Digestion and Absorption of Nutrients-205 (1)

Instructor Information

• Associate Professor Dr. Victor Markus

• Department of Medical Biochemistry

• Near East University

• Faculty of Medicine

Major Processes in Digestion

• Digestion:

  • Hydrolysis of complex food substances into simpler units:

    • Monosaccharides

    • Amino acids

    • Fatty acids

    • Glycerol

• Absorption:

  • Transport of the products of digestion, vitamins, minerals, and water across the intestinal epithelium into the lymphatic or blood circulatory systems.

Nutrient Handling by the Digestive System

• The digestive system processes nutrients differently:

  • Large organic molecules (carbohydrates, lipids, proteins) require hydrolysis by specific digestive enzymes before absorption.

  • Water, electrolytes, vitamins can be absorbed without processing and may need special transport mechanisms.

The Role of the Pancreas

• Pancreas: Major organ synthesizing digestive enzymes.

  • Aqueous Portion:

    • Induced by SECRETIN (released by the duodenum)

    • Contains Na+, K+, HCO3-, and water.

    • HCO3- provides proper pH (approx. 7) for enzymatic action.

  • Enzymatic Portion:

    • Induced by CHOLECYSTOKININ (released by the duodenum)

    • Enzymes include:

      • Amylase

      • Lipase

      • Trypsinogen

      • Chymotrypsinogen

      • Elastase

      • Procarboxypeptidase A and B

Enzymatic Action in the Duodenum

• Pancreatic enzymes and bile are released into the duodenal lumen.

• Digestion of peptides and disaccharides occurs through enzymes in the luminal plasma membranes of enterocytes.

• Di- and tripeptides are further hydrolyzed within the cytoplasm of enterocytes.

Carbohydrate Digestion

• Polysaccharides (e.g., starch, glycogen) are broken down into:

  • Maltotriose, α-limit dextrins, maltose, and glucose by α-amylase (found in saliva and pancreas).

  • α-amylase attacks internal α-1,4-glucosidic bonds; does not hydrolyze α-1,6 bonds.

• Final carbohydrate digestion into monosaccharides occurs in intestinal mucosal cells via enzymes such as:

  • Sucrase

  • Maltase

  • Isomaltase

  • Lactase

  • Trehalase

Human Inability to Digest Cellulose

• Humans lack β(1→4)-glucosidases; cellulose is indigestible and exits the colon unabsorbed.

• Cellulose is excreted in feces.

Digestion Conditions

• No carbohydrate digestion occurs in the stomach due to high acidity inactivating salivary α-amylase.

• Continued carbohydrate digestion in the small intestine by pancreatic α-amylase occurs after stomach contents are neutralized by bicarbonate secreted by the pancreas.

Monosaccharide Absorption

• Absorption takes place in intestinal mucosal cells (duodenum, jejunum) via:

  • Carrier-mediated transport processes:

    • Glucose and galactose: absorbed via Na+-dependent cotransport (SGLT-1).

      • Requires energy, coupled to the Na+-K+ pump.

    • Fructose: absorbed via Na+-independent, facilitated transport (GLUT-5).

Transporters in Intestinal Cells

• GLUT 2 and GLUT 5 are transporter proteins on the:

  • Apical side of the membrane (for absorption into mucosal cells)

  • Basolateral side of the membrane (for transport into portal circulation).

Abnormal Carbohydrate Digestion

• In a healthy individual, all digestible dietary carbohydrates should be absorbed by the lower jejunum.

• Defects in disaccharidase activity can lead to undigested carbohydrates in the large intestine, causing osmotic diarrhea, bacterial fermentation, cramps, and flatulence.

Digestive Enzyme Deficiencies

• Can be genetic or acquired; common causes include:

  • Disaccharide intolerance

  • Intestinal diseases

  • Malnutrition

  • Certain drugs leading to mucosal damage.

• Up to 90% of adults of African or Asian descent may have lactase deficiency, reducing lactose metabolism.

Lipid Digestion

• Triacylglycerol (TAG) constitutes 90% of dietary lipids.

• Lipids are absorbed as:

  • Cholesterol

  • Phospholipids

  • Free fatty acids in small droplets known as micelles (4-6 nm in diameter).

• Fat-soluble vitamins (A, D, E, K) are absorbed with lipids.

• Absorption diminishes on a very low-fat diet.

Hormonal Regulation of Lipid Digestion

• Hormones such as Cholecystokinin (CCK) and Secretin helps regulate lipid digestion:

  • CCK stimulates bile release from gallbladder, pancreatic enzyme release, and decreases gastric motility.

  • Secretin promotes bicarbonate secretion from the pancreas to maintain appropriate pH for lipid digestion.

Lipid Emulsification and Absorption

• Emulsification breaks down large fat globules:

  • Bile acids facilitate this process, increasing the surface area for action by lipases.

• Lipid digestion products in the jejunum include free fatty acids, monoacylglycerols, and free cholesterol, forming mixed micelles.

• Short- and medium-chain fatty acids can pass into portal blood without further modification; long-chain fatty acids are transported into enterocytes and re-esterified into TAGs.

Chylomicron Formation and Release

• Chylomicrons, containing resynthesized TAGs and cholesteryl esters, are transported through the lymphatic system and enter the blood circulation, providing a milky appearance (chyle) to lymph after lipid-rich meals.

• Conditions impacting lipid digestion:

  • Steatorrhea occurs when lipids, including fat-soluble vitamins, are not absorbed, leading to their excretion in feces.

Protein Digestion

• Proteins must be hydrolyzed into di- and tripeptides and amino acids for absorption; proteolytic enzymes (proteases) are produced by the stomach, pancreas, and small intestine.

Types of Proteases

• Endopeptidases:

  • Hydrolyze peptide bonds within the protein (e.g., Pepsin, Trypsin, Chymotrypsin).

• Exopeptidases:

  • Catalyze the removal of amino acids from peptide ends (e.g., Carboxypeptidases, Aminopeptidases).

Protein Digestion Process

• Begins in the stomach with hydrochloric acid (HCl), which denatures proteins and activates pepsinogen to pepsin.

• Pepsin, along with pancreatic proteases, digests proteins into smaller peptides and amino acids.

• These are then absorbed through sodium-dependent active transport processes in enterocytes.

Absorption of Amino Acids

• Free amino acids along with di- and tripeptides are absorbed into the hepatic portal vein after being hydrolyzed within mucosal cells.

Impacts of Digestive Disorders

• Pancreatic deficiency (e.g., chronic pancreatitis, cystic fibrosis) can lead to steatorrhea and undigested protein in feces due to incomplete digestion.

• Conditions such as celiac disease result in malabsorption due to immune-mediated intestinal damage from gluten.

Water and Ion Absorption in the Intestine

• Water absorption occurs depending on osmotic gradients; about 90% is absorbed in the small intestine.

• Ions such as sodium, potassium, calcium, and magnesium are also actively transported, often coupled with nutrient absorption.

• Calcium absorption is regulated by vitamin D and parathyroid hormone (PTH).