More than 80 to 90% of the dietary lipids are triacylglycerols besides a small amount of phospholipids, cholesterol esters, free cholesterol, and free fatty acids. The main site of digestion and absorption of lipids is in the small intestinal lumen.
Digestion of lipids in the mouth
Although there is a small amount of lipase secreted from the dorsal surface of the tongue, still in normal adults practically, we can say that there is no digestion of lipids in the mouth.
Digestion of lipids in the stomach
In the stomach also there is a lipase called gastric lipase, but its optimum pH is towards neutral. This gastric lipase, together with lingual lipase, can hydrolyze short and medium-chain fatty acids, and this becomes a significant way of lipid digestion in infants and those patients who suffer from pancreatic insufficiency.
Digestion of lipids in the small intestine
The product of the digestion from the stomach passes into the small intestine, and it is called acidic chyme. Acidic chyme stimulates duodenal mucosal cells to release two local hormones called secretin and cholecystokinin. Secretin stimulates the pancreas to release bicarbonate, which in the small intestine neutralizes the gastric HCl and restores the pH to 7. Cholecystokinin [CCK] stimulates the pancreas to release lipolytic enzymes, namely, pancreatic lipase, phospholipase A2, and cholesterol esterase. CCK also stimulates the gallbladder to release bile, which is essential for the emulsification of fat. Emulsification is a process where large lipid goblets converted to smaller lipid droplets by bile salts. This process helps to provide a hydrophilic surface for the action of lipolytic enzymes.
Digestion of dietary triacylglycerols
Pancreatic lipase is the main enzyme for the digestion of most of the triacylglycerols. This enzyme hydrolyzes triacylglycerols to produces fatty acids, glycerol, and monoacylglycerols. Monoacylglycerol further hydrolyzed by lipase, or a small amount can remain like that.
Digestion of dietary phospholipids
Pancreatic phospholipase A2 hydrolyzes dietary phospholipids. It hydrolyzes ester bond at the second carbon atom of phospholipid and forms lysophospholipid and fatty acid.
Digestion of dietary cholesterol esters
Cholesterol esterase hydrolyzes dietary cholesterol ester to free cholesterol and fatty acid. The presence of bile salt increases the activity of cholesterol esterase.
Absorption of lipids into intestinal mucosal cells
In the small intestinal lumen, we have free fatty acids, free cholesterol, 2-monoacylglycerol, and lysophospholipid. Along with bile salts, fat-soluble vitamins these products of lipid digestion to form mixed micelles. These mixed micelles approach the brush border of the jejunal mucosal cells where they are absorbed into intestinal epithelium leaving behind bile salts. The bile salts are reabsorbed through the ilium and reach by enterohepatic circulation to the liver.
Formation of chylomicron in the intestinal mucosal cells
In the jejunal mucosal cells, all absorbed lipids reesterified back to triacylglycerols, phospholipids, and cholesterol esters. These lipids, along with apolipoprotein B-48, aggregate in a definite shape known as a chylomicron. The chylomicron is a type of lipoprotein, and it is spherical. Triacylglycerols, cholesterol esters occupy dense central core and in the periphery monolayer of phospholipids, free cholesterol, and apo B-48. These chylomicrons released into the lymphatics. Later these chylomicrons pass into portal circulation through the thoracic duct.