Nutrition, Metabolism, and Body Temperature Regulation 24 Suggested Lecture Outline I. Nutrition (pp. 942–954, Figs. 24.1–24.2, Tables 24.1–24.3) A. A nutrient is a substance in food that is used by the body to promote normal growth, maintenance, and repair. 1. There are six categories of nutrients: carbohydrates, lipids, proteins, vitamins, minerals, and water. 2. Essential nutrients are those that cannot be made by the body and must be obtained in the diet. B. Carbohydrates 1. Except for milk sugar (lactose) and small amounts of glycogen found in meats, all the carbohydrates we ingest are derived from plants. 2. Glucose is the carbohydrate molecule ultimately used by the body as fuel to make ATP. 3. The current recommendation is 125–175 grams of carbohydrates daily with the emphasis on complex carbohydrates. C. Lipids 1. The most common dietary lipids are the neutral fats, triglycerides or triacylglycerols, which occur as saturated fats and unsaturated fats. 2. Cholesterol is another dietary lipid that is found in egg yolk, meats, and milk products. 3. Dietary fats are essential as the major source of fuel for hepatocytes and skeletal muscle, for absorption of fat- soluble vitamins, and as components of the myelin sheaths and cellular membranes of the body. D. Proteins 1. Animal products contain the highest-quality proteins, those with the greatest amount and best ratio of amino acids. a. Proteins in eggs, milk, and meats are considered to be complete proteins that meet all the body’s amino acid requirements for tissue maintenance and growth. b. Legumes, nuts, and cereals are protein-rich but not complete, but when cereal grains and legumes are ingested together they provide all the essential amino acids. 2. Proteins are important as structural materials of the body, enzymes, and hormones. a. All amino acids needed to make a particular protein must be present in a cell at the same time and in sufficient amounts for the protein to be made. b. For optimal protein synthesis the diet needs sufficient carbohydrate or fat calories for ATP production. c. The body is in nitrogen balance when the amount of nitrogen ingested in proteins is equal to the amount lost in urine and feces. d. Anabolic hormones accelerate protein synthesis and growth. 3. The amount of protein a person needs reflects his or her age, size, metabolic rate, and current state of nitrogen balance. E. Vitamins are potent organic compounds needed in small amounts for growth and good health, most of which function as coenzymes. 1. Fat-soluble vitamins (A, D, E, and K) bind to ingested lipids and are absorbed along with their digestion products. 2. Water-soluble vitamins (B complex vitamins and vitamin C) are absorbed along with water from the gastrointestinal tract, and excess amounts are excreted from the body in urine. 3. Vitamins A, C, and E are antioxidants that disarm tissue-damaging free radicals. F. The body requires moderate amounts of seven minerals and trace amounts of about a dozen others that are used by the body to add strength to structures or to act as ions in the blood and cells. II. Metabolism (pp. 955–982, Figs. 24.3–24.22, Tables 24.4–24.7) A. Overview of Metabolic Processes 1. Anabolism is the general term for all reactions in which larger molecules or structures are built from smaller ones. 2. Catabolism refers to all processes that break down complex structures to simpler ones. 3. In oxidation-reduction reactions one substance is oxidized and loses energy by losing electrons, while another substance is reduced and gains energy and electrons that are transferred from the oxidized substance. 4. Our cells use two mechanisms to capture some of the energy liberated through oxidation-reduction reactions to make ATP. a. Substrate-level phosphorylation occurs when high-energy phosphate groups are transferred directly from phosphorylated substrates to ADP. b. Oxidative phosphorylation is carried out by electron transport, which occurs in the cristae of the mitochondria and couples the movement of substances across membranes to chemical reactions. B. Carbohydrate Metabolism 1. Oxidation of glucose a. Glycolysis is a series of ten steps where glucose is converted into two pyruvic acid molecules in the cytosol of cells. b. Krebs cycle occurs in the matrix of the mitochondria where the pyruvic acid is passed through a series of reactions that generate reduced electron carrier molecules, NADH 1 H 1 and FADH2. c. Electron transport chain passes electrons through a series of oxidative phosphorylation reactions that generate 28 ATP molecules. 2. Glycogenesis is the formation of glycogen, the animal storage form of glucose, that occurs when excess glucose is ingested. 3. Glycogenolysis is the breakdown of glycogen into individual glucose molecules that occurs when the blood sugar levels drop. 4. Gluconeogenesis is the process of forming new glucose from noncarbohydrate molecules that occurs in the liver using glycerol and amino acids. C. Lipid Metabolism 1. Most body cells easily convert glycerol to glyceraldehyde phosphate, a glycolysis intermediate that enters the Krebs cycle. 2. Beta oxidation is the first phase of fatty acid metabolism where fatty acid chains are split into two carbon acetic acid fragments and coenzymes are reduced. 3. Lipogenesis is the reformation of triglycerides from unused glycerol and fatty acid chains for storage in the body. 4. Lipolysis is the breakdown of stored fats into glycerol and fatty acids to be used by the body for fuel. D. Protein metabolism occurs whenever proteins have reached the end of their life span and must be broken down to replace the “aged” proteins. 