- Ingestion and initial digestion
- Most dietary fat is in the form of triglycerides (TAGs), along with smaller amounts of cholesterol esters, phospholipids, and fat-soluble vitamins (A, D, E, K)
- Mechanical digestion (chewing and stomach churning) breaks fat into large droplets, increasing surface area for enzymatic action
- Emulsification
- Occurs in the duodenum via the action of bile salts (produced in the liver, stored in the gallbladder)
- Bile salts coat fat droplets, breaking them into smaller micelles and increasing surface area for enzyme access
- This process is physical, not chemical
- Enzymatic digestion
- Pancreatic lipase, with the help of colipase, hydrolyzes triglycerides into:
- 2 free fatty acids and 1 monoglyceride
- Colipase helps anchor lipase to emulsified fat droplets, especially in the presence of bile
- Cholesterol esterase and phospholipase A2 break down cholesterol esters and phospholipids, respectively
- Micelle formation
- Bile salts form micelles, which are small, water-soluble aggregates
- Micelles contain:
- Monoglycerides, free fatty acids, cholesterol, lysophospholipids, and fat-soluble vitamins
- Micelles ferry lipid products across the unstirred water layer to the enterocyte brush border
- Absorption into enterocytes
- Lipids passively diffuse from micelles into enterocytes (mostly in the proximal jejunum
- Bile salts remain in the lumen and are recycled via enterohepatic circulation
- Re-esterification
- Inside enterocytes, lipids are re-esterified in the smooth endoplasmic reticulum:
- Free fatty acids + monoglycerides → triglycerides
- Cholesterol → cholesterol esters
- Lysophospholipids → phospholipids
- Chylomicron formation
- Re-esterified lipids are packaged with apolipoproteins (mainly ApoB-48) into chylomicrons
- Chylomicrons are released by exocytosis into lacteals (lymphatic vessels in the villi)
- They travel through the thoracic duct into the systemic circulation, bypassing the portal system
- Transport and utilization
- In peripheral tissues, chylomicrons are acted upon by lipoprotein lipase (LPL) on capillary endothelium
- LPL hydrolyzes triglycerides into free fatty acids (taken up by muscle and adipose tissue) and glycerol (taken up by the liver)
- The remaining chylomicron remnant is cleared by the liver