Lesson 1, Topic 1
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06-Metabolic and Eating Disorders

March 28, 2021

Metabolic imbalances
Disorders characterized by a disruption or imbalance of normal metabolism can be caused by several different factors.
For example, inborn errors of metabolism are a group of genetic conditions involving a deficiency or absence of a particular enzyme. Specific enzymes are required by cells to carry out each step of every metabolic reaction. Although an abnormal genetic code may affect the production of only a single enzyme, the resulting abnormal metabolism may have widespread effects. Specific diseases resulting from inborn errors of metabolism, such as phenylketonuria (PKU), are discussed in Chapter 25.
A number of metabolic disorders are complications of other conditions. For example, you may recall from Chapter 12 that both hyperthyroidism and hypothyroidism have profound effects on the BMR. Diabetes mellitus affects metabolism 543throughout the body when an insulin deficiency limits the amount of glucose available for use by the cells.

Eating disorders
Some metabolic disorders result from disruptions of normal mechanisms in the body that maintain homeostasis. For example, the body has several mechanisms that maintain a relatively constant level of glucose in the blood—glucose that is required by cells for life-sustaining catabolism. As mentioned earlier in this chapter, during starvation or in certain eating disorders, these mechanisms may become unbalanced as they attempt to maintain blood glucose homeostasis.
A few of the more well-known eating and nutrition disorders are briefly described in the following sections.

Anorexia nervosa
A behavioral disorder characterized by chronic refusal to eat, anorexia nervosa often results from an abnormal fear of becoming obese. This condition is most commonly seen in teenage girls and young adult women and is often linked to emotional stress.
Treatment plans are usually directed at solving the resulting nutritional deficit, while at the same time dealing with the underlying behavioral problem.

Bulimia
Bulimia is a behavioral disorder characterized by insatiable craving for food alternating with periods of self-deprivation. The self-deprivation that follows a “food binge” is often accompanied by depression.
People with a form of this disorder called bulimarexia purposely induce the vomiting reflex to purge themselves of food they just ate. Excessive vomiting in this way can have a variety of consequences, including damage to the esophagus, pharynx, mouth, and teeth by stomach acid.

Obesity
Obesity is not an eating disorder itself but may be a result of chronic overeating behavior. Like anorexia nervosa and bulimia, eating disorders characterized by chronic overeating usually have an underlying emotional cause.

Obesity is defined as an abnormal increase in the proportion of fat in the body. Most of the excess fat is stored in the subcutaneous tissue and around the viscera. Obesity is a risk factor for a variety of chronic life-threatening diseases, including diabetes mellitus, many forms of cancer, and heart disease.

Protein-calorie malnutrition
Protein-calorie malnutrition (PCM) is an abnormal condition resulting from a deficiency of calories in general and protein in particular. PCM is likely to result from reduced intake of food but may also be caused by increased nutrient loss or increased use of nutrients by the body. Table 19-6 summarizes a few of the wide variety of conditions that may lead to PCM.

 

Marasmus results from an overall lack of calories and proteins, such as when sufficient quantities of food are not available. Marasmus is characterized by progressive wasting of muscle and subcutaneous tissue accompanied by fluid and electrolyte imbalances.

Kwashiorkor results from a diet that has sufficient calories, but is deficient in protein—as when a child is weaned from milk to low-protein foods. Kwashiorkor also causes wasting of tissues, but unlike marasmus, it is accompanied by pronounced ascites (abdominal bloating) and flaking dermatitis. The ascites results from a deficiency of plasma proteins, which changes the osmotic balance of the blood and thus promotes osmosis of water from the blood into the peritoneal space (see Figure 18-24).

Nutrition disorders, including many specific deficiency diseases, are summarized in Appendix A at evolve.elsevier.com