26.2 dig.pdf

Chapter 26 Nutrition & Metabolism Nutrition Nutrient: a substance in food that promotes normal growth, maintenance, and repair Major nutrients – Carbohydrates, lipids, and proteins Other nutrients – Vitamins and minerals (and, technically speaking, water) Nutrition Carbohydrates Dietary sources – St...

Chapter 26 Nutrition & Metabolism Nutrition Nutrient: a substance in food that promotes normal growth, maintenance, and repair Major nutrients – Carbohydrates, lipids, and proteins Other nutrients – Vitamins and minerals (and, technically speaking, water) Nutrition Carbohydrates Dietary sources – Starch (complex carbohydrates) in grains and vegetables – Sugars in fruits, sugarcane, sugar beets, honey and milk – Insoluble fiber: cellulose in vegetables; provides roughage – Soluble fiber: pectin in apples and citrus fruits; reduces blood cholesterol levels Carbohydrates Uses – Glucose is the fuel used by cells to make ATP Neurons and RBCs rely almost entirely upon glucose Excess glucose is converted to glycogen or fat and stored Lipids Dietary sources – Triglycerides Most abundant dietary lipids of Americans Saturated fats in meat, dairy foods, and tropical oils Unsaturated fats in seeds, nuts, olive oil, and most vegetable oils – Cholesterol in egg yolk, meats, organ meats, shellfish, and milk products Essential fatty acids – Fats that must be ingested in our diet – Omega 3’s and Omega 6’s Lipids Essential uses of lipids in the body – Help absorb fat-soluble vitamins – Major fuel of hepatocytes and skeletal muscle – Phospholipids are essential in myelin sheaths and all cell membranes Functions of fatty deposits (adipose tissue) – Protective cushions around body organs – Insulating layer beneath the skin – Concentrated source of energy Lipids Regulatory functions of prostaglandins – Smooth muscle contraction – Control of blood pressure – Inflammation Functions of cholesterol – Stabilizes cell membranes – Used in steroid hormones Proteins Dietary sources – Eggs, milk, fish, and most meats contain complete proteins – Legumes, nuts, and cereals contain incomplete proteins (lack some essential amino acids) – Legumes and cereals together contain all essential amino acids Proteins In a normal balanced diet proteins are used for – Production of hormones – Production of enzymes, clotting factor, and antibodies – Formation of hemoglobin – Structural materials: keratin, collagen, elastin, muscle proteins Proteins Use of amino acids in the body 1. All-or-none rule All amino acids needed must be present for protein synthesis to occur 2. Adequacy of caloric intake Protein will be used as fuel if there is insufficient carbohydrate or fat available Proteins 3. Nitrogen balance In a healthy body the rate of protein synthesis equals the rate of protein breakdown Positive nitrogen balance is when the protein synthesis in the tissues is greater than the amount broken down for energy (normal in children and tissue repair) Negative nitrogen balance is when breakdown exceeds synthesis (e.g., stress, burns, infection, injury, or starvation) Deamination (breakdown) of protein forms urea and is released from the body in urine 4. Hormonal controls Anabolic hormones (GH, sex hormones) accelerate protein synthesis Vitamins Organic compounds Crucial in helping the body use nutrients Most function as coenzymes Vitamins D, some B, and K are synthesized in the body Vitamins Two types, based on solubility 1. Water-soluble vitamins B complex and C are absorbed with water B12 absorption requires intrinsic factor Not stored in the body 2. Fat-soluble vitamins A, D, E, and K are absorbed with lipid digestion products Stored in the body, except for vitamin K *Vitamins A, C, and E act as antioxidants Minerals Seven required in moderate amounts: – Calcium, phosphorus, potassium, sulfur, sodium, chloride, and magnesium Others required in trace amounts Work with nutrients to ensure proper body functioning Uptake and excretion must be balanced to prevent toxic overload Minerals Examples – Calcium, phosphorus, and magnesium salts harden bone – Iron is essential for oxygen binding to hemoglobin – Iodine is necessary for thyroid hormone synthesis – Sodium and chloride are major electrolytes in the blood Metabolism Metabolism: biochemical reactions inside cells involving nutrients Two types of reactions – Anabolism: synthesis of large molecules from small ones – Catabolism: hydrolysis of complex structures to simpler ones Cellular respiration: catabolism of food fuels and capture of energy to form ATP in cells Stages of Metabolism Processing of nutrients 1. Digestion, absorption and transport to tissues 2. Cellular processing (in cytoplasm) Synthesis of lipids, proteins, and glycogen, or Catabolism (glycolysis) into intermediates 3. Oxidative (mitochondrial) breakdown of intermediates into CO2, water, and ATP Stages of Metabolism Pathways of Cellular Respiration for Carbohydrate Metabolism Oxidation of glucose C6H12O6 + 6O2 → 6H2O + 6CO2 + 36 ATP + heat Glucose is catabolized in three pathways 1. Glycolysis 2. Krebs cycle 3. Electron transport chain and oxidative phosphorylation Glycolysis Glycolysis – catabolic reaction based upon the conversion of glucose into two molecules of pyruvic acid Anaerobic Occurs in the cytoplasm Final products of glycolysis – 2 pyruvic acid Converted to lactic acid if O2 not readily available Enter aerobic pathways if O2 is readily available – 2 NADH + H+ (reduced NAD+) – Net gain of 2 ATP Glycolysis Krebs Cycle Occurs