Comprehensive Notes: Digestion and Absorption of Macronutrients

Carbohydrate Digestion and Absorption

  • Forms of carbohydrate in foods

    • Monosaccharides: glucose, fructose, galactose

    • Disaccharides: sucrose, lactose, maltose, trehalose

    • Oligosaccharides: naturally present in foods (α-galactosides, β-fructans); processed foods (maltodextrins: glucose/corn syrups)

    • Polysaccharides: starches (a form of CHO stored in plants)

    • Sugar alcohols: sorbitol and mannitol

  • Structure of starches

    • Amylose: mainly α-1,4 glycosidic bonds

    • Amylopectin: α-1,6 glycosidic bonds (branching)

  • Digestion of carbohydrates: where and which enzymes

    • Where digestion occurs:

    • Mostly in the duodenum

    • Some digestion occurs in the mouth

    • Enzymes involved in carbohydrate digestion:

    • Starch digestions: α-amylase produced by salivary glands and the pancreas

      • Reactants/products: starch → smaller glucose polymers (maltose, maltotriose, limit dextrins)

    • Digestion of oligosaccharides and disaccharides: brush-border enzymes produced by enterocytes

      • Enzymes: α-amylase is in lumen; brush-border/disaccharidases: sucrase, α-dextrinase, glucoamylase, trehalase, β-galactosidase, lactase

      • Also includes enzymes for oligosaccharides.convert to monosaccharides

  • Final products of carbohydrate digestion

    • Monosaccharides absorbed: glucose, galactose, fructoseglucose, \, galactose, \, fructose

  • How final digested products are absorbed

    • Transport into enterocytes:

    • Glucose and galactose via secondary active transport through the transporter SGLT1SGLT1 (Na$^+$-dependent)

    • Fructose via facilitated diffusion through GLUT5GLUT5

    • Transport out of enterocytes into portal blood:

    • All three monosaccharides exit via facilitated diffusion through GLUT2GLUT2

    • Na$^+$ gradient maintenance:

    • Na$^+$/K$^+$-ATPase on the basolateral membrane maintains gradient for SGLT1-mediated uptake

    • Overall pathway: lumen → enterocyte via specific transporters → portal vein via basolateral transporter

  • Transport of carbohydrates across enterocytes

    • Final products absorbed: glucose, galactose, fructose

    • Key transporters: SGLT1, GLUT5, GLUT2

  • Quick recap of transport sites and flow

    • Lumen → Enterocyte: Na$^+$-dependent co-transport for glucose/galactose (SGLT1); fructose via facilitated diffusion (GLUT5)

    • Enterocyte → Portal blood: glucose/galactose/fructose via GLUT2

    • Na$^+$ gradient generated by Na$^+$-K$^+$-ATPase on basolateral membrane

  • Notes on absorption efficiency and relevance

    • Absorption is rapid for monosaccharides; disruptions (lactose intolerance, lactase deficiency) affect glucose absorption and osmotic balance

Protein Digestion and Absorption

  • Starting molecules

    • Proteins and polypeptides

  • Enzymes and where they act (protein digestion steps)

    • Stomach:

    • Chief cells secrete pepsinogen; activated by HCl to form pepsin

    • Pancreas:

    • Zymogens secreted: trypsinogen, chymotrypsinogen, proelastase, procarboxypeptidases A and B

    • Small intestine (mucosa):

    • Enteropeptidase (enterokinase) activates trypsinogen to trypsin, which activates other zymogens

    • Cholecystokinin (CCK) from mucosal cells stimulates secretion of pancreatic zymogens

    • Brush-border (enterocytes):

    • Aminopeptidases and carboxypeptidases present on brush border and in cytosol

  • Summary of zymogens and active enzymes

    • Inactive zymogens: trypsinogen, chymotrypsinogen, proelastase, procarboxypeptidases A and B

    • Active proteases: trypsin, chymotrypsin, elastase, carboxypeptidases A and B

    • Other related enzymes/peptidases: aminopeptidases, dipeptidases on brush border

  • Where digestion occurs

    • Step I (stomach): hydrolysis of peptide bonds in proteins by pepsin

    • Step II (small intestine lumen): digestion of polypeptides to smaller peptides by pancreatic proteases (trypsin, chymotrypsin, elastase)

    • Step III (brush border): digestion of oligopeptides by brush-border peptidases

    • Step IV (enterocytes): digestion of di- and tripeptides by cytosolic peptidases to amino acids

    • Step V (basolateral exit): transport of amino acids into enterocytes and then into venous capillaries/portal blood

  • Where digestion occurs (location references)

    • Stomach, lumen of the small intestine, brush-border membrane of enterocytes, the enterocytes themselves

