy expenditure.
Anorexia describes any state in which the severe depletion of body nutrients fails to lead to adaptive behavior. The appetite is commonly impaired in systemic disorders and disorders of the digestive tract, including neoplasms, pancreatitis, hepatitis, and colitis. The release of tumor necrosis factor alpha, interleukins, and corticotropin-releasing hormone in these disorders contributes to anorexia. Once poor intake has caused calorie deficiency, ketone excess may lead to further anorexia and food deprivation. The hormone orexin has an important contribution to impaired appetite in systemic diseases. Severe nutritional deficiency leading to pancreatic and epithelial atrophy can further exacerbate inadequate intake with malabsorption.
Several psychiatric disorders can lead to impaired, even life-threatening, inadequate food intake, including bulimia and anorexia nervosa. Anorexia nervosa is loss of appetite amounting to a disgust or distaste for food and a fear of gaining weight. The patient has intense concerns about the body habitus and a phobia about being overweight or gaining weight. Severe forms of anorexia lead to nutritional and metabolic deficiencies, fluid and electrolyte deficiencies, cachexia, osteoporosis, infertility, amenorrhea, heart damage due to a beriberi type of condition, and death. Gastric emptying is often delayed in such malnourished patients but should not be interpreted as the primary disorder.
Bulimia is a related condition in which the patient restricts nutritional intake by self-induced vomiting after a meal, often in a surreptitious manner, or by purging with laxatives. Rumination syndrome and cyclic vomiting are related disorders. Management should always include psychiatric consultation by experts in eating disorders. Hospitalization and enteral feeding may be necessary in severe cases. Although the causes are multifactorial and incompletely understood, genetic factors are often involved.
Decreased appetite resulting from food aversion is known as sitophobia. This is common in patients with painful swallowing (odynophagia) and other conditions that result in pain in response to food intake, such as gastritis and gastric ulcers. Odynophagia results from breaks in the oropharyngeal or esophageal mucosa.
Impairment of appetite with excessive smoking may be due in part to impairment of taste sensations. Impaired appetite is also common in patients with xerostomia following irradiation or with Sjögren syndrome.
Street drugs, particularly cocaine, methamphetamines, and other stimulants, are potent appetite suppressants and should be considered in the differential diagnosis of all patients with anorexia. Similarly, marijuana and other forms of tetrahydrocannabinols commonly lead to cyclic vomiting that mimics bulimia.
Parorexia is an abnormal desire for certain substances, such as the craving for salt in uncontrolled Addison disease or for chalk in calcium deficiency states. The desire in early pregnancy for sour foodstuffs or other selective and often unusual foods is another example. Its mechanism is incompletely understood.