Copyright (c) The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 1 CHAPTER 42 CONTROL OF ENERGY BALANCE, METABOLIC RATE, AND BODY TEMPERATURE Prepared by Brenda Leady, University of Toledo

2 Nutrient utilization and storage Nutrient utilization divided into 2 alternating phases  Absorptive phase – occurs when ingested nutrients enter blood stream from GI tract Some of ingested nutrients used for immediate energy needs, others stored  Postabsorptive stage – occurs when GI tract empty of nutrients so body’s stores used

3 Absorbed carbohydrates Chief monomers absorbed are glucose, galactose, and fructose Glucose is 1 of 2 major energy sources during absorptive phase (other is fat) Much of absorbed glucose used to synthesize ATP Skeletal muscle major consumer of glucose  Can store as glycogen Excess glucose stored as glycogen in liver or triglycerides

4 Absorptive phase

5 Triglycerides Too large to diffuse across plasma membrane of intestinal epithelial cells  Digested into monoglycerides and fatty acids  Diffuse into intestinal epithelial cells  Resynthesized into triglycerides  Packaged into chylomicrons for blood transport Lipoprotein lipase releases fatty acids to diffuse into cells of the body Some ingested fat used during absorptive phase for energy

6 Amino acids Taken up by all body cells Used to synthesize proteins Excess amino acids not stored as protein Excess converted by liver cells into carbohydrate or fat

7 Postabsorptive period Synthesis of glycogen and fat slows and breakdown begins Blood glucose concentration maintained  Reactions that provide glucose to blood  Using fat instead of glucose for energy (glucose sparing)

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9 Reactions that provide glucose to blood  Glycogenolysis – glycogen hydrolysis in liver  Gluconeogenesis – liver converts noncarbohydrates into glucose Lipolysis releases fatty acids (alternate energy source to glucose) and glycerol (converted to glucose by liver)  Proteins can also be converted to glucose in prolonged fasting

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12 Table 42.1

13 Glucose sparing Most organs and tissues increase fat utilization for energy Reserves glucose for use by nervous system Essential step is lipolysis  Fatty acids used by cells for energy Liver uses fatty acids to make ketones  Prolonged energy source for many tissues, including  brain  Less protein breakdown required

14 Regulation of phases Insulin  Made by pancreas  Binds to cell surface receptor, stimulates signaling pathway, facilitates glucose diffusion into cells  Glucose transporters (GLUTs) in intracellular vesicles move to the surface  Also inhibits glycogenolysis and gluconeogenesis in the liver

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GLUT Proteins Transport Glucose in Animal Cells All animal cells require transporters to move glucose across plasma membranes GLUTs make up a family of at least 14 related proteins Expressed in different tissues Structures similar across phyla – gene duplication of common ancestral gene Different GLUTS vary in ability to bind glucose  High affinity – binds even at low glucose concentrations Muscle and fat have low affinity GLUT4 (also only one requiring insulin) Brain cells have high affinity GLUT

18 Controlling insulin secretion Blood glucose rises during absorptive state so insulin secretion rises Blood glucose levels fall during postabsorptive state so insulin secretion falls Negative feedback Input from nerve cells also plays role  Eating meal increases insulin secretion  Stressful situation decreases insulin secretion

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20 Maintaining blood glucose levels Blood glucose must remain within the normal homeostatic range Too low and even high affinity GLUT receptors in the brain will not take in glucose Glucagon – from the pancreas stimulates glycogenolysis, gluconeogenesis, and ketone synthesis

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22 Food intake Satiety (fullness) – involved in short term control of feeding Satiety signals – remove sensation of hunger and set time period before hunger returns again  Stretch receptors in the stomach and small intestines send signals to the brain  Stomach and small intestine release hormones to suppress appetite

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24 Long-term control of food intake mediated by many different brain molecules, by hormones, and by emotional state, particularly in humans Leptin – produced in adipose tissue in proportion to fat mass  As more fat is stored in the body, more leptin is secreted into the blood  Acts on brain centers to reduce appetite and increase metabolic rate  Decreased fat mass decreases leptin which decreases BMR and increases appetite

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Coleman Revealed a Satiety Factor in Mammals Parabiosis – surgical connection between bloodstreams of 2 animals 1 st experiment – connected wild type mouse to ob mouse  Ob mouse ate less and gained less weight than usual  Blood of wild type mouse contained factor signaling sufficient fat stores 2 nd experiment – connected wild type mouse to db mouse  Db mouse continued to gain weight while wild type mouse lost weight Obesity phenotype the same – factor was absent in ob mice and present but unable to act in db mice Most obese humans like db mice – produce leptin but fail to respond to it

29 Countercurrent heat exchange  Conserves heat  Dolphin flipper or bird legs  Heat moves from warm arteries to adjacent veins carrying cooler blood  Reduces heat lost to environment  Fish keep muscles warm without losing heat through gills

30 Impact on public health Obesity  Body Mass Index (BMI) of 30 or greater Lack of physical activity reduces daily energy expenditure  “Apples” versus “pears” – location of fat has health consequences  Genetic factors – thrifty genes may have been survival mechanism Underweight  Anorexia nervosa – reduce food intake to the point of starving, biological and psychological causes  Bulimia nervosa – recurrent episodes of fasting and overeating, may include self induced vomiting or laxative use  Treatments rely on counseling, nutritional education and medication