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