  • Final digestion products

    • Free amino acids, dipeptides, and tripeptides (and sometimes residual oligopeptides before final hydrolysis)

  • Transport of amino acids and peptides into enterocytes

    • Na$^+$-dependent transport systems (ion-coupled):

    • System B: neutral amino acids (brush border)

    • ASC: alanine, serine, threonine, cysteine, glutamine

    • IMINO: proline and imino acids (brush border)

    • XAG-: aspartate and glutamate

    • Peptide transport into enterocytes:

    • PepT1 transporter (H$^+$-activated carrier) on the apical membrane; di- and tri-peptides transported by PepT1

    • Post-entry processing inside enterocytes:

    • Di- and tri-peptides hydrolyzed by cytosolic aminopeptidases and carboxypeptidases to free amino acids

  • Fate of absorbed amino acids

    • Free amino acids exit the enterocyte via the basolateral membrane into the portal vein

    • Some amino acids (e.g., glutamine, glutamate, aspartate) are further metabolized by enterocytes

  • Absorption site and efficiency

    • Absorption greatest in the jejunum

Lipid Digestion and Absorption

  • Dietary lipids and structures

    • Lipids: dietary TAGs, cholesterol/cholesterol esters, phospholipids

    • TAG structure: glycerol backbone with three fatty acids at sn-1, sn-2, and sn-3 positions (sn numbering)

  • Free vs esterified lipids

    • Free fatty acids (FFA) and monoacylglycerols (MAG) are products of digestion; esterified lipids include TAGs and cholesterol esters

  • Lipid digestion: where and which enzymes

    • Stomach: digestion of TAG begins with gastric lipase (10–25% of TAG digestion)

    • Small intestine (duodenum): pancreatic lipase dominates digestion

    • Specifics of cleavage:

    • Gastric lipase preferentially cleaves the sn-3 position, yielding 1,2-diacylglycerols (1,2-DAG) and fatty acids

    • Pancreatic lipase cleaves the sn-1 and sn-3 positions, yielding 2-monoacylglycerol (2-MAG) and free fatty acids

  • Digestion of phospholipids and cholesterol

    • Phospholipids: Phospholipase A2 (pancreas) and Phospholipase B (brush border) hydrolyze phospholipids (e.g., phosphatidylcholine) to fatty acids and lysophosphatidylcholine

    • Cholesterol esters: Cholesterol esterase (pancreas) hydrolyzes cholesteryl esters to free cholesterol; it can also affect other lipid esters and vitamins

  • Absorption of lipids

    • Lipids are transported to enterocytes via micelles and liposomes; concentration gradient drives simple diffusion across the brush-border membrane into enterocytes

    • Inside enterocytes, re-esterification occurs to keep intracellular lipid concentrations low and maintain gradient

  • Intracellular transport and re-esterification in enterocytes

    • Fatty acid-binding proteins (FABPs) facilitate fatty acid transport to the endoplasmic reticulum (ER)

    • Sterol carrier proteins SCP-1 and SCP-2 transport cholesterol to the ER

    • Re-esterification in the ER: 2-MAG and fatty acids are reconstituted into TAGs

    • Enzymes involved in TAG re-esterification:

    • MGAT (monoacylglycerol acyltransferase) and DGAT (diacylglycerol acyltransferase)

    • Pathways: 2-MAG pathway (approximately 80%) versus glycerol-3-phosphate pathway (less prominent)

  • Export of lipids from enterocytes

    • Lipids are packaged into lipoproteins for export via exocytosis: chylomicrons and VLDL (triacylglycerol-rich)

    • Transport routes:

    • Chylomicrons and VLDL exit the enterocytes and enter the lymphatic system

    • The liver and adipose tissue are final destinations for absorbed lipid chylomicrons and VLDL remnants

  • Specific transport routes of lipids

    • Lipids transported to the lymphatic system; medium- and short-chain fatty acids (MCFAs and SCFAs) can be transported to the portal blood

  • Summary of lipid digestion products and transport

    • Lipid digestion products include MAG, FFA, lysophosphatidylcholine, cholesterol

    • Micelles facilitate delivery to the enterocyte surface; diffusion and re-esterification occur inside enterocytes

  • Key pathway schematic notes

    • Dietary TAG → digestion to 2-MAG + FFA (stomach/pancreatic lipases) → MICELLES/enterocyte uptake → re-esterification to TAG via MGAT/DGAT → chylomicron assembly via MTP → lymphatic transport → liver/adipose tissues

    • Lipid transport distinctions: MCFA/SCFA go to portal vein; long-chain lipids go to lymphatics

