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This document contains Chapters 4 to 5 Chapter 4 Human Digestion and Absorption Overview Chapter 4 introduces students to the organization of the human body, from atoms and molecules to organ systems. The main focus on the chapter is the digestive system. Students will learn about the anatomy of the gastrointestinal tract in detail, including the purpose of the digestive organs, enzymes, hormones, and secretions. The processes of digestion, absorption, and circulation of nutrients are described. The chapter concludes with a discussion of the symptoms, causes, and treatments for various disorders of the digestive processes, such as heartburn, constipation, diarrhea, and food intolerances. Learning Objectives 1. Outline the basic anatomy and functions of the digestive system organs. 2. Describe how the digestive tract processes foods and propels its contents from mouth to anus. 3. Describe the function of key enzymes and hormones required for digestion and absorption. 4. Explain the processes of nutrition absorption and how nutrients enter the circulatory system. 5. Identify major nutrition-related gastrointestinal disorders and typical approaches to prevention and treatment. 6. Explain why diarrhea represents a serious health challenge to infants and young children around the world and how it can be prevented. Teaching Strategies, Activities, Demonstrations, and Assignments 1. Have students trace the digestion of major nutrients (carbohydrates, fats, and proteins) in a meal or a food such as whole milk, pizza, tacos, chocolate pie, or a Quarter Pounder. This should include nutrients involved, site of action, mechanical action, enzymes and bile, hormones, and products. 2. Bring a garden hose to lecture to demonstrate the length (@ 18 feet) of the digestive tract. This visual gives students a tangible example they can more readily comprehend. 3. Use the Take Action activity to make students aware of their dietary patterns that help them maintain a healthy GI tract. Have the students discuss in small groups what specific changes they can make to their diet to improve their GI tract health. 4. Have the class discuss implications of having a condition in which one could not release the appropriate gastrointestinal hormones. Ask how the processes of digestion and absorption might be altered. 5. Divide students into groups of three or four individuals. Have each group devise a one-day menu plan for the following digestive complications: ulcers, heartburn, constipation, hemorrhoids, irritable bowel syndrome, and diarrhea. Have each group share their menu with the class; critique for accuracy. 6. Use the Take Action activity and visit the website www.gluten.net to determine if certain flours and grains contain gluten. Have the students discuss their findings and develop healthy diets that are free of gluten. Lecture Outline Organization of the Human Body Chemical level: atoms form molecules Cells Smallest functional unit of the human body Grow, absorb nutrients, use energy, conduct metabolic and physiological functions, excrete wastes Require water, building supplies (e.g., amino acids, minerals), chemical regulators (e.g., vitamins), and energy in the form of adenosine triphosphate (ATP) Tissues: groups of similar cells working together to perform a specific task Epithelial tissue: covers surfaces inside and outside body Absorbs nutrients Secretes substances Excretes waste Protects underlying tissue Connective tissue Supports and protects body Stores fat Produces blood cells Muscle tissue: permits movement by contracting and relaxing Nervous tissue: transmits nerve impulses Organs: tissue structures that perform specific functions Organ systems: several organs working together to perform a specific function (see Table 4-1) Digestive Nervous Cardiovascular Endocrine Lymphatic and Immune Urinary Integumentary Skeletal Muscular Respiratory Reproductive Organism Digestive System Overview General Digestion: mechanical and chemical processes of breaking down foods into smaller components Absorption: uptake of nutrients from the GI tract into the blood or lymph Controlled by hormones and nervous system GI tract is barrier to entry of harmful microorganisms into the body and produces immune components Healthy bacteria in the large intestine keep pathogenic bacteria under control, synthesize nutrients and short-chain fatty acids Mouth and salivary glands Chew food Detect taste molecules