GI Lipid Absorption
- As described in GI Lipid Digestion the primary products of lipid digestion are fatty acids, 2-monoglycerides, and cholesterol. Absorption of these molecules past the small intestine epithelium occurs by distinct mechanisms on the luminal and basolateral surface as described below.
- All of the products of lipid digestion are highly hydrophobic and can easily diffuse across cellular plasma membranes. Therefore, close proximity of these lipids to the enterocyte luminal membrane will result in their crossing into the enterocyte cytosol. This close proximity is achieved with the aid of bile acids which can surround small aggregations of lipids to form ~5 nanometer-sized globular "Micelles". Because bile acids are amphipathic they can coordinate a number of lipids within the micellar core using their hydrophobic region and maintain the lipid globule within aqueous solution at the micellar surface using their hydrophilic region. In this way micelles can achieve very close proximity with the small intestine brush border at which point the lipid products of digestion simply exit the micelles and enter the enterocyte cytosol by directly diffusing across the enterocyte plasma membrane. Because amphipathic bile acids cannot cross the plasma membrane they are left within the lumen and can simply return to collect more lipid products of digestion and ferry them to the small intestine brush border.
- Free fatty acids and cholesterol can be toxic if simply released into the blood at high concentrations. Therefore, within the enterocyte cytosol they are used to resynthesize triglycerides and cholesterol esters. These lipids are then packaged within large macro-molecular structures known as chylomicrons which are subsequently released from the basolateral membrane and into a specialized lymphatic vessel known as a lacteal from where they eventually enter the blood.