17.4:
Lipid Digestion
In the mouth, lipid digestion begins as the glands of the tongue release lingual lipase, an enzyme that starts the process of breaking down large fats, such as triglycerides, the most abundant dietary type.
As the lipids travel towards the stomach, additional enzymes, gastric lipases are secreted. Because lipids are hydrophobic, they tend to aggregate in these aqueous environments, preventing the lipases from breaking down the masses.
Once inside the duodenum of the small intestine, bile salts coat the fat droplets and cause the globules to disperse, creating an emulsion, an aqueous suspension. With increased surface area available, the pancreatic lipases present can hydrolyze the triglycerides into smaller components, specifically free fatty acids and monoglycerides.
Now these smaller products are ready to be absorbed by the small intestine for further processing.
17.4:
Lipid Digestion
Lipids are large molecules that are generally not water-soluble. Since most of the digestive enzymes in the human body are water-based, there are specific steps the body must take to break down lipids and make them available for use.
Lingual Lipase and Gastric Lipase
Lingual lipase is an enzyme secreted by the acinar cells of the sublingual gland that aids lipid digestion. Although found in saliva, it plays only a minimal role in breaking down lipids in the mouth. Interestingly, lingual lipase has a pH optimum of 3.5-6.0 and is not activated until chewed food enters the acidic environment of the stomach. Gastric lipase is an acidic lipase that is secreted by the gastric chief cells in the lining of the stomach.
Acidic Lipases in Adult Humans and Neonates
Lingual lipase and gastric lipase comprise the two acidic lipases found in the human digestive system. These lipases are active in the stomach but rapidly inactivated by bile acids in the duodenum. Together, gastric lipase and lingual lipase account for 10-30% of lipid hydrolysis that occurs in human adults, with gastric lipase contributing the most. Given the low concentrations of pancreatic lipase and bile salts in the neonatal phase, the acidic lipases are critical for lipid digestion and account for 50% of lipid hydrolysis in neonates.
Bile
Bile contains bile salts, lecithin, and cholesterol-derived substances, so it acts as an emulsifier in the duodenum of the small intestine. The emulsification of the fat droplets in the duodenum increases their surface area over a thousand-fold, thus making them more accessible to pancreatic lipases.
Suggested Reading
Wang, Tony Y., Min Liu, Piero Portincasa, and David Q.-H. Wang. “New Insights into the Molecular Mechanism of Intestinal Fatty Acid Absorption.” European Journal of Clinical Investigation 43, no. 11 (November 2013): 1203–23. [Source]