28.12 : Lipid Absorption

Dietary triglycerides from chyme in the duodenum are mixed with bile salts produced by the liver to emulsify fats. As a result, large droplets are broken down into smaller ones, increasing the surface area for enzymatic action. Once emulsified, pancreatic lipases hydrolyze the triglycerides into free fatty acids and monoglycerides.

These breakdown products bind with bile salts and lecithin to form micelles, which quickly pass between microvilli to come in close contact with the apical enterocyte membrane. The fatty acids and monoglycerides at the membrane leave the micelles and enter the enterocytes via simple diffusion. Once inside, they undergo re-esterification back into triglycerides.

The newly formed triglycerides combine with proteins and other phospholipids within the enterocytes to create chylomicrons, large lipoprotein particles. These chylomicrons exit enterocytes by exocytosis to enter lacteals, specialized lymphatic vessels in the intestinal villi. They travel through the lymphatic system and enter the bloodstream via the thoracic duct.

In the blood, lipoprotein lipase associated with capillary endothelium hydrolyzes the triglycerides in the chylomicrons to free fatty acids and glycerol. These can be used for energy production or stored as fat. The remaining chylomicron remnant is then taken up by the liver, where it is broken down. Its components—cholesterol, phospholipids, and proteins—are repurposed for the synthesis of other lipoproteins or bile acids, which are important for lipid digestion and transport.