Comprehensive Guide to Nutrient Digestion and Absorption

The Three Phases of Digestion 1. Cephalic Phase (Neurogenic Phase) Duration : Before and during eating Stimuli : Sight, smell, taste, thought of food Mechanism : Vagal stimulation (parasympathetic) Effects : Salivary secretion increases Gastric acid secretion begins (30% of total) Pancreatic enzyme secretion starts Gastrin release from G cells 2. Gastric Phase Duration : When food enters stomach Stimuli : Gastric distension, amino acids, peptides Mechanism : Local reflexes + vagal reflexes Effects : Gastric acid secretion peaks (60% of total) Pepsinogen secretion Gastrin release continues Gastric motility increases 3. Intestinal Phase Duration : When chyme enters duodenum Stimuli : Amino acids, fatty acids, glucose in duodenum Mechanism : Hormonal (CCK, secretin, GIP) Effects : Inhibits gastric acid secretion (10% of total) Stimulates pancreatic enzyme and bicarbonate secretion Stimulates bile release Regulates gastric emptying --- CARBOHYDRATE DIGESTION AND ABSORPTION Step-by-Step Digestion Process 1. Mouth Enzyme : Salivary α-amylase (ptyalin) Action : Hydrolyzes α-1,4-glycosidic bonds in starch Products : Maltose, maltotriose, α-limit dextrins pH : Optimal at 6.8 2. Stomach Process : Salivary amylase inactivated by gastric acid Result : Minimal carbohydrate digestion 3. Small Intestine - Lumen Enzyme : Pancreatic α-amylase Action : Continues starch breakdown Products : Maltose, maltotriose, α-limit dextrins pH : Optimal at 8.0 4. Small Intestine - Brush Border Enzymes and Actions : Maltase : Maltose → Glucose + Glucose Isomaltase (α-dextrinase) : α-limit dextrins → Glucose Sucrase : Sucrose → Glucose + Fructose Lactase : Lactose → Glucose + Galactose Trehalase : Trehalose → Glucose + Glucose Absorption Mechanisms Glucose and Galactose Absorption Luminal Membrane : Transporter : SGLT1 (Sodium-Glucose Linked Transporter 1) Mechanism : Secondary active transport (Na+-dependent cotransport) Stoichiometry : 2 Na+ : 1 glucose/galactose Energy : Uses Na+ gradient created by Na+-K+ ATPase Basolateral Membrane : Transporter : GLUT2 (Glucose Transporter 2) Mechanism : Facilitated diffusion Direction : Cell → Blood → Portal circulation Fructose Absorption Luminal Membrane : Transporter : GLUT5 Mechanism : Facilitated diffusion (Na+-independent) Limitation : Cannot be absorbed against concentration gradient Basolateral Membrane : Transporter : GLUT2 Mechanism : Facilitated diffusion Key Point : SGLT2 is found in the kidneys (proximal tubule), not intestines! --- PROTEIN DIGESTION AND ABSORPTION Step-by-Step Digestion Process 1. Stomach Enzyme : Pepsin (from pepsinogen) Activation : Pepsinogen → Pepsin (by gastric HCl) Action : Endopeptidase (cleaves interior peptide bonds) pH : Optimal 1.5-3.5 Products : Large peptides and polypeptides 2. Small Intestine - Pancreatic Enzymes Activation Cascade : Enterokinase (brush border) → Trypsinogen to Trypsin Trypsin → Activates all other pancreatic proteases Enzymes : Trypsin : Endopeptidase (cleaves after basic amino acids) Chymotrypsin : Endopeptidase (cleaves after aromatic amino acids) Elastase : Endopeptidase (cleaves after small amino acids) Carboxypeptidase A : Exopeptidase (removes aromatic/basic AA from C-terminus) Carboxypeptidase B : Exopeptidase (removes basic AA from C-terminus) 3. Small Intestine - Brush Border Enzymes : Aminopeptidases, dipeptidases, tripeptidases Products : Free amino acids, dipeptides, tripeptides Absorption Mechanisms Free Amino Acids Luminal Membrane : Transporters : Four separate Na+-dependent cotransporters Neutral amino acids : System B⁰ (broad specificity) Acidic amino acids : System X⁻ₐᴳ Basic amino acids : System b⁰⁺ Imino acids : System IMINO Mechanism : Secondary active transport Basolateral Membrane : Mechanism : Facilitated diffusion Transporters : Various amino acid transporters Dipeptides and Tripeptides Luminal Membrane : Transporter : PEPT1 (Peptide Transporter 1) Mechanism : H+-dependent cotransport Advantage : Faster absorption than free amino acids Intracellular : Process : Cytoplasmic peptidases hydrolyze to amino acids Exit : Amino acids exit via basolateral facilitated diffusion --- LIPID DIGESTION AND ABSORPTION Step-by-Step Digestion Process 1. Mouth Enzyme : Lingual lipase Action : Hydrolyzes short- and medium-chain triglycerides Contribution : Minimal (continues in stomach) 2. Stomach Enzyme : Gastric lipase Action : Hydrolyzes triglycerides → fatty acids + diglycerides Contribution : ~15% of total fat digestion Physical : Churning creates lipid droplets 3. Small Intestine - Bile Acid Action Source : Bile acids from liver/gallbladder Function : Emulsification (↑ surface area) Process : Large lipid droplets → smaller droplets 4. Small Intestine - Pancreatic Enzymes Pancreatic lipase : Triglycerides → 2-monoglycerides + fatty acids Cholesterol ester hydrolase : Cholesterol esters → cholesterol + fatty acids Phospholipase A2 : Phospholipids → lysophospholipids + fatty acids Cofactor : Colipase (helps lipase bind to lipid