Recording 55-53

Overview of Lipid Digestion

• Lipids and Solubility

  • Lipids are generally insoluble in water.

  • Enzymes that digest lipids are secreted in aqueous solutions, necessitating a specialized system.

• Digestion Pathway

  • Starts in the mouth with lingual lipase, continues in the stomach with gastric lipase.

  • These lipases preferentially hydrolyze triglycerides containing short and medium-chain fatty acids.

  • The majority of dietary fats are long-chain fatty acids; 70% of fat entering the duodenum is composed of triglycerides.

• Partial Lipolysis Result

  • Remaining mixture in the duodenum includes partially hydrolyzed lipids, free fatty acids, and 1,2-DAGs (diacylglycerol).

  • 1,2-DAGs result from the hydrolysis of triglycerides at the 1 and 2 positions of glycerol.

Role of Colipase and Bile

• Colipase Function

  • Colipase assists lipase by anchoring it to lipid droplets that bile emulsifies.

  • Bile emulsifies lipids, increasing the surface area for lipase action.

• Emulsification Process

  • Lipids are broken into smaller droplets, preventing them from re-agglomerating.

  • Emulsified droplets allow lipase access to all lipid components for digestion.

Hydrolysis and Absorption of Lipids

• Microvilli and Micelles Formation

  • Post-digestion, lipids are formed into micelles, allowing for absorption by enterocytes (small intestine cells).

  • Components include 1-MEGs, 2-MEGs, long-chain fatty acids, and cholesterol.

• Transport into Enterocytes

  • Lipids can enter via diffusion or transport proteins (FATP 1-4, CD36 for fatty acids, NPC1L1 for cholesterol).

Chylomicron Formation

• Reconstruction of Triglycerides

  • Inside enterocytes, free fatty acids and monoglycerides are reassembled into triglycerides and packaged into chylomicrons.

  • Chylomicrons also carry fat-soluble vitamins (A, D, E, K) and are marked by APOB48.

Chylomicron Function and Transport

• Chylomicron Release

  • Chylomicrons are released into the lymphatic system before entering circulation.

  • Due to their size, they cannot enter blood capillaries directly but can pass through lymphatic vessels.

• Lipoprotein Interaction

  • Chylomicrons interact with HDL in circulation to acquire APO CII and APO E for efficient delivery of lipids to tissues.

Lipid Delivery to Tissues

• Use of Lipoprotein Lipase (LPL)

  • LPL, activated by APO CII, hydrolyzes triglycerides in the capillaries, allowing fatty acids to enter tissues for energy storage or use.

• Adipose Tissue Function

  • Fatty acids are converted back into triglycerides for storage.

  • Glycerol produced can enter the liver for gluconeogenesis.

  • Insulin is crucial for lipid storage; absence directs fatty acids to oxidation instead.

Chylomicron Remnants

• Post-Delivery Processing

  • After fat delivery, chylomicrons become chylomicron remnants, exchanging proteins again with HDL.

  • These remnants signal the liver for uptake via receptors, after which they are disassembled for recycling of their components.

Conclusion on Triglyceride Management

• Importance of Triglyceride Breakdown

  • Essential for proper absorption of lipids and preventing potential fatty liver disease if triglycerides accumulate.