Moisten food with saliva Start digestion of starch with amylase enzyme Esophagus Move food to stomach (peristalsis) Stomach Secretes gastric juice containing acid and enzymes Mixes food with gastric juice, converting it to liquid chyme Starts digestion of protein with pepsin enzyme Kills microorganisms with acid Secretes intrinsic factor, a protein needed for vitamin B-12 absorption Slowly releases chyme to the small intestine Liver: produces bile to aid fat digestion and absorption Gallbladder: store, concentrate, and release bile into small intestine Pancreas: secretes pancreatic juice containing digestive enzymes and bicarbonate into the small intestine Small intestine Mixes chyme with bile and pancreatic juice to complete digestion Absorbs nutrients and other compounds in foods Transports remaining residue to large intestine Large intestine Absorbs water and electrolytes Forms and stores feces Houses most of the gut microbiota Rectum: holds feces and expels via anus Anatomy of the GI Tract General Long, hollow, muscular tube that extends ~15 feet from mouth to anus Nutrients pass through walls of the GI tract Mucosa: innermost layer, lined with epithelial cells and glands, not smooth and has tiny, fingerlike structures that project into the lumen Submucosa: loose connective tissue, glands, blood vessels, and nerves Muscle: inner layer of circular smooth muscle and outer layer of longitudinal muscle fibers (stomach has third layer that runs diagonally) Serosa: outermost, protective layer; secretes fluid that cushions GI tract and reduces friction Sphincters: ring-like muscle that opens and closes like valves to control the flow of GI contents Lower esophageal sphincter: prevents backflow of stomach contents into the esophagus Pyloric sphincter: controls flow of stomach contents into the small intestine Hepatopancreatic sphincter: controls flow of bile and pancreatic juice from common bile duct and pancreatic ducts into small intestine Ileocecal valve: prevents contents of large intestine from reentering small intestine Anal sphincters: controls defecation until person desires to do so GI Motility: Mixing and Propulsion Peristalsis: coordinated wave of contraction and relaxation that moves the contents through the intestinal tract toward the anus Segmentation: back-and-forth action in the small intestine that breaks apart contents of the small intestine into smaller pieces and mixes with digestive juices Stomach has 3 opposing layers of muscle that contract 3 times per minute Small intestine contracts every 4 to 5 seconds Large intestine has slower peristaltic waves that occur 2 or 3 times per day; leads to occasional mass movements, which propel fecal matter toward the rectum for elimination Vomiting: reverses normal digestive tract flow; can be triggered by toxins or irritants in the GI tract, rapid changes in body position, and stomach distention Digestive Enzymes and Other Secretions Saliva: secreted in mouth Begins starch digestion Facilitates swallowing by lubrication Mucus: thick fluid secreted in mouth, stomach, small and large intestine Protects GI tract cells Lubricates food Enzymes (amylases, lipases, and proteases): secreted in mouth, stomach, small intestine, and pancreas Break down carbohydrates, fats, and protein Acid (HCl): secreted by stomach Protein digestion Destruction of microorganisms Increases solubility of minerals Bile: produced by liver, stored in gallbladder Emulsifies fat to aid fat digestion Bicarbonate: secreted by pancreas and small intestine Neutralizes stomach acid when it reaches small intestine Hormones: secreted by stomach, small intestine, and pancreas Regulate digestion and absorption Moving through the GI Tract: Mouth and Esophagus General Food preparation begins the process of digestion Teeth tear and grind food into smaller pieces to increase surface area for exposure to saliva (see Figure 4-7) Tongue mixes food with saliva, creating a bolus Saliva: dilute, watery fluid containing Mucus (lubrication) Lysozyme (kills bacteria) Amylase (digest starch) Lingual lipase (digest fats, mainly during infancy) Substances that prevent tooth decay (antibacterial agents, minerals, neutralizes acids) Taste and Smell Taste-forming compounds in food dissolve in saliva Taste buds on tongue and soft palate detect taste compounds in food 5 basic taste sensations Salty (table salt, potassium chloride) Sour (acid) Sweet (sugars) Bitter (caffeine, quinine, other compounds in fruits and vegetables; many