NUTRITION AND METABOLISM

Chapter 23: Nutrition and Metabolism Dr. Paul Davis Everyday questions What are the basic nutrients every human needs to survive?  What does a vegetarian need to be considerate of?  What is good and bad cholesterol?  How does cyanide work?  Why is kidney failure bad?  Nutrition Nutrient – a substance that promotes normal growth, maintenance, and repair  Major nutrients – carbohydrates, lipids, and proteins  Other nutrients – vitamins and minerals (and technically speaking, water)  Nutrition Figure 23.1b Carbohydrates Complex carbohydrates (starches) are found in bread, cereal, flour, pasta, nuts, and potatoes  Simple carbohydrates (sugars) are found in soft drinks, candy, fruit, and ice cream  Carbohydrates Glucose is the molecule ultimately used by body cells to make ATP  Neurons and RBCs rely almost entirely upon glucose to supply their energy needs  Excess glucose is converted to glycogen or fat and stored  Lipids    The most abundant dietary lipids, triglycerides, are found in both animal and plant foods Essential fatty acids – linoleic and linolenic acid, found in most vegetables, must be ingested Dietary fats:  Help the body to absorb vitamins  Are a major energy fuel of hepatocytes and skeletal muscle  Are a component of myelin sheaths and all cell membranes Proteins  Proteins supply:  Essential amino acids, the building blocks for nonessential amino acids  Nitrogen for nonprotein nitrogen-containing substances Proteins Complete proteins that meet all the body’s amino acid needs are found in eggs, milk, milk products, meat, and fish  “Incomplete” proteins are found in legumes, nuts, seeds, grains, and vegetables   Meaning, as a whole, the proteins are deficient in one or more necessary amino acids Essential Amino Acids Figure 23.2 Proteins: Synthesis and Hydrolysis  All-or-none rule  All amino acids needed must be present at the same time for protein synthesis to occur will be used as fuel if there is insufficient carbohydrate or fat available (“dirty burn”)  Hydrolysis  Protein Vitamins Organic compounds needed for growth and good health  They are crucial in helping the body use nutrients and often function as coenzymes  Only vitamins D, K, and B are synthesized in the body; all others must be ingested  Water-soluble vitamins (B-complex and C) are absorbed in the gastrointestinal tract   B12 additionally requires gastric intrinsic factor to be absorbed Minerals  Seven minerals are required in moderate amounts  Calcium, phosphorus, potassium, sulfur, sodium, chloride, and magnesium Dozens are required in trace amounts  Minerals work with nutrients to ensure proper body functioning  Calcium, phosphorus, and magnesium salts harden bone  Metabolism Metabolism – all chemical reactions necessary to maintain life  Cellular respiration – food fuels are broken down within cells and some of the energy is captured to produce ATP  reactions – synthesis of larger molecules from smaller ones  Catabolic reactions – hydrolysis of complex structures into simpler ones  Anabolic Stages of Metabolism  Energy-containing nutrients are processed in three major stages – breakdown of food; nutrients are transported to tissues  Anabolism & catabolism  Digestion Anabolism: Built into lipids, proteins, and glycogen  Catabolism: Broken down by catabolic pathways to pyruvic acid and acetyl CoA   Oxidative breakdown – nutrients are catabolized to carbon dioxide, water, and ATP Figure 23.3 Carbohydrate Metabolism     Since all carbohydrates are transformed into glucose, it is essentially glucose metabolism Oxidation of glucose is shown by the overall reaction: C6H12O6 + 6O2  6H2O + 6CO2 + 36 ATP + heat Glucose is catabolized in three pathways  Glycolysis  Krebs cycle  The electron transport chain and oxidative phosphorylation Krebs Cycle  An eight-step cycle in which each acetic acid is decarboxylated and oxidized, generating: molecules of NADH + H+  One molecule of FADH2  Two molecules of CO2  One molecule of ATP  Three  For each molecule of glucose entering glycolysis, two molecules of acetyl CoA enter the Krebs cycle Pyruvic acid from glycolysis Glycolysis Krebs cycle Electron transport chain and oxidative phosphorylation Cytosol CO2 CoA Acetyl CoA NAD+ NADH+H+ Mitochondrion (fluid matrix) ATP ATP ATP Oxaloacetic acid NADH+H+ NAD+ Malic acid (pickup molecule) Citric acid CoA (initial reactant) Isocitric acid NAD+ Krebs cycle CO2 NADH+H+ a-Ketoglutaric acid CO2 CoA NAD+ NADH+H+ Fumaric acid FADH2 FAD Key: = Carbon atom Pi = Inorganic phosphate CoA = Coenzyme A ADP ATP Succinic acid CoA GTP Succinyl-CoA GDP + Pi Figure 23.7 Electron Transport Chain  Food (glucose) is oxidized and the released hydrogens:  Are transported by coenzymes NADH and FADH2 (made available by Kreb’s cycle)  Enter a chain of proteins bound to metal atoms (cofactors)  Combine with molecular oxygen to form water  Release energy  The energy released is harnessed to attach inorganic phosphate groups (Pi) to ADP, making ATP by oxidative phosphorylation Cyanide Figure 23.9 Electronic Energy Gradient    The transfer of energy from NADH + H+ and FADH2 to oxygen releases large amounts of energy This energy is released in a stepwise manner through the electron transport chain (else, spontaneous combustion would occur) Within the mitochondrion, low H+ results.  