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

Biochemical Digestion of Macronutrients

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

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.

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.

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

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

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.

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.

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

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

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.

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

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

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.

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