Porth's Essentials of Pathophysiology, 4e

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Disorders of Nutritional Status

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E, and K. They require dietary fat for absorption and transport. The nine water-soluble vitamins are vitamin C and the B vitamins, which are thiamine, riboflavin, niacin, vitamin B 6 , pantothenic acid, vitamin B 12 , folate, and bio- tin. Because excess amounts of the water-soluble vitamins are excreted in the urine, it is less likely that they may become toxic to the body, but the fat-soluble vitamins are stored in adipose tissue and may reach toxic levels. Minerals are inorganic elements, not compounds; however, they often function as components of cer- tain enzyme systems, vitamins, and hormones. They are involved in energy metabolism, acid-base balance, maintenance of normal hemoglobin levels, and play a role in nervous system function, are involved in muscle contraction and skeletal development and maintenance, and are major components of bone tissue. Minerals that are present in relatively large amounts in the body are called major minerals . These include calcium, phospho- rus, sodium, chloride, potassium, magnesium, and sul- fur. The remainder are classified as trace minerals ; they include iron, manganese, copper, iodine, zinc, cobalt, fluoride, chromium, molybdenum, and selenium. Fiber Fiber cannot be digested by the human gastrointestinal system and is not classified as a nutrient; however, it increases stool bulk and facilitates bowel movements. Soluble fiber, the type that produces a gel in the intestinal tract, binds with cholesterol and prevents it from being absorbed by the body. Soluble fiber also lowers blood glucose. More studies are needed to establish whether fiber prevents colon cancer and promotes weight loss. The adequate daily intake for fiber for adult men and women up to age 50 is 38 and 25 g, respectively. Adults over age 50 should consume 30 and 21 g, respectively, each day. The recommendation for children ranges from 19 to 31 g, and the recommendation for teenagers is similar to that for adults. 7 Regulation of Food Intake Stability of body weight and composition over time requires that energy intake matches energy utilization. Environmental, cultural, genetic, and psychological fac- tors all influence food intake and energy expenditure. In addition, body weight is tightly controlled by various physiologic feedback control systems that contribute to the regulation of hunger and food intake. Hunger, Appetite, and Satiety Hunger and appetite are closely associated with food intake. The sensation of hunger is associated with sev- eral sensory perceptions, such as the rhythmic contrac- tions of the stomach and that “empty feeling” in the stomach that stimulates a person to seek food. A per- son’s appetite is the desire for a particular type of food. It is useful in helping the person determine the type of food that is eaten. Satiety is the feeling of fullness or decreased desire for food.

The hypothalamus contains the feeding center for hunger and satiety 1 (Fig. 10-1). It receives neural input from the gastrointestinal tract, which provides informa- tion about stomach distention, chemical signals from nutrients (glucose, amino acids, and fatty acids) in the blood, and input from the cerebral cortex regarding the smell, sight, and taste of the food. Centers in the hypo- thalamus also control the secretion of several hormones (e.g., thyroid and adrenocortical hormones) that regu- late energy balance and metabolism.

– + –

Hypothalamus

Vagus nerve

Fat cells

Stomach

Ghrelin

Leptin

GLP-1

Insulin

Pancreas

Small intestine

CCK

Ghrelin

Large intestine

FIGURE 10-1. Feedback mechanisms for control of food intake. Black lines leading to − indicate feedback signals that decrease appetite and feeding, and blue lines with + indicate feedback signals that increase appetite and feeding. Stretch receptors in the stomach activate sensory afferent vagal pathways that inhibit food intake. Glucagon-like peptide-1 (GLP-1), cholecystokinin (CCK), and insulin are gastrointestinal hormones that are released by the ingestion of food and suppress further feeding. Ghrelin, which is released by the stomach and small intestine, especially during fasting, stimulates appetite. Leptin is a hormone produced by increasing amounts of fat cells as they increase in size; it inhibits food intake. (Modified from Guyton AC, Hall JE. Textbook of Medical Physiology. 11th ed. Philadelphia, PA: Elsevier Saunders; 2006:868.)