This gradient is now used for the last stage of respiration. Structure of ATP Synthase Figure 23.10 Summary of ATP Production Figure 23.11 Lipid Metabolism  Lipids are necessary for life  Lipids form the cell membrane lipid bilayer, serve as protective layers in viscera, and are required for neuronal function  Too much lipid content in blood is associated with cardiac or cardiovascular disease Plasma Cholesterol Levels  The liver produces cholesterol:  At a basal level of cholesterol regardless of dietary intake  In response to saturated fatty acids Saturated fatty acids decrease cholesterol excretion  Unsatured fatty acids increase cholesterol excretion   Some (omega-3) lower both saturated fats & cholesterol Cholesterol  Carried in two liposomes (lipid bodies)  LDL=  “bad” cholesterol Present when the body tries to maintain and distribute cholesterol to the organs  HDL=“good”  cholesterol Essentially, serve as “carriers” or “trash bins” to collect/scavenge and ultimately excrete cholesterol  High LDL:HDH levels are associated with heart attack and stroke Non-Dietary Factors Affecting Cholesterol Stress, cigarette smoking, alcohol, and coffee drinking increase LDL levels  Aerobic exercise increases HDL levels  Triglycerides  Triglycerides are fats which are obtained in the diet  They are independently associated with increased risk of heart disease  Reduction occurs through dietary changes (alcohol, fat, carbohydrates) and aerobic exercise Standard Blood levels HDL Cholesterol Level (Good) Less than 40 mg/dL Low HDL cholesterol. A major risk factor for heart disease. 60 mg/dL and above High HDL cholesterol. An HDL of 60 mg/dL and above is considered protective against heart disease. Total Cholesterol Less than 200 mg/dL Desirable level that puts you at lower risk for coronary heart disease. 200 to 239 mg/dL Borderline 240 mg/dL and above High blood cholesterol. A person with this level has more than twice the risk of coronary heart disease as someone whose cholesterol is below 200 mg/dL. Triglyceride Level Less than 150 mg/dL Normal 150–199 mg/dL Borderline high 200–499 mg/dL High 500 mg/dL and above Very high LDL Cholesterol Level (Bad) Less than 100 mg/dL Optimal 100 to 129 mg/dL Near or above optimal 130 to 159 mg/dL Borderline high 160 to 189 mg/dL High 190 mg/dL and above Very high Body Energy Balance Bond energy released from catabolized food equals the total energy output  Energy intake – equal to the energy liberated during the oxidation of food  Energy output includes the energy:   Immediately lost as heat (about 60% of the total)  Used to do work (driven by ATP)  Stored in the form of fat and glycogen Body Energy Balance Nearly all energy derived from food is eventually converted to heat  Cells cannot use this energy to do work, but the heat:   Warms the tissues and blood  Helps maintain the homeostatic body temperature  Allows metabolic reactions to occur efficiently Regulation of Body Temperature Body temperature – balance between heat production and heat loss  At rest, the liver, heart, brain, and endocrine organs account for most heat production  During vigorous exercise, heat production from skeletal muscles can increase 30–40 times  Regulation of Body Temperature Normal body temperature is 36.2C (98.2F); optimal enzyme activity occurs at this temperature  Temperature spikes above this range denature proteins and depress neurons  Feeding Behaviors  Feeding behavior and hunger depend on one or more of five factors  Neural signals from the digestive tract  Bloodborne signals related to the body energy stores  Hormones, body temperature, and psychological factors Role of the Hypothalamus The main thermoregulation center is the preoptic region of the hypothalamus  The heat-loss and heat-promoting centers comprise the thermoregulatory centers  The hypothalamus:   Receives input from thermoreceptors in the skin and core  Responds by initiating appropriate heat-loss and heat-promoting activities Heat-Promoting Mechanisms  Low external temperature or low temperature of circulating blood activates heat-promoting centers of the hypothalamus to cause:  Vasoconstriction of cutaneous blood vessels  Increased metabolic rate  Shivering  Enhanced thyroxine release Heat-Loss Mechanisms  When the core temperature rises, the heat-loss center is activated to cause:  Vasodilation of cutaneous blood vessels  Enhanced sweating  Voluntary measures commonly taken to reduce body heat include:  Reducing activity and seeking a cooler environment  Wearing light-colored and loose-fitting clothing Chapter 25 Fluid, Electrolyte, and AcidBase Balance Body Water Content Infants have low body fat, low bone mass, and are 73% or more water  Total water content declines throughout life  Healthy males are about 60% water; healthy females are around 50%  Body Water Content  This difference reflects females’:  Higher body fat  Smaller amount of skeletal muscle  In old age, only about 45% of body weight is water Fluid Compartments    Water occupies two main fluid compartments Intracellular fluid (ICF) – about two thirds by volume, contained in cells Extracellular fluid (ECF) – consists of two major