Lipid Digestion and Absorption
High-Yield Summary
- Bile emulsifies fat (increases surface area) but is NOT a digestive enzyme — it breaks no chemical bonds.
- Pancreatic lipase + colipase converts triglycerides into 2-monoacylglycerols and free fatty acids; cholesterol esterase releases free cholesterol.
- Micelles (bile salts + fatty acids + monoglycerides + cholesterol) ferry hydrophobic digestion products through the watery intestinal lumen to enterocytes; bile salts stay behind, reabsorbed in the ileum via enterohepatic circulation.
- Inside enterocytes: lipids reassembled into triglycerides and cholesteryl esters, packaged into chylomicrons, which exit via lacteals (lymphatic system) → thoracic duct → bloodstream (NOT portal circulation).
- Exception: short-chain fatty acids are water-soluble enough to skip micelles/chylomicrons entirely — absorbed directly into blood via portal circulation.
Key Terms
- Emulsification
- Bile's action of breaking large fat droplets into smaller ones to increase surface area for enzymes.
- Micelle
- Small water-soluble sphere (bile salts + lipids) with hydrophilic exterior, hydrophobic interior, ferrying lipids through the gut lumen.
- Chylomicron
- Large lipoprotein particle assembled in enterocytes to carry dietary lipids into the lymphatic system.
- Enterohepatic circulation
- Recycling loop where bile salts are reabsorbed in the ileum and returned to the liver.
Lipid Digestion and Absorption Pathway
- 1Bile emulsifies dietary fat into smaller droplets (increases surface area)
- 2Pancreatic lipase/colipase and cholesterol esterase break triglycerides and cholesterol esters into absorbable products
- 3Fatty acids, monoglycerides, cholesterol combine with bile salts to form micelles
- 4Micelles deliver lipid cargo to enterocytes; lipids diffuse in, bile salts stay in the lumen
- 5Enterocytes reassemble triglycerides and cholesteryl esters, package into chylomicrons
- 6Chylomicrons exit via lacteals → lymphatic system → thoracic duct → bloodstream
Absorption Route by Fatty Acid Chain Length
| Fatty Acid Type | Route / Destination |
|---|---|
| Short-chain | Direct diffusion into blood, no micelle/chylomicron needed / Liver via portal circulation |
| Long-chain | Micelle → enterocyte → chylomicron → lacteal / Systemic circulation via lymphatics |
Common MCAT Trap
- Bile is not an enzyme — don't credit it with breaking ester bonds; that's pancreatic lipase's job.
- Chylomicrons enter the lymphatic system (lacteals → thoracic duct), NOT the portal vein — unlike short-chain fatty acids and most other absorbed nutrients.
- Bile salts don't enter the enterocyte with the lipid cargo — they stay in the lumen and get recycled via enterohepatic circulation.
Quick Recall
What does bile do to fat, and is it an enzyme?
How do chylomicrons reach the bloodstream?
Which fatty acids skip micelle/chylomicron packaging entirely?