Nutrient Absorption: Why It Happens in the Small Intestine, Not the Mouth

Digestive Process Overview

1. Mouth: Mechanical breakdown (chewing), chemical initiation (salivary amylase, lipase). No significant absorption (except some drugs/substances buccally/sublingually).
2. Stomach: Acid/pepsin digestion; minimal absorption (alcohol, aspirin, water).
3. Small intestine: Primary site of nutrient absorption (95%+).
4. Large intestine: Water, electrolytes, SCFAs, vitamins (K, B12 from bacteria).

Small Intestine Adaptations

• Length: ~6 m (duodenum 25 cm, jejunum 2.5 m, ileum 3.5 m).
• Surface amplification: Plicae circulares → villi → microvilli = ~300 m².
• Enterocytes with specific transporters (SGLT1, GLUT2, PepT1, NPC1L1, etc.).
• Rich vascular and lymphatic supply (lacteals for chylomicrons).
• Tight junctions regulate paracellular transport.

Absorption Mechanisms by Nutrient Class

Nutrient	Mechanism	Key Transporter
Glucose, galactose	Secondary active (Na⁺-coupled)	SGLT1 (apical), GLUT2 (basolateral)
Fructose	Facilitated diffusion	GLUT5, GLUT2
Amino acids	Secondary active (multiple systems)	B⁰AT1, PAT1, etc.
Fatty acids, 2-MAG	Passive diffusion + protein-facilitated	FABP, CD36
Water, electrolytes	Osmosis, active transport	ENaC, CFTR, NKCC, aquaporins
Vitamins/minerals	Specific carriers	CUBN (B12-IF), DMT1 (Fe), etc.

Why Not the Mouth?

• Short transit time (~10–30 sec).
• Stratified squamous epithelium (barrier, not absorptive).
• No villi/microvilli, no specialized transporters.
• Saliva dilutes; swallowing moves bolus onward.