bitter compounds are toxic, but some are beneficial phytochemicals) Umami (mushrooms, cooked tomatoes, MSG, etc.) 6 million olfactory cells in the nose enhance the sense of taste Genetic variation Affected by diseases, drugs, and aging Swallowing (see Figure 4-8) Bolus of food is pushed by the tongue against the hard palate, moves toward the pharynx Epiglottis: flap at top of esophagus that prevents food from entering the trachea by covering the larynx Esophageal muscle contractions push bolus toward stomach Moving through the GI Tract: Stomach Bolus moves past lower esophageal sphincter (a.k.a. cardiac sphincter), which prevents reflux of stomach contents into the esophagus (LES malfunction leads to heartburn) Stomach Capacity About 2 oz when empty Expands to 4 to 6 cups after a meal Can hold 16 cups when extremely full Little digestion occurs in stomach Some absorption can occur Water Few forms of fat 20% of alcohol Secretions: 2 c/d of gastric juice Hydrochloric acid (HCl) from parietal cells inactivates proteins, destroys pathogens, dissolves minerals, converts pepsinogen to pepsin (active protein-digesting enzyme) Pepsinogen (inactive protein-digesting enzyme) from chief cells Gastric lipase from chief cells Gastrin (hormone) controls release of HCl and pepsinogen; secretion is highest at the beginning of a meal and declines as meal progresses Mucus from mucous cells on gastric mucosa lubricates and protects stomach cells from digestion by HCl and pepsin Heavy use of nonsteroidal anti-inflammatory drugs can damage stomach wall because they inhibit prostaglandin production Glucose-dependent insulinotropic peptide (hormone) helps slow release of chyme into the small intestine Intrinsic factor is required for absorption of vitamin B-12 Contractions of 3 muscle layers mix food with gastric secretions to transform solid food into chyme Pyloric sphincter between stomach and duodenum controls flow of chyme into small intestine Moving through the GI Tract: Small Intestine and Accessory Organs General (see Figure 4-12) 1” diameter Duodenum: 10” of length Jejunum: 4’ of length Ileum: 5’ of length Circular folds, villi, and microvilli increase surface area 600X Goblet cells produce mucus Endocrine cells produce hormones and hormone-like substances Enterocytes produce digestive enzymes and absorb nutrients Brush border of microvilli covered with digestive enzyme-containing glycocalyx Digestion occurs primarily in duodenum and upper part of jejunum, requiring secretions from enterocytes, pancreas, liver, and gallbladder 6 c/d of mucus, enzymes, and hormones Liver, Gallbladder, and Pancreas (accessory organs; see Figure 4-15) Common bile duct and pancreatic duct empty into the duodenum at the hepatopancreatic sphincter Liver secretes 2 to 4 cups of bile per day Cholesterol-containing yellow-green fluid Emulsifies fat into micelles Reabsorbed in ileum and returned to the liver (enterohepatic circulation) Gallbladder stores bile until needed Pancreas produces 5 to 6 cups of pancreatic juice per day Sodium bicarbonate (NAHCO3) Pancreatic amylase digests starch Pancreatic lipase digests fat Proteases digest protein Gastrointestinal Hormones: A Key to Orchestrating Digestion (see Table 4-4) Gastrin (stomach and duodenum): triggers release of HCl and pepsinogen, stimulates gastric and intestinal motility Cholecystokinin (small intestine): stimulates release of pancreatic enzymes and bile Secretin (small intestine): stimulates release of pancreatic bicarbonate Motilin (small intestine): regulates motility of the GI tract Glucose-dependent insulintropic peptide (GIP) (small intestine): inhibits gastric acid secretion; stimulates insulin release Peptide YY (ileum and large intestine): inhibits gastric and pancreatic secretions Somatostatin (stomach, small intestine, and pancreas): inhibits release of GI hormones, slows gastric emptying, GI motility, and blood flow Absorption in the Small Intestine Absorptive cells migrate from crypts (base of villi) to surface of villi, maturing as they migrate; shed into the lumen when damaged (2 - 5 days) Because absorptive cells undergo such rapid turnover, nutrient deficiencies have a dramatic effect on the health and digestive capacity of the small intestine Sites of absorption (see Figure 4-16) Stomach Alcohol (20% of total) Water (minor amount) Small intestine Calcium, magnesium, iron, other minerals Glucose Amino acids Fats Vitamins Water (70 - 90% of total) Alcohol (80% of total) Bile acids Large