Normal Physiologic Control of Metabolic Rate
Caloric use is largely a function of the basal metabolic rate, which is controlled by the net action of several hormones. Thyroid hormones set the basal metabolic rate by increasing carbohydrate, amino acid, and lipid uptake by cells. They increase oxygen consumption and heat production within the mitochondria. Thyroid hormones work in combination with growth hormone to stimulate normal growth and development in adolescents. An increase in serum thyroid hormones (as occurs in hyperthyroidism) causes increased caloric use and severe weight loss, whereas a decrease in thyroid hormones (as occurs in hypothyroidism) causes decreased caloric utilization and weight gain if dietary calories are not reduced.
Insulin is released by the beta cells of the pancreas to maintain blood glucose levels. Insulin has anabolic effects by mediating the intracellular movement of glucose, amino acids, and fatty acids. Insulin stimulates the conversion of these substances into storage products such as glycogen, triglycerides, and proteins. Obesity induces insulin resistance through various receptor and postreceptor mechanisms. Therefore, more insulin must be released to maintain normal blood glucose levels, which in turn cause more storage of triglycerides in adipocytes, worsening the obesity. Eventually this may lead to exhaustion or destruction of the beta cells and culminates in a diabetic state. This pathologic mechanism plays an important role in the obesity–diabetes connection.
Glucocorticoids are important counterregulatory hormones to the actions of insulin. They stimulate gluconeogenesis and glycogen storage while causing protein catabolism and redistribution of fat into the liver and abdomen. Therefore, excessive glucocorticoids induce insulin resistance, muscle wasting, and hepatomegaly due to the accumulation of glycogen and fat. Catecholamines, glucagon, somatostatin, and growth hormones are also counterregulatory hormones that oppose insulin action.
Weight Loss
Weight loss is a physical condition that results from a negative caloric balance such as when metabolic utilization and excretion of essential nutrients exceed the supply. Weight loss does not necessarily imply malnutrition; however, in many disease states this is actually what occurs. When the nutritive deficiency is restricted to calories, stored fats may be lost, which may be desirable as in obesity. Weight loss may result from the loss of body fluids as in dehydration or from the elimination of ascites or edema. In this chapter, only weight loss due to undernutrition (calorie deficiency, protein deficiency, or both) will be discussed. The reader is referred to other chapters dealing with the problems of edema and ascites.
Weight loss is considered significant when a 10% decrease in normal body weight occurs unassociated with loss of body fluids. Certainly, undernutrition is present when the small reserves of body protein are gone and body tissue protein is utilized for calories. Previous weights and body condition scores should be consulted when one considers the significance of any weight loss. In addition, body conformation, muscle mass, and bone structure characteristics of the different breeds should be considered.
Emaciation is extreme weight loss due to severe undernutrition and is characterized by prominence of the skeleton due to catabolism of body fat and protein. Cachexia is a state of extreme ill health associated with weight loss, anorexia, weakness, and mental depression.
Pathophysiology of Weight Loss
Weight loss, unrelated to loss of water, is caused by a negative caloric balance. Several basic mechanisms can cause negative caloric balance including increased caloric utilization, increased loss of nutrients, decreased intake of calories, or malassimilation of nutrients (see Table 10-1). Weight loss may be beneficial, as in the treatment of obesity, or pathologic.
Table 10-1. Causes of weight loss
| 1. Decreased Caloric Intake |
| a. Anorexia |
| b. Oropharyngeal disease |
| c. Starvation |
| d. Dietary deficiency |
| e. Vomiting and regurgitation |
| 2. Malassimilation (maldigestion/malabsorption) |
| a. Exocrine pancreas—pancreatic enzyme deficiency |
| b. Small intestine—villous atrophy, inflammatory bowel disease, lymphosarcoma, and lymphagiectasia |
| 3. Loss of caloric nutrients |
| a. Protein-losing enteropathy (albumin and globulins) |
| b. Protein-losing nephropathy (albumin) |
| c. Diabetes mellitus (glucose) |
| d. Burns (serum proteins) |
| e. Intestinal parasites |
| f. Chronic blood loss |
| 4. Increased caloric use (increased metabolism) |
| a. Hyperthyroidism |
| b. Malignancy |
| c. Fever |
| d. Heart failure |
| e. Infection/sepsis |
| f. Burns |
| g. Trauma |
| 5. Decreased caloric production a. Hepatic failure |
| a. Hepatic failure |
| 6. Physiologic causes |
| a. Pregnancy |
| b. Lactation |
| c. Extreme exercise |
| d. Increased thermogenesis (decrease ambient temperature) |
Causes of Weight Loss
While there are numerous causes of weight loss, there are five basic mechanisms commonly involved: decreased caloric intake, malassimilation of nutrients, increased caloric loss, increased caloric usage, and decreased caloric production. One or any combination of these mechanisms may be responsible for weight loss.
Decreased Caloric Intake.
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