Digestive System Overview and Cross-Cnut Notes

  • Digestive timing and site differences among macronutrients

    • Carbohydrates and lipids: digestion largely occurs in the lumen of the small intestine

    • Proteins: initial digestion in the stomach with pepsin; subsequent digestion and absorption occur intracellularly and at brush border

  • Transport and destination of final digestion products

    • Carbohydrates and proteins: transported via the portal vein to the liver

    • Lipids: transported via the lymphatic system (with the exception of MCFA/SCFA to portal blood) to systemic circulation, eventually reaching liver and adipose tissue

  • Practical or clinical correlations

    • Defects in brush-border enzymes (e.g., lactase deficiency) lead to malabsorption of specific monosaccharides

    • Pancreatic insufficiency affects lipase and protein-digesting enzyme supply, impairing digestion and absorption

    • Malabsorption syndromes can be diagnosed by symptoms related to specific nutrient malabsorption (carbohydrates, proteins, lipids) and by targeted enzyme deficiency tests

Key Terms and Pathway Highlights (at-a-glance)

  • Transporters and enzymes:

    • SGLT1: Na$^+$-dependent secondary active transport of glucose and galactose into enterocytes

    • GLUT5: facilitated diffusion transport of fructose into enterocytes

    • GLUT2: facilitated diffusion transporter for glucose, galactose, and fructose out of enterocytes into portal blood

    • PepT1: H$^+$-coupled transport of di- and tri-peptides into enterocytes

    • Brush-border enzymes: sucrase, lactase, maltase, α-dextrinase, trehalase, β-galactosidase, aminopeptidases, carboxypeptidases

    • Zymogens and activators: trypsinogen, chymotrypsinogen, proelastase, procarboxypeptidases; activated by enteropeptidase and trypsin

  • Lipid processing:

    • TAG digestion products: 2-MAG and FFA; lysophosphatidylcholine; cholesterol

    • Re-esterification in ER via MGAT and DGAT; TAG packaging by MTP into chylomicrons

    • Transport routes: chylomicrons via lymphatics; MCFA/SCFA via portal vein

  • General principles:

    • Digestion and absorption are tightly linked to transporter expression and gradient maintenance; injury or disease can disrupt these processes and lead to nutrient deficiencies

Connections to Foundational Principles

  • Digestion relies on enzyme specificity and organ compartmentalization (stomach vs. small intestine; lumen vs. enterocyte cytosol)

  • Absorption depends on transporter types and ion gradients, particularly Na$^+$-coupled and proton-coupled transport systems

  • Lipid absorption uniquely uses lymphatic transport due to chylomicron packaging, highlighting the body's adaptation to lipid-rich nutrients

  • Real-world relevance: understanding these pathways informs nutritional therapies, management of malabsorption, and interpretation of metabolic disorders

GLUT2 and GLUT4 are both glucose transporters, but they differ significantly in their tissue distribution and how they respond to blood glucose and insulin levels.

  • GLUT2:

    • Tissue Distribution: Primarily found in the liver, pancreatic beta cells, kidneys, and the basolateral membrane of intestinal enterocytes (as mentioned in the provided notes for carbohydrate absorption).

    • Response to Blood Glucose: Has a low affinity for glucose but a high capacity. This means it becomes active when glucose levels are high. In the pancreas, it senses high blood glucose to trigger insulin secretion. In the liver, it allows glucose to enter or exit, depending on concentration gradients, playing a role in maintaining glucose homeostasis.

    • Response to Insulin: Generally, GLUT2's activity is not directly regulated by insulin. Its presence on the cell membrane is constitutive (always there at high levels), and its primary function is to transport large amounts of glucose when concentrations are high, or allow bidirectional flow based on gradients.

    • Role: Facilitates glucose entry into pancreatic beta cells to sense glucose levels and trigger insulin release; mediates glucose uptake and release in hepatocytes for glucose homeostasis; facilitates glucose, galactose, and fructose exit from enterocytes into the portal blood.

  • GLUT4:

    • Tissue Distribution: Primarily found in insulin-sensitive tissues such as skeletal muscle, cardiac muscle, and adipose tissue.

    • Response to Blood Glucose: Its activity is primarily regulated by insulin. In the absence of insulin, most GLUT4 transporters are stored in vesicles within the cell cytoplasm.

    • Response to Insulin: When insulin binds to its receptor on the cell surface, it triggers a signaling cascade that causes GLUT4-containing vesicles to translocate and fuse with the plasma membrane. This increases the number of GLUT4 transporters on the cell surface, thereby increasing glucose uptake into these cells from the bloodstream. This is a crucial mechanism for lowering blood glucose after a meal.

    • Role: Mediates insulin-stimulated glucose uptake into muscle and fat cells, removing glucose from the circulation and storing it.