Biochemical processes in the digestion of carbohydrates, lipids and proteins.

Biochemical Digestion of Macronutrients

1. Carbohydrates

➤ Overview

Carbohydrates are primarily digested into monosaccharides (glucose, fructose, galactose) for absorption.

➤ Digestion Process

• Mouth:
  • Enzyme: Salivary α-amylase initiates starch breakdown into maltose and dextrins.
• Stomach:
  • Low pH inactivates salivary amylase; minimal carbohydrate digestion occurs.
• Small Intestine:
  • Pancreatic α-amylase: Continues starch digestion into maltose, isomaltose, and oligosaccharides.
  • Brush border enzymes:
    • Maltase: Maltose → 2 glucose.
    • Sucrase: Sucrose → glucose + fructose.
    • Lactase: Lactose → glucose + galactose.

➤ Absorption

• Monosaccharides are absorbed in the small intestine via specific transporters:
  • SGLT1: Glucose and galactose via active transport.
  • GLUT5: Fructose via facilitated diffusion.
  • GLUT2: Transports all three monosaccharides into the bloodstream.

➤ Clinical Note

• Lactose intolerance results from lactase deficiency, leading to gastrointestinal symptoms upon lactose ingestion.

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2. Lipids

➤ Overview

Lipids are hydrophobic molecules requiring emulsification and enzymatic action for digestion into absorbable units.

➤ Digestion Process

• Mouth:
  • Enzyme: Lingual lipase begins triglyceride breakdown.
• Stomach:
  • Enzyme: Gastric lipase continues lipid digestion, especially in infants.
• Small Intestine:
  • Bile salts: Emulsify fats, increasing surface area for enzyme action.
  • Pancreatic lipase: Hydrolyzes triglycerides into monoglycerides and free fatty acids.

➤ Absorption

• Monoglycerides and free fatty acids form micelles with bile salts, facilitating transport to enterocytes.
• Within enterocytes, lipids are re-esterified and packaged into chylomicrons for lymphatic transport.

➤ Clinical Note

• Steatorrhea (fatty stools) can result from impaired bile or pancreatic enzyme secretion, leading to fat malabsorption.

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3. Proteins

➤ Overview

Proteins are digested into amino acids and small peptides for absorption.

➤ Digestion Process

• Stomach:
  • Enzyme: Pepsin (activated from pepsinogen by HCl) initiates protein digestion into polypeptides.
• Small Intestine:
  • Pancreatic enzymes:
    • Trypsin: Cleaves at lysine and arginine residues.
    • Chymotrypsin: Targets aromatic amino acids.
    • Carboxypeptidase: Removes terminal amino acids from carboxyl end.
  • Brush border enzymes:
    • Aminopeptidase: Removes amino acids from amino end.
    • Dipeptidase: Splits dipeptides into free amino acids.

➤ Absorption

• Amino acids and small peptides are absorbed via active transport mechanisms into enterocytes and then into the bloodstream.

➤ Clinical Note

• Pancreatic insufficiency can lead to protein malabsorption, resulting in muscle wasting and edema.

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📚 References

• Lehninger Principles of Biochemistry, 7th Edition.
• Harper's Illustrated Biochemistry, 31st Edition.
• Lumen Learning. "Chemical Digestion and Absorption: A Closer Look."
• LibreTexts. "Chemical Digestion of Carbohydrates, Proteins, Lipids, and Nucleic Acids."
• BYJU'S Biology. "Digestion and Absorption.

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For visual learners, diagrams illustrating these processes can be found in the referenced textbooks and online resources.