intestine Sodium Potassium Some fatty acids Vitamin K and biotin Gases Water (10 - 30% of total) Types of absorption Passive diffusion: concentration of nutrient is higher in lumen than in absorptive cells, nutrient moves into absorptive cells by diffusion (e.g., fats, water, some minerals) Facilitated diffusion: nutrients move down concentration gradient, but require carrier proteins to move into absorptive cells (e.g., fructose) Active absorption: allows for concentration of nutrients on either side of the cell membrane and requires energy for absorption (e.g., amino acids, glucose) Endocytosis Phagocytosis: absorptive cells engulf compounds, form vesicle Pinocytosis: absorptive cells engulf liquids, form vesicle e.g., immune substances from breast milk Global Perspective: Diarrhea in Infants and Children In developing countries, diarrhea is the leading killer of children Caused by pathogenic microorganisms found in water, food, and human and animal waste Rotavirus Replicates rapidly in the epithelial cells of intestinal mucosae Toxins cause the epithelial cells to slough off faster than they can be replaced Fluids and electrolytes are excreted rapidly, leading to dehydration One-third of deaths due to diarrhea are caused by rotavirus Malnutrition increases risk Intestinal mucosa becomes thin, damaged, and leaky, allowing pathogens to invade more easily Immune function declines Repeated bouts of diarrhea can make malnutrition worse Prevention and treatment Oral rehydration therapy Supplemental zinc Rotavirus vaccine Moving Nutrients around the Body: Circulatory Systems Cardiovascular System (see Figure 4-18) Absorption of short- and medium-chain fatty acids and water-soluble nutrients Proteins Carbohydrates B-vitamins Vitamin C Components Heart Blood vessels (arteries, capillaries, veins) Blood Flow of cardiovascular system Right side of the heart accepts oxygen-depleted venous blood Blood is pumped out of the right side of the heart to the lungs Blood picks up oxygen and releases carbon dioxide at the lungs Left side of the heart receives oxygen-rich blood from the lungs Oxygenated blood from the left side of the heart to the body cells Blood reaches small intestine, supplies oxygen and nutrients, picks up nutrients from digestion of food Nutrient-rich venous blood leaves the small intestine and travels via the portal vein to the liver At the kidney, waste products, nutrient excesses, and water are removed into urine Lymphatic System Absorption of fat-soluble nutrients Most fats Fat-soluble vitamins Large molecules that cannot be absorbed by the capillary beds Flow of lymphatic system Lacteals within villi transport nutrients to large lymphatic vessels Lymphatic vessels merge with circulatory system through thoracic duct at left subclavian vein Moving through the GI Tract: Large Intestine General Small intestine empties into large intestine through ileocecal valve Normally, only water, some minerals, and undigested food fibers and starches are left Sections of large intestine (see Figure 4-19) Cecum Ascending colon Transverse colon Descending colon Sigmoid colon Rectum Anus 3 main functions Houses gut microbiota Absorbs water and electrolytes Forms and expels feces Gut Microbiota 100 trillion microbial cells reside in colon (10x number of human cells in body) Balance between beneficial and pathogenic bacteria may be disrupted by: Antibiotic treatment Radiation therapy Surgery Diseases Roles of beneficial bacteria Vitamin K synthesis Biotin synthesis Lactose digestion Fermentation of fibers and starches generates short-chain fatty acids used as energy source in the colon Ileocecal valve prevents migration of bacteria into small intestine Probiotics and Prebiotics Probiotics Live microorganisms that provide health benefits Sources include yogurt, kefir (a type of fermented milk), miso (fermented soybean paste), capsules, and powders Examples include Lactobacilli and Bifidobacteria bacteria and Saccharomyces yeast Thought to help prevent diarrhea and food allergies, treat diarrhea, irritable bowel syndrome, and inflammatory bowel disease Prebiotics Non-digestible food ingredients that promote the growth of beneficial bacteria in the large intestine Examples include inulin (found in chicory, wheat, onions, garlic, asparagus, bananas) and resistant starch (found in unprocessed whole grains, seeds, legumes, unripe fruit and cooked and chilled pasta, potatoes and rice) Fermentation produces short-chain fatty acids and other