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HNES 351 Metabolic Basis of Nutrition Exam 1 Study Guide Chapters 2 and 4

Digestion and Absorption Organs

  • Mouth: mechanical digestion (chewing); chemical digestion begins with saliva; saliva contains salivary amylase; formation of a bolus for swallowing.
  • Esophagus: carries food to stomach via peristaltic waves; lower esophageal sphincter prevents reflux.
  • Stomach: reservoir and mixer; acid environment (HCl); pepsinogen secreted by chief cells, activated to pepsin; mucus protects mucosa; intrinsic factor for vitamin B12 absorption; churning creates chyme.
  • Small intestine: major site of digestion and absorption; sections include duodenum, jejunum, ileum; pancreatic enzymes and bile acids enter here; extensive surface area via villi and microvilli for absorption.
  • Liver: produces bile and participates in nutrient metabolism and detoxification.
  • Gallbladder: stores and concentrates bile; releases bile into the small intestine as needed.
  • Pancreas: exocrine secretions include digestive enzymes (amylase, proteases, lipase) and bicarbonate to neutralize chyme; ductal flow into the small intestine.
  • Large intestine (colon): absorbs water and electrolytes; houses colonic bacteria; ferment undigested carbohydrates; forms and expels feces.

Major Digestive Enzymes: Secretion, Site of Action, Major Function

  • Amylase (carbohydrate digestion)
    • Secretion site: saliva (salivary amylase) and pancreas (pancreatic amylase)
    • Site of action: mouth for salivary amylase; small intestine for pancreatic amylase
    • Function: begins and continues digestion of starches into maltose and limit dextrins; ultimately contributes to glucose availability.
  • Proteases (protein digestion)
    • Pepsin: secretion site = stomach (as pepsinogen from chief cells, activated by acid)
    • Trypsin, chymotrypsin, carboxypeptidase: secretion site = pancreas; action = small intestine
    • Function: break proteins into peptides and amino acids for absorption
  • Lipase (fat digestion)
    • Secretion site: pancreas; action site: small intestine
    • Function: hydrolyzes triglycerides into free fatty acids and monoglycerides; works in concert with bile salts to emulsify fats
  • Brush-border enzymes (final digestion at enterocyte surface)
    • Examples: lactase, maltase, sucrase, aminopeptidases
    • Secretion site: enterocytes (brush border) in the small intestine
    • Function: digest disaccharides into monosaccharides; further breakdown of peptides into amino acids
  • Other important digestive enzymes
    • Enteropeptidase (enterokinase): activates pancreatic trypsinogen to trypsin in the small intestine
    • Bile acids are not enzymes but essential for fat digestion by forming micelles with lipids to enable lipase action

Components of Gastric Juice and Their Functions

  • HCl (hydrochloric acid): secreted by parietal cells; creates acidic environment; denatures proteins; activates pepsinogen to pepsin
  • Pepsinogen/Pepsin: secreted by chief cells as pepsinogen and activated in acidic stomach; begins protein digestion
  • Mucus: secreted by goblet cells; protects mucosa from acid and digestive enzymes; provides lubrication
  • Intrinsic factor: secreted by parietal cells; essential for vitamin B12 absorption in the ileum
  • Water and electrolytes: maintain the gastric environment and aid in digestion

Gastrointestinal Hormones (Regulatory Peptides) and Their Actions

  • Gastrin: released by G-cells in the stomach
    • Stimulates gastric acid (HCl) secretion by parietal cells and promotes mucosal growth
    • Triggered by peptides in the stomach and stomach distention
  • Secretin: released by S-cells in the duodenum
    • Stimulates pancreatic bicarbonate secretion; inhibits gastric acid secretion
  • Cholecystokinin (CCK): released by I-cells in the duodenum/jejunum
    • Stimulates pancreatic enzyme secretion and gallbladder contraction; slows gastric emptying; induces satiety
  • GIP (Gastric Inhibitory Peptide, now called glucose-dependent insulinotropic peptide): released by K-cells in the duodenum
    • Incretin effect: stimulates insulin secretion in response to glucose; inhibits gastric emptying and acid secretion
  • Motilin: released in the small intestine during fasting
    • Stimulates migrating motor complex (MMC) activity and GI motility
  • Ghrelin: released primarily by stomach
    • Increases appetite and stimulates growth hormone release
  • Somatostatin: released by D-cells in the GI tract
    • Inhibits release of several other hormones and slows GI activity

