Digestion and absorption of essential nutrients.
1. Overview
Digestion breaks down macronutrients and micronutrients into absorbable units.
Absorption occurs mainly in the small intestine, with some nutrients absorbed in the stomach or large intestine. Enzymes, bile salts, and transporters facilitate this process.
2. Carbohydrates
Digestion:
- Begins in mouth: Salivary amylase breaks starch → maltose.
- Small intestine: Pancreatic amylase continues starch digestion → maltose.
- Brush-border enzymes (maltase, sucrase, lactase) break disaccharides → monosaccharides.
Absorption:
- Monosaccharides (glucose, galactose) → active transport via SGLT1 transporter (active, sodium-dependent)
- Fructose → facilitated diffusion via GLUT5 transporter
- All monosaccharides exit enterocytes into blood via GLUT2.
Proteins
Digestion:
- Begins in stomach: Pepsin breaks proteins → peptides.
- Small intestine: Pancreatic proteases (trypsin, chymotrypsin, carboxypeptidase) break peptides → smaller peptides.
- Brush-border peptidases break peptides → amino acids and di/tripeptides.
Absorption:
- • Amino acids: Absorbed actively using sodium-dependent transporters.
- • Di- and tripeptides: Taken up by PepT1 transporter, then broken down into amino acids inside enterocytes.
- • To blood: Amino acids exit enterocytes and enter blood via facilitated diffusion.