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.
