Digestion and absorption

Digestion and absorption

1. Carbohydrate digestion and absorption

   • Small intestine
     • Monomers
   • Digestion
     • Luminal hydrolysis
       • salivary α-amylase
         • pancreatic α-amylase
     • membrane digestion
       • Small intestine
         • Brush edge
           • Lactase Maltase Sucrase-isomaltase.
   • Absorption
     • Glucose, galactose and fructose
       • Absorption in the small intestine
         • Glucose uptake and galactose in the apical membrane
           • (SGLT1)
             • Does not transport fructose
               • (GLUT5) Transport fructose
           • Coordinated exit through the basolateral membrane
             • (GLUT2)
               • Glucose, galactose and fructose
             • Facilitated dissemination

2. Lipid digestion and absorption

   • Digestion
     • Core process: Hydrolysis
       • Lingual and gastric lipases initiate digestion
         • Hydrolysis in the duodenum and jejunum
           • Pancreatic lipase and colipase (gastric juice secretion)
             • Hydrolyzes mainly ester bonds by TAG
               • Products: Two fatty acids and a single sn2-MAG (2-MAG)
     • Milk lipase is important for the digestion of fat in breastfed infants
   • Absorption
     • Decomposition of emulsion droplets into mixed micelles
       • Entrance to the enterocito as monomers. (LCFA, MAG, lysophospholipids and cholesterol)
         • Reesterification of lipid components and assembles them into chylomicrons
           • Chylomicron segregation to capillaries lymphatics during VLDL ingestion and secretion during the fast

3. Protein digestion and absorption

   • Digestion
     • Luminal digestion
       • Pancreatic Proenzymes
         • 1. Trypsinogen, 2. Chymotrypsinogen, 3. Proelastase, 4. Procarboxypeptidase A, 5. Procarboxypeptidase B
           • Pancrearic enzimes
             • Endopeptidases
               • 1. Trypsin, 2. Chymotrypsin, 3. Elastase,
                 • Product: Oligopeptides
             • Exopeptidases
               • 4. Carboxypeptidase A, 5. Carboxypeptidase B
                 • Products: amino acids
     • Brush border peptidases
       • Affinity for oligopeptides relatively longer
     • Cytoplasmic peptidases
       • Hydrolyze dipeptides and tripeptides
   • Absorption
     • Neonatal period
       • Intact protein absorption by apical endocytosis
     • Apical absorption of dipeptides, tripeptides and tetrapeptides
       • Oligopeptide uptake by the H counterscarrier (PepT1)
         • Hydrolyzation by cytoplasmic peptidases to their constituent amino acids
           • Absorption of amino acids in the small intestine

4. General mechanisms of digestion and absorption

   • without digestion
   • Luminal hydrolysis of polymers to monomers
   • Hydrolysis at the brush edge
   • Luminal hydrolysis followed by intracellular synthesis
   • Intracellular hydrolysis

5. Digestion and absorption of vitamins and minerals

   • Intestinal absorption of fat-soluble vitamins follows the pathways of lipid absorption and transport
     • Intake
       • Proteolysis
         • Binding to other lipid products (proximal small intestine)
           • Enterocyte uptake (simple diffusion or by conveyors)
             • Association with lipid drops forming chylomicrons and VLDL
               • Transport to lymph by exocytosis
   • Ca2 + absorption
     • Active transport in the duodenum and by diffusion throughout the small intestine
   • Mg2 + absorption
     • Process active in the ileum