Overview
This lecture explains the digestion and absorption of dietary lipids, focusing on the roles of specific enzymes, bile, and the formation of micelles and chylomicrons in the gastrointestinal tract.
Types and Sources of Dietary Lipids
- Main dietary lipids: triglycerides, cholesterol esters, and phospholipids.
- Triglycerides are the most abundant lipid in fatty foods.
- Polymers (large molecules) are broken down into monomers (small units) via catabolism.
Enzymes and Chemical Digestion of Lipids
- Digestion begins in the mouth with lingual lipase from sublingual and parotid glands.
- In the stomach, chief cells secrete gastric lipase.
- Both lingual and gastric lipase break triglycerides’ ester bonds, yielding monoacylglycerol and free fatty acids, but have limited effects due to short exposure.
Emulsification and Enzymatic Hydrolysis in the Small Intestine
- Most lipid digestion occurs in the small intestine (duodenum).
- Bile, mainly from the liver/gallbladder, contains bile salts and phospholipids (lecithin) acting as emulsifiers.
- Emulsification increases surface area of fat droplets for enzyme action.
- Pancreatic enzymes involved:
- Pancreatic lipase (breaks triglycerides into monoacylglycerol and free fatty acids, aided by co-lipase).
- Phospholipase A2 (breaks phospholipids into fatty acids and glycerol).
- Cholesterol ester hydrolase (breaks cholesterol esters into cholesterol and fatty acids).
Formation and Role of Micelles
- Bile salts and phospholipids form micelles that contain monoacylglycerols, free fatty acids, cholesterol, and fat-soluble vitamins (A, D, E, K).
- Micelles transport lipid breakdown products to the enterocyte (intestinal cell) membrane for absorption.
Absorption and Re-synthesis of Lipids
- At the enterocyte membrane, bile salts are recycled (enterohepatic circulation).
- Lipid monomers diffuse into enterocytes and are re-esterified into triglycerides and cholesterol esters in the smooth endoplasmic reticulum.
- Lipids are combined with proteins (apoB48) in the rough endoplasmic reticulum to form chylomicrons.
Transport of Chylomicrons and Further Lipid Distribution
- Chylomicrons are exocytosed from enterocytes into lacteals (lymphatic vessels), not directly into blood.
- Chylomicrons travel through the lymphatic system and enter the bloodstream via the thoracic duct.
- Capillary lipoprotein lipase releases fatty acids from chylomicrons for uptake by muscle and adipose tissue.
- Remaining chylomicron remnants are taken up by the liver.
Key Terms & Definitions
- Triglyceride — main storage form of fat, made of glycerol and three fatty acids.
- Ester bond — chemical link between fatty acids and alcohol groups in lipids.
- Emulsification — process of breaking large fat globules into smaller droplets.
- Micelle — tiny sphere formed by bile salts that transports lipid digestion products.
- Chylomicron — lipoprotein particle that transports absorbed lipids via lymphatic system.
- Lipoprotein lipase — enzyme that releases fatty acids from circulating chylomicrons.
- Enterocyte — intestinal cell responsible for absorbing nutrients.
Action Items / Next Steps
- Review the structure and function of bile, micelles, and chylomicrons.
- Study the specific enzymes involved in lipid digestion.
- Watch the lipoprotein metabolism video for details on chylomicron metabolism.