organic acids May lead to flatulence, bloating, and other GI distress Absorption of Water and Electrolytes Of 10 liters per day of water received by the GI tract, only 1% is excreted in feces Large intestine is the major site of sodium and potassium absorption Elimination of Feces 12 - 24 hours from ingestion of meal Peristaltic waves and mass movements push feces toward rectum Presence of feces in rectum stimulates defecation Internal sphincter is relaxed External sphincter is under voluntary control Composition: 75% water, 25% solids (indigestible plant fibers, tough connective tissue from animal foods, bacteria from large intestine) When Digestive Processes Go Awry Heartburn and Gastroesophageal Reflux Disease Half of U.S. adults occasionally experience heartburn (acid indigestion) Occurs when stomach acid backs up into the esophagus Experiencing heartburn 2x/week may signal gastroesophageal reflux disease (GERD) Symptoms of GERD may include: Heartburn Hoarseness Trouble difficulty Coughing Gagging Nausea Severe effects of GERD may include: Weight loss Ulceration Bleeding Anemia Increased risk of adenocarcinoma of the esophagus Potential causes of GERD Immature digestive tract (infants, children) Hiatal hernia: protrusion of stomach through diaphragm into chest cavity Alcohol use Overweight Smoking Pregnancy Large meals and certain foods (chocolate, caffeine, citrus fruits, fatty fried foods, garlic, onion, spicy foods, tomato-based foods) Treatment for GERD Small meals Avoidance of foods that cause reflux Delay of laying down after eating Weight loss Smoking cessation Limiting alcohol intake Medications Antacids H2 blockers Proton pump inhibitors Prokinetic drugs Surgery to strengthen LES Ulcers Small erosion in top layer of cells in stomach or duodenum Potential causes Helicobacter pylori infection weakens mucus coating that protects the stomach and duodenum Heavy use of NSAID medications (aspirin, ibuprofen) use suppresses prostaglandin synthesis, weakening mucous barrier Alcohol use Smoking Symptoms Gnawing or burning pain in the stomach region between meals or during the night; often relieved by eating or antacids Nausea/vomiting Loss of appetite Weight loss Complications Bleeding, leading to anemia and fatigue Perforation, leading to infection Treatment Antibiotic treatment for H. pylori infection H2 blockers, proton pump inhibitors Bismuth subsalicylate Smoking cessation Limiting NSAID use Avoidance of irritating foods Gallstones Affect 10 - 20% of U.S. adults Cholesterol and bile pigments in bile form crystal-like particles Related to slow gallbladder motility and bile composition (too little bile, too little phospholipids, too much cholesterol) Factors that increase risk for gallstones High calorie, low fiber diet Prolonged fasting Overweight and obesity Rapid weight loss (>3 lbs/week) and weight loss surgery Type 2 diabetes Sedentary lifestyle Medications (e.g., estrogen replacement therapy, birth control pills) Female gender Pregnancy Age over 60 years Family history of gallstones Ethnicity (e.g., Native American, Mexican-American) Prevention of gallstones Maintenance of healthy weight Avoiding rapid weight loss Choosing plant instead of animal protein Eating high-fiber diet Using unsaturated fats Regular physical activity Symptoms (attacks occur when stones block the bile ducts) Many with gallstones are asymptomatic Intermittent pain in right upper abdomen Pain between shoulder blades or near right shoulder Nausea Vomiting Gas Bloating Treatment of gallstones Surgical removal of gallbladder Food Intolerances Inability to digest certain food components (e.g., due to low enzymes) Food intolerances ≠ food allergies Symptoms vary widely Common causes Digestive enzyme deficiency (e.g., lactase) Sensitivity to food components (e.g., gluten) Synthetic compounds (e.g., coloring agents, sulfites, MSG) Medication and chemical residues in production of livestock and crops Toxic contaminants (e.g., mold, bacteria) Intestinal Gas (flatulence) Humans produce 1 to 4 pints per day, pass gas about 13 to 21 times per day Flatulence is composed of carbon dioxide, oxygen, nitrogen, hydrogen, methane, and small amounts of sulfur-containing gas Sources Swallowed air Fermentation of undigested carbohydrates by bacteria in the large intestine Treatment Beano Lactase Slowly increasing gas-forming foods in the diet Constipation Difficult or infrequent (<3x/week) bowel movements caused by slow movement of fecal material through the large