Digestion and Transport of Dietary Lipids

• Triacylglycerols are the major fat source in the human diet.
• Lipases (lingual lipase in the saliva and gastric lipase in the stomach) perform limited digestion of triacylglycerol prior to entry into the intestine.
• Cholecystokinin is released by the intestine as food enters, which signals the gallbladder to release bile acids and the exocrine pancreas to release digestive enzymes.
• Within the intestine, bile salts emulsify fats, which increases their accessibility to pancreatic lipase and colipase.
• Triacylglycerols are degraded to form free fatty acids and 2-monoacylgylcerol by pancreatic lipase and colipase.
• Dietary phospholipids are hydrolyzed by pancreatic phospholipase A2 in the intestine.
• Dietary cholesterol esters (cholesterol esterified to a fatty acid) are hydrolyzed by pancreatic cholesterol esterase in the intestine.
• Micelles, consisting of bile acids and the products of fat digestion, form within the intestinal lumen and interact with the enterocyte membrane. Lipid-soluble components diffuse from the micelle into the cell.
• Bile salts are resorbed farther down the intestinal tract and returned to the liver by the enterohepatic circulation.
• The intestinal epithelial cells resynthesize triacylglycerol and package them into chylomicrons for release into the circulation.
• Once in circulation the nascent chylomicrons interact with high-density lipoprotein particles and acquire two additional protein components; apolipoproteins C-II and E.
• ApoCII activates lipoprotein lipase on capillary endothelium of muscle and adipose tissue, which digests the triglycerides in the chylomicron. The fatty acids released from the chylomicron enter the muscle for energy production or the fat cell for storage. The glycerol released is metabolized only in the liver.
• As the chylomicron loses triglyceride, its density increases, and it becomes a chylomicron remnant. Chylomicron remnants are removed from circulation by the liver through specific binding of the remnant to apolipoprotein E receptors on the liver membrane.
• Once in the liver the remnant is degraded, and the lipids are recycled.