1. Oxidation of Amino Acids a. Transamination is the process of transferring an amine group to alpha-ketoglutaric acid to make glutamic acid. b. Oxidative deamination occurs in the liver and removes the amine group of glutamic acid as ammonia and regenerates alpha-ketoglutaric acid. c. Keto acid modification is used to produce molecules that can be oxidized in the Krebs cycle or converted to glucose from keto acids produced through transamination. E. The body exists in a dynamic catabolic-anabolic state, where substances are continually being broken down and rebuilt. F. Absorptive and Postabsorptive States 1. Absorptive state is the time during and shortly after eating when nutrients are moving into the blood from the GI tract. a. Absorbed monosaccharides are delivered directly to the liver where they are converted into glucose and either used by the cells of the body, stored as glycogen, or converted into fats to be stored. b. Triglycerides are either used for anabolic purposes or stored in adipose tissue. c. Amino acids are delivered to the liver, which deaminates some and uses others to make plasma proteins, but most remain in the blood to be distributed to body cells. d. Insulin directs all events of the absorptive state. 2. Postabsorptive state is the period when the GI tract is empty and energy resources are supplied by the body reserves. a. Sources of blood glucose include glycogen in the liver, skeletal muscle cells, adipose tissues, and cellular proteins. b. Glucose sparing is the increased use of noncarbohydrate fuel molecules for energy to save glucose during times of fasting. c. The sympathetic nervous system and several hormones interact to control the postabsorptive state. G. The Metabolic Role of the Liver 1. The hepatocytes carry out over 500 metabolic functions. 2. Cholesterol serves as the structural basis for bile salts, steroid hormones, vitamin D; as a component of the plasma membrane; and as a signaling molecule in embryonic development. 3. About 85% of cholesterol is made in the liver and other body cells, and is lost from the body in bile salts in feces. 4. Cholesterol is insoluble in water and must be transported in the body bound to small lipid-protein complexes called lipoproteins. 5. Factors Regulating Plasma Cholesterol Levels a. Severe restriction of dietary cholesterol does not lead to a steep reduction in plasma cholesterol levels. b. Saturated fatty acids stimulate liver synthesis of cholesterol and inhibit its excretion from the body. c. Unsaturated fatty acids enhance excretion of cholesterol from the body. d. Trans fatty acids cause a greater increase in LDL levels and a greater reduction in HDL levels than saturated fatty acids. e. Smoking, coffee drinking, and stress increase LDL levels; regular aerobic exercise appears to reduce LDL levels and increase HDL levels. III. Energy Balance (pp. 982–989, Figs. 24.23–24.27) A. A dynamic exists within the body between the energy intake and energy output. 1. Energy intake is the energy liberated during food oxidation. 2. Energy output includes energy lost as heat, energy used to do work, and energy that is stored as fat or glycogen. B. Regulation of Food Intake 1. When energy intake and energy output are balanced weight remains stable; when not balanced weight is either lost or gained. 2. Factors that control eating include: neural signals from the digestive system, bloodborne signals related to body energy stores, hormones, body temperature, and psychological factors. C. The body’s rate of energy output is called the metabolic rate, which is the total heat produced by all the chemical reactions and mechanical work of the body. 1. The basal metabolic rate (BMR) reflects the energy the body needs to perform only its most essential activities. 2. Factors influencing BMR include body surface area, age, gender, stress, and hormones. 3. The total metabolic rate (TMR) is the rate of kilocalorie consumption needed to fuel all ongoing activities both involuntary and voluntary. D. Body temperature regulation represents a balance between heat production and heat loss. 1. The body’s core (organs within the skull, thoracic and abdominal cavities) has the highest temperature and its shell (the skin) has the lowest temperature in most circumstances. 2. Mechanisms of Heat Exchange a. Radiation is the loss of heat in the form of infrared waves (thermal energy). b. Conduction is the transfer of heat from a warmer object to a cooler one when the two are in direct contact with each other. c. Convection occurs when the warm air surrounding the body expands and rises and is replaced by cooler air molecules. d. Evaporation removes large amounts of body heat when water absorbs heat before vaporizing. 3. The hypothalamus is the brain’s main integrating center for thermoregulation, containing the heat-loss center and the heat-promoting center. 4. Heat-promoting mechanisms are triggered when the external temperature is low, or blood temperature falls and the heat-promoting center is activated. a. Vasoconstriction of cutaneous blood vessels b. Increase in metabolic rate c. Shivering d. Enhanced thyroxine release e. Behavioral modifications 5. Heat-loss mechanisms protect the body from excessively high temperatures. a. Vasodilation of cutaneous blood vessels allows the body to lose heat through radiation, conduction, and convection. b. Enhanced sweating is used if the environment becomes so hot it cannot be lost by other means than evaporation. 6. Fever is controlled hyperthermia, usually resulting from an infection somewhere in the body. IV. Developmental Aspects of Nutrition and Metabolism (pp. 989–990) A. Embryological 1. Good nutrition is essential in utero for the growth of fetal tissues and brain. 2. Inadequate calories during the first three years of life, a time of brain growth, will lead to mental deficits or learning disorders. 3. Proteins are needed for muscle and bone growth, and calcium is required for strong bones. B. Aging 1. By middle age and old age non-insulin-dependent diabetes mellitus becomes a problem, especially in the obese. 2. Metabolic rate declines as we age. 3. Muscle and bones deteriorate, and the efficiency of the endocrine system decreases.