intestine Increases with age because of slowed gastric motility Causes Ignoring urge to defecate Complications of diabetes mellitus Irritable bowel syndrome Depression Pregnancy Medications (e.g., antacids, antidepressants, calcium and iron supplements) Low-fiber diet Treatment High-fiber diet (25 to 38 g per day) including whole grains, beans, fruits, vegetables) Increased fluid intake Develop more regular bowel habits Relaxation and daily exercise Laxatives Bulk-forming laxatives Osmotic laxatives Stimulant laxatives Stool softeners Lubricant laxatives (not recommended due to decreased absorption of fat-soluble vitamins) Enema: insertion of fluid into rectum and colon Diarrhea Loose, watery stools occurring more than 3 times per day Causes Bacterial or viral infection Parasites Food intolerances Medications (e.g., magnesium-containing antacids, some antibiotics) Megadoses of vitamin C supplements Intestinal diseases IBS Excessive consumption of poorly absorbed substances (e.g., sorbitol) Treatment Fluid and electrolyte replacement Normal diet, but avoid greasy, high-fiber, and very sweet foods Requires prompt treatment In children 110 mg/dl) Abdominal obesity (e.g., waist circumference >35” for women or 40” for men) High blood triglycerides (e.g., >150 mg/dl), high LDL, low HDL (e.g., <40 mg/dl for men or 130/85 mmHg) Increased inflammatory blood proteins (e.g., C-reactive protein) Diagnostic criteria are not standardized; American Heart Association and National Heart, Lung, and Blood Institute suggest diagnosis based on presence of 3 or more of the above criteria Treatments include lifestyle modification focused on weight loss, decreased dietary fat intake, and increased physical activity Hypoglycemia Can occur with or without diabetes Among people with diabetes, hypoglycemia may occur with use of too much insulin, low food intake, or exercising without increasing carbohydrate intake Reactive hypoglycemia Exaggerated insulin response after eating Symptoms include irritability, sweating, anxiety, weakness, headache, and confusion occurring 2 - 5 hours after a meal Fasting hypoglycemia Low blood glucose after fasting for 8 hours or more Usually caused by underlying medical condition (e.g., cancer, liver disease, renal disease) Diagnosis requires classic symptoms of hypoglycemia and BG 126 mg/dl Affects over 8% of North Americans, leads to over 200,000 deaths/year 35% of North Americans show evidence of pre-diabetes (FBG 100 - 126 mg/dl) Type 1 Diabetes (insulin-dependent, juvenile-onset) Classic symptoms of hyperglycemia Increased hunger Excessive thirst Excessive urination Weight loss Caused by autoimmune attack on pancreas, leading to insufficient insulin production, thus regulating blood glucose levels Genetic predisposition Excess glucose excreted in urine Complications of type 1 diabetes CVD Kidney disease Nerve disease (constipation, diarrhea, amputations) Blindness Infections Treatment Insulin (injections, pumps) Diet Regular meals and snacks Regulated ratio of carbohydrates to protein and fat in order to maximize insulin action and moderate glucose excursions Ample fiber Balance calorie intake with expenditure Meets overall nutritional needs Carbohydrate counting and diabetic exchange system are tools to regulate dietary intake Exercise enhances glucose uptake by muscles independent of insulin action Diabetes Control and Complications Trial demonstrated that development of CVD and nerve damage can be delayed with aggressive blood glucose control Type 2 Diabetes (non-insulin-dependent, adult-onset) 90% of all cases of diabetes Characterized by insulin resistance, wherein cells become resistant to the action of insulin, thereby decreasing uptake of glucose by cells Risk factors Strong genetic predisposition Obesity Physical inactivity Ethnicity (e.g., Latino/Hispanic, African, Asian, Native American, Pacific Islander) Metabolic syndrome Pre-diabetes Treatment Diet Energy-controlled Meets nutrient needs Regular mealtimes Limit total and saturated fat Moderate alcohol (1 drink/day) may increase HDL to reduce CVD risk, but must be consumed in conjunction with meals to avoid hypoglycemia Exercise Oral medications Reduce glucose production by liver Increase insulin synthesis by pancreas Slow intestinal absorption of glucose Decrease insulin resistance Insulin (in advanced cases) Instructor Manual for Wardlaw's Perspectives in Nutrition Carol Byrd-Bredbenner, Gaile Moe , Jacqueline Berning , Danita Kelley 9780078021411