Immune System Protection of the Gastrointestinal Tract

  • Gut-associated lymphoid tissue (GALT): extensive immune tissue protecting the mucosa
  • Peyer’s patches and M cells: sample antigens and help initiate immune responses; maintain tolerance
  • Immunoglobulin A (IgA): secreted across mucosal surfaces; neutralizes pathogens
  • Mucus barrier and tight junctions: physical barrier preventing pathogen entry; maintain selective permeability
  • Antimicrobial peptides and secretions: defensins and other compounds that inhibit pathogens
  • Role of resident microbiota: barrier function, competition with pathogens, and immune modulation

Regulation of Gastric Secretion and Gastric Emptying

  • Phases of gastric secretion
    • Cephalic phase: sight/smell/thought of food triggers vagal stimulation and acid secretion
    • Gastric phase: food presence stimulates gastrin release and acid secretion; continued digestion
    • Intestinal phase: chyme in duodenum inhibits gastric secretion via hormones (secretin, CCK) and neural reflexes
  • Gastric emptying regulation
    • Slowed by fats, proteins, hypertonic solutions, large volumes; influenced by chyme composition and duodenal feedback
    • Neural (enteric and vagal) and hormonal mechanisms coordinate pace to small intestine

Nutrient Absorption: Types and Mechanisms

  • Passive diffusion: substances move along concentration gradient without energy; small lipophilic molecules
  • Facilitated diffusion: requires carrier proteins but does not use energy; e.g., fructose via GLUT5
  • Active transport: requires energy and transporters; moves substances against gradient; e.g., SGLT1 for glucose and galactose
  • Endocytosis: uptake of larger molecules or immune components; less common for macronutrient absorption in adults
  • Transcellular vs. paracellular routes: across enterocytes (transcellular) or between cells (paracellular)
  • Lipids absorption route: bile salts emulsify fats; formation of micelles; uptake by enterocytes; reassembly into triglycerides and incorporation into chylomicrons for lymphatic transport
  • Vitamin and mineral absorption specifics: B12 requires intrinsic factor; iron absorption regulated by body needs

Role of Bile in Digestion and Absorption

  • Bile acids/emulsification: emulsify fats to increase surface area for pancreatic lipase
  • Micelle formation: allows fat-soluble vitamins (A, D, E, K) and fatty acids to be transported to enterocytes
  • Enterohepatic circulation: bile acids reabsorbed in the ileum and recycled back to the liver
  • Importance for fat digestion and absorption efficiency

Colonic Bacteria, Prebiotics, and Probiotics

  • Colonic bacteria (microbiota): ferment undigested carbohydrates to short-chain fatty acids (SCFA) like acetate, propionate, and butyrate; provide vitamins; support gut barrier and immune function
  • Prebiotics: non-digestible fibers that promote growth/activity of beneficial bacteria (e.g., inulin, fructooligosaccharides, resistant starch)
  • Probiotics: live beneficial microbes (e.g., Lactobacillus, Bifidobacterium) that confer health benefits when consumed in adequate amounts

Dietary Fiber and Disease Relationships: Cardiovascular Disease, Diabetes, Weight Control, and Colon Cancer

  • Cardiovascular disease (CVD)
    • Mechanisms: soluble fibers bind bile acids and cholesterol, reducing LDL cholesterol; may improve lipid profile and blood pressure; slower glucose absorption supports better lipid and glucose homeostasis
  • Diabetes
    • Mechanisms: soluble fiber slows gastric emptying and glucose absorption; reduces postprandial glucose spikes; improves insulin sensitivity and glycemic control
  • Weight control
    • Mechanisms: fiber increases satiety, reduces energy density of meals, may decrease overall energy intake; promotes longer meal intervals
  • Colon cancer
    • Mechanisms: increased stool bulk and reduced transit time; fermentation to butyrate provides colonocyte energy and anti-inflammatory effects; dilution and binding of potential carcinogens; overall reduced exposure to carcinogens

Examples of Soluble, Insoluble, and Functional Fiber

  • Soluble fiber (dissolves in water; forms viscous solutions)

    • Examples: oats, barley, apples, citrus fruits, psyllium, pectin, beta-glucans, gums, some legumes

  • Insoluble fiber (does not dissolve; adds bulk)

    • Examples: cellulose, lignin, wheat bran, whole grains, seeds

  • Functional fiber (isolated or manufactured and added to foods)

    • Examples: psyllium husk, inulin, resistant starch, guar gum

  • Connections to prior content and real-world relevance

    • Digestion and absorption principles underpin all topics; hormones regulate secretions and motility to optimize digestion
    • Immune protection and microbiota influence nutrient utilization, barrier integrity, and disease risk
    • Dietary fiber and bile acid metabolism link nutrition to cardiovascular and metabolic health

  • Ethical, philosophical, and practical implications

    • Choosing whole-food sources vs. processed fiber additives; balance between dietary patterns and disease risk; consideration of individual variability in digestion and microbiota