in mitochondrial matrix Does not directly use O2 Utilizes the 2 pyruvic acids from glycolysis Products from each pyruvic acid: – – – – 3 NADH + H+ 1 FADH2 2 CO2 1 ATP Krebs Cycle (Citric Acid Cycle) Electron Transport Chain and Oxidative Phosphorylation Occurs on the inner mitochondrial membrane The part of metabolism that directly uses oxygen Utilizes the hydrogen atoms from NADH + H+ and FADH2 from glycolysis and Krebs Cycle Products of oxidative phosphorylation – About 28 ATP Electron Transport Chain and Oxidative Phosphorylation Cellular Respiration Cellular Respiration Glycogenesis and Glycogenolysis Gluconeogenesis – Glucose is formed from non-carbohydrate precursors – Amino acids and fatty acids are converted to glucose – Occurs when the body has undergone prolonged fasting Glycogenesis – Glycogen formation when glucose supplies exceed need for ATP synthesis – Mostly in liver and skeletal muscle Glycogenolysis – Glycogen breakdown to release glucose in response to low blood glucose Lipid Metabolism Lipogenesis- formation of triglycerides (fat) when ATP and glucose levels are high Lipolysis- when triglycerides are broken down into glycerol and fatty acids Triglycerides are routinely oxidized for energy (ATP) Lipid Metabolism Beta Oxidation: when fatty acids are broken down for the production of ATP – Occurs in the mitochondria – Two-carbon fragments at a time are used from the fatty acid chains and enter Krebs cycle – Reduced coenzymes, which enter the electron transport chain Protein Metabolism When dietary protein is in excess, amino acids are – Oxidized for energy – Converted into fat for storage Nutrient Pools Three interconvertible pools – Amino acids – Carbohydrates – Fats Metabolism Pathways Lipoproteins Types of lipoproteins – HDLs (high-density lipoproteins) The highest protein content High levels of HDL are thought to protect against heart attack – LDLs (low-density lipoproteins) Cholesterol-rich High levels of LDL, especially lipoprotein (a) increase the risk of heart attack – VLDLs (very low density lipoproteins) Mostly triglycerides – Chylomicrons Plasma Cholesterol Levels Saturated fatty acids – Solid fats – Heart unhealthy fats – Stimulate liver synthesis of cholesterol – Inhibit cholesterol excretion from the body Unsaturated fatty acids – Liquid fats – More heart healthy fats – Enhance excretion of cholesterol Trans fats – Found in processed foods – More harmful to the heart and blood vessels than saturated fats – Increase LDLs and reduce HDLs Plasma Cholesterol Levels Unsaturated omega-3 fatty acids (found in cold-water fish) – Lower the proportions of saturated fats and cholesterol – Have antiarrhythmic effects on the heart – Help prevent spontaneous clotting – Lower blood pressure Non-Dietary Factors Affecting Cholesterol Stress, cigarette smoking, and coffee lower HDL levels Aerobic exercise and estrogen increase HDL levels and decrease LDL levels Body shape – “Apple”: Fat carried on the upper body is correlated with high cholesterol and LDL levels – “Pear”: Fat carried on the hips and thighs is correlated with lower cholesterol and LDL levels Energy Balance Heat energy – Cannot be used to do work – Warms the tissues and blood – Helps maintain the homeostatic body temperature – Allows metabolic reactions to occur efficiently Obesity Body mass index (BMI) = wt (lb)  705/ht (inches)2 Considered overweight if BMI is 25 to 30 Considered obese if BMI is greater than 30 – Higher incidence of atherosclerosis, diabetes mellitus, hypertension, heart disease, and osteoarthritis Metabolic Rate Basal metabolic rate (BMR) – Reflects the energy the body needs to perform its most essential activities – Thyroxine is the most important hormone affecting BMR Factors that Influence BMR As the ratio of body surface area to volume increases, BMR increases Decreases with age Increases with temperature or stress Males have a disproportionately higher BMR Thyroxine increases oxygen consumption, cellular respiration, and BMR Regulation of Body Temperature Body temperature reflects the balance between heat production and heat loss At rest, the liver, heart, brain, kidneys, and endocrine organs generate most heat During exercise, heat production from skeletal muscles increases dramatically Normal body temperature = 37C (98.6F) Optimal enzyme activity occurs at this temperature Increased temperature denatures proteins and depresses neurons Core and Shell Temperature Organs in the core have the highest temperature Blood is the major agent of heat exchange between the core and the shell Core temperature is regulated Core temperature remains relatively constant, while shell temperature fluctuates substantially (20C– 40C) Homeostatic Imbalance Hyperthermia – Elevated body temperature depresses the hypothalamus – Positive-feedback mechanism (heat stroke) begins at core temperature of 41C – Can be fatal if not corrected Hypothermia – Low body temperature where vital signs decrease – Shivering stops at core temperature of 30 - 32C – Can progress to coma a death by cardiac arrest at ~ 21C Fever Controlled hyperthermia Due to infection (also cancer, allergies, or CNS injuries) Hyperthermia steps: 1. Macrophages release interleukins (“pyrogens”) that are carried to the hypothalamus 2. Interleukins make the hypothalamus release prostaglandins 3. Prostaglandins reset the hypothalamic thermostat higher Natural body defenses or antibiotics reverse the disease process; cryogens reset the thermostat to a lower (normal) level