脂質の消化

Lipid digestion begins in the mouth when the glands of the tongue release lingual lipase, an enzyme that starts breaking down large fats like triglycerides, the most abundant dietary lipid type. As lipids travel to the stomach, additional gastric lipases are secreted to continue the process.

Lipids are hydrophobic, meaning they repel water and tend to clump together in aqueous environments like saliva and gastric juices. This aggregation prevents lipases from accessing and breaking down the fat masses effectively, limiting enzyme activity in early digestion stages.

Bile salts coat fat droplets in the duodenum and cause them to disperse, creating an emulsion—an aqueous suspension that increases surface area over a thousandfold. This dramatically improves accessibility for pancreatic lipases to hydrolyze triglycerides into free fatty acids and monoglycerides.

In adults, lingual lipase and gastric lipase together account for 10-30% of lipid hydrolysis, with gastric lipase contributing most. In neonates, acidic lipases are critical and account for 50% of lipid hydrolysis due to low pancreatic lipase and bile salt concentrations during the neonatal phase.

Pancreatic lipases hydrolyze triglycerides into smaller components: free fatty acids and monoglycerides. These smaller products are then ready to be absorbed by the small intestine through the anatomy of the intestines and microvilli for further processing and nutrient utilization by the body.

Emulsification breaks large fat droplets into smaller dispersed particles, dramatically increasing their surface area available for enzyme action. This expanded surface area allows pancreatic lipases greater access to triglycerides, making enzymatic breakdown significantly more efficient and enabling complete lipid digestion in the small intestine.

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. Although found in saliva, it plays only a minimal role in breaking down lipids in the mouth due to the neutral pH of the oral cavity.