Porth's Essentials of Pathophysiology, 4e

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Integrative Body Functions

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Regulatory Mechanisms The control of food intake can be divided into short- term regulation, which is concerned with the amount of food that is consumed at a meal or snack, and inter- mediate and long-term regulation, which is concerned with the maintenance of constant stores of nutrients in the tissues, preventing them from becoming too low or too high. 1 Short-Term Regulation. The short-term regulation of food intake provides a person with the feeling of sati- ety and turns off the desire for eating when adequate food has been consumed. It requires rapid feedback mechanisms that signal the adequacy of food intake before digestion has taken place and nutrients have been absorbed into the blood. These mechanisms include stretch receptors that monitor filling of the gastrointes- tinal tract, oral receptors that monitor food intake, and gastrointestinal tract hormones that suppress or increase food intake. Stretch receptors in the gastrointestinal tract monitor gastrointestinal filling and send inhibitory impulses by way of the vagus nerve to the feeding center to suppress the desire for food. Oral receptors monitor the amount of food that passes through the mouth and sends impulses to the feeding center to suppress food intake. This effect occurs despite the fact that the gastro- intestinal tract has not become the least bit filled and is thought to result from various oral factors such as tast- ing, chewing, and swallowing that meter food intake. 1 However, the inhibition caused by this mechanism is considerably less intense and of shorter duration than that caused by gastrointestinal filling. The gastrointestinal tract hormones that contribute to the short-term regulation of food intake include cho- lecytokinin (CCK), glucagon-like peptide 1 (GLP-1), and ghrelin. 10,11 Cholecytokinin, which is released in response to fat in the duodenum, and GLP-1, which is released from the lower small bowel in response to nutrients, espe- cially carbohydrates, have a strong suppressant effect on the hypothalamic feeding center. Ghrelin is a hormone released mainly from the stomach and to a lesser extent by the intestine. Its levels peak just before eating and then fall after a meal, suggesting that it may also stimulate appetite and eating. 10 The presence of food in the stomach increases the release of insulin and glucagon, both of which sup- press the neurogenic feeding signals from the brain. 1 Intermediate and Long-Term Regulation. The inter- mediate and long-term regulation of food intake is determined by the amount of nutrients that are in the blood and in storage sites. It has long been known that a decrease in blood glucose causes hunger. In contrast, an increase in breakdown products of lipids such as ketoac- ids produces a decrease in appetite. A ketogenic weight loss diet (the Atkins diet) relies partly on the appetite suppressant effects of ketones in the blood. Recent evidence suggests that the hypothalamus also senses the amount of energy through a hormone called leptin that is produced by fat cells. 1 The stimulation of leptin receptors in the hypothalamus has been shown to

produce a decrease in appetite and food intake as well as an increase in metabolic rate and energy consumption. It also produces a decrease in insulin release from the beta cells of the pancreas, which decreases energy storage in fat cells. Assessment of Energy Stores and Nutritional Status The nutritional status of an individual can be assessed in a number of ways including a history of weight gain or loss, dietary intake, gastrointestinal symptoms that affect food intake, functional capacity, and physical signs of fat loss and muscle wasting. This information is usually combined with other objective measures of fat stores and skeletal muscle mass. Anthropometric Measurements Anthropometric measurements provide a means for assessing body composition, particularly fat stores and skeletal muscle mass. 12 This is done by measuring height, weight, body circumferences, and thickness of various skinfold areas. These measurements commonly are used to determine growth patterns in children and appropri- ateness of current weight in adults. Body weight is the most frequently used method of assessing nutritional status; it should be used in combi- nation with measurements of body height to establish whether a person is underweight or overweight. For weight measurement, subjects should ideally be in light clothing and bare feet, fasting, and with an empty blad- der. Repeat measurements are best made at the same time of the day. The body mass index (BMI) uses height and weight to determine healthy weight (Table 10-2). It is calcu- lated by dividing the weight in kilograms by the height in meters squared (BMI = weight [kg]/height [m 2 ]). 13 A BMI less than 18.5 is classified as being underweight and one between 25 and 29.9 is considered over- weight. 13 A BMI at or greater than 30.0 is diagnosed as obesity and is furthered classified into classes I (BMI 30.0 to 34.9), II (BMI 35.0 to 39.9), and III or extreme obesity (BMI > 40). Body weight reflects both lean body mass and adipose tissue and cannot be used as a method for describing body composition or the percentage of fat tis- sue present. Statistically, the best percentage of body fat for men is between 12% and 20%, and for women, it is between 20% and 30%. 14 During physical training, body fat usually decreases, and lean body mass increases. Among the methods used to estimate body fat are body circumferences, skinfold thickness, bioelectrical impedance, computed tomography (CT), and magnetic resonance imaging (MRI). 12 The measurement of body circumferences has received attention because excess visceral (or intra-abdominal) fat is closely associated with metabolic syndrome (i.e., a syndrome described by a collection of cardiovascular risk factors). 15,16 Measurements of skinfold thickness can provide a rea- sonable assessment of body fat, particularly if taken at