Why Salivary and Gastric Lipases Play Minor Roles in Fat Digestion

Lipolysis initiates in the mouth and stomach but pancreatic lipase handles >90% of dietary fat digestion in the duodenum.

Salivary Lipase (Lingual Lipase)

• pH mismatch: Optimal pH 4.5-5.5; saliva pH 6.8-7.4 → low activity.
• Low concentration: Trace amounts; no emulsification capacity.
• Substrate access: Large fat globules; no bile salts in mouth.
• Physiologic role: More critical in neonates (milk fat digestion; gastric pH higher).

Gastric Lipase

• pH inactivation: Optimal pH 4-5; stomach pH 1-2 → denatured.
• Protease interference: Pepsin degrades lipase protein.
• No emulsification: Gastric lipase acts on surface of large droplets; no bile.
• Contribution: ~10-30% of TG hydrolysis (mainly sn-3 position); more important in infants.

Pancreatic Lipase: The Powerhouse

• Secreted into duodenum at pH 6-7 (bicarbonate neutralizes chyme).
• Requires colipase (anchors to lipid-water interface) and bile salts (emulsify → ↑ surface area 1000x).
• Hydrolyzes sn-1,3 positions → 2-MAG + 2 FAs → absorbed.

Clinical Relevance

• Pancreatic insufficiency (CF, chronic pancreatitis) → steatorrhea; treated with PERT (pancreatic enzyme replacement therapy).
• Orlistat inhibits pancreatic lipase → weight loss via fecal fat loss.

Salivary/gastric lipases are evolutionary vestiges with minor roles; pancreatic lipase-bile salt complex is the digestive engine.