subdivisions – the fluid portion of the blood  Interstitial fluid (IF) – fluid in spaces between cells  Plasma  Other ECF – lymph, cerebrospinal fluid, eye humors, synovial fluid, serous fluid, and gastrointestinal secretions Extracellular and Intracellular Fluids  Proteins, phospholipids, cholesterol, and neutral fats account for:  90% of the mass of solutes in plasma  60% of the mass of solutes in interstitial fluid  97% of the mass of solutes in the intracellular compartment Water Balance and ECF Osmolality To remain properly hydrated, water intake must equal water output  Water intake sources   Ingested fluid (60%) and solid food (30%)  Metabolic water or water of oxidation (10%) Water Intake and Output Figure 25.4 Regulation of Water Intake  The hypothalamic thirst center is stimulated:  By a decline in plasma volume of 10%–15%  By increases in plasma osmolality of 1–2%  Via baroreceptor input, angiotensin II, and other stimuli Electrolyte Balance Electrolytes are salts, acids, and bases, but electrolyte balance usually refers only to salt balance  Salts are important for:   Neuromuscular excitability  Secretory activity  Membrane permeability  Controlling fluid movements  Salts enter the body by ingestion and are lost via perspiration, feces, and urine Acid-Base Balance  Normal pH of body fluids  Arterial blood is 7.4  Venous blood and interstitial fluid is 7.35  Intracellular fluid is 7.0 Alkalosis or alkalemia – arterial blood pH rises above 7.45  Acidosis or acidemia – arterial pH drops below 7.35 (physiological acidosis)  Sources of Hydrogen Ions  Most hydrogen ions originate from cellular metabolism  Anaerobic respiration of glucose produces lactic acid (too little oxygen)  Fat metabolism yields organic acids and ketone bodies (too much fat (“dirty”) burning)  Transporting carbon dioxide as bicarbonate releases hydrogen ions (excessive CO2 production) Hydrogen Ion Regulation  Concentration of hydrogen ions is regulated sequentially by: buffer systems – act within seconds  The respiratory center in the brain stem – acts within 1-3 minutes  Renal mechanisms – require hours to days to effect pH changes  Chemical Chemical Buffer Systems One or two molecules that act to resist pH changes when strong acid or base is added  Three major chemical buffer systems   Bicarbonate buffer system  Phosphate buffer system  Protein buffer system  Any drifts in pH are resisted by the entire chemical buffering system Bicarbonate Buffer System A mixture of carbonic acid (H2CO3) and its salt, sodium bicarbonate (NaHCO3) (potassium or magnesium bicarbonates work as well)  If strong acid is added:   Hydrogen ions released combine with the bicarbonate ions and form carbonic acid (a weak acid)  The pH of the solution decreases only slightly Bicarbonate Buffer System  If strong base is added:  It reacts with the carbonic acid to form sodium bicarbonate (a weak base)  The pH of the solution rises only slightly  This system is the only important ECF buffer Physiological Buffer Systems   During carbon dioxide unloading, hydrogen ions are incorporated into water When hypercapnia (too much CO2 in blood) :   Leads to acidosis (too much H+ in blood), defined as pH <7.35 Deeper and more rapid breathing expels more carbon dioxide  Hydrogen ion concentration is reduced  Alkalosis (pH >7.45) causes slower, more shallow breathing, causing H+ to increase Respiratory system impairment causes acid-base imbalance (respiratory acidosis or respiratory alkalosis)  Renal Mechanisms of AcidBase Balance    Chemical buffers can tie up excess acids or bases, but they cannot eliminate them from the body Only the kidneys can rid the body of metabolic acids (phosphoric, uric, and lactic acids and ketones) and prevent metabolic acidosis The ultimate acid-base regulatory organs are the kidneys The lungs can eliminate carbonic acid by eliminating carbon dioxide  Respiratory Acidosis and Alkalosis Result from failure of the respiratory system to balance pH  PCO2 is the single most important indicator of respiratory inadequacy  PCO2 levels   Normal mm Hg  Values above 45 mm Hg signal respiratory acidosis  Values below 35 mm Hg indicate respiratory alkalosis PCO2 fluctuates between 35 and 45 Metabolic Acidosis    All pH imbalances except those caused by abnormal blood carbon dioxide levels Metabolic acid-base imbalance – bicarbonate ion levels above or below normal (22-26 mEq/L) Metabolic acidosis is the second most common cause of acid-base imbalance  Typical causes are ingestion of too much alcohol and excessive loss of bicarbonate ions  Other causes include accumulation of lactic acid, shock, ketosis in diabetic crisis, starvation, and kidney failure Metabolic Alkalosis Rising blood pH and bicarbonate levels indicate metabolic alkalosis  Typical causes are:   Vomiting of the acid contents of the stomach  Intake of excess base (e.g., from antacids)  Constipation, in which excessive bicarbonate is reabsorbed Respiratory and Renal Compensations  Acid-base imbalance due to inadequacy of a physiological buffer system is compensated for by the other system  The respiratory system will attempt to correct metabolic acid-base imbalances  The kidneys will work to correct imbalances caused by respiratory disease