Lecture 24: Lipid Digestion + Absorption Flashcards
8 D’s of lipid digestion
Luminal:
1. Dispersion
2. Digestion
3. Dissolve
4. Diffusion
Enterocytic:
5. Distribution
6. Delivery
Vascular:
7. aDipose uptake
8. die Leber (liver)
Lipid dispersion
Emulsification w/ emulsifying agents
Lipid digestion
Lipolysis via pancreatic lipase
Lipid dissolution
MG + FAs dissolve; solubilized w/ bile salt micelles
Lipid diffusion
MG + FAs diffuse passively into enterocytes
Lipid distribution
Enterocytes resynthesize TG and package as chylomicra
Lipid delivery
Chylomicron exocytosis to lymph then to blood
Lipid aDipose uptake
Chylomicra are “digested” through LPL TG hydrolysis
Lipid die Leber (liver)
Liver clears remnant chylomicra after adipose uptake via LPL
Critical Micelle Concentration
[bile salts] at which micelles start to form/aggregate, where surface tension stops dropping
Mixed micelle
Micelle (bile salts) + cholesterol + phospholipids. Form in which gall bladder stores bile salts.
Factors for gall stone development
- Excess cholesterol vs bile salts, lecithin (p-lipids)
- Excess H2O/electrolyte resorption
- Stasis
- Genetics
Bile = balance of bile salts, cholesterol, lecithin
Requirements for fat emulsification
- Emulsifying agents to prevent reaggregation
- Mechanical disruption to break aggregates (maximizes surface area for lipases; GI motility)
Stimulation of bile secretion
CCK -> pancreatic enzyme release + gall bladder contraction + sphincter relaxation
Secretin -> bicarb carrying secretions
Pancreatic lipase
- 1,3 cleavages of TG, secreted in active form
- Optimal pH in duodenum/jejunum (6-7)
- Labile enzyme destroyed by acid, inactivated by bile acids w/o colipase
Fatty diet and cholesterol absorption
As micelle gets more FAs, cholesterol capacity increases. Fatty diet potentiates cholesterol solubilization/absorption
Micelle shuttle service
Micelles serve as holding station for lipid digestion products, maintaining passive diffusion flux of MGs/FAs from emulsion drops to enterocytes.
Bile salt absorption
Ileal Bile Acid Transporter (IBAT) active transport in distal ileum (passive transport in colon)
Enterocyte lipid events
- FA/MG uptake
- Resynth. of TGs
- Chylomicron formation in Golgi for exocytosis
3a. Esterification
3b. Surface stabilization
3c. Lipoprotein addition - Secretion to lacteals
Alternate absorption path for medium chain FAs
- Lipase hydrolysis to MCFAs
- Diffusion w/o micelles into enterocyte; solubility is sufficient
Cholesterol + fat soluble vitamin absorption
- Micellar solubilization w/ bile acids
- Mixed micelles diffuse to enterocytes; no digestion via lipase req.
Colipase
Holds pancreatic lipase at surface of emulsified lipid droplets for enzymatic activity
