Lipid Digestion and Absorption

Lipid Digestion

• Lipid digestion is a more complicated process compared to sugars and starches
• Triglycerides and phospholipids need to be broken down by special enzymes called lipases before they can be absorbed
• Salivary lipase in saliva mixes with food and breaks down lipids in the mouth
• Gastric lipase in the stomach breaks down some fat before it enters the duodenum
• The small intestine is the primary site of lipid digestion

CCK and Bile Salts

• Cholecystokinin (CCK) stimulates the gallbladder to contract and release bile
• CCK also signals the pancreas to secrete digestive enzymes, including pancreatic lipase, into the duodenum
• Bile salts enhance digestion and absorption by keeping lipids dispersed in the watery environment of the small intestine (emulsification)

Role of Bile in Lipid Digestion

• Bile salts surround lipid particles, forming micelles, which reduce the size of large fat droplets and increase the surface area of lipids
• Pancreatic lipase gains greater access to lipid molecules and digests them more readily
• If bile is not secreted into the duodenum, lipids in chyme clump together, making lipid digestion less efficient

Intestinal Enzyme Action

• Pancreatic lipase digests triglycerides by removing two fatty acids from each triglyceride molecule, converting them into monoglycerides
• Phospholipase digests phospholipids, removing two fatty acids from a phospholipid molecule
• Glycerol, fatty acids, monoglycerides, and phospholipid fragments are the end products of lipid digestion

Lipid Absorption

• Micelles transport fat-soluble end products of digestion to villi in the small intestine
• Fatty acids, monoglycerides, and cholesterol leave micelles and move into villi
• The small intestine digests and absorbs nearly all triglycerides and phospholipids in food, but only about half of dietary cholesterol is absorbed

Lipid Transport and Elimination

• Chylomicrons are formed in the small intestine and transport lipids through the bloodstream
• Lipoprotein lipase breaks down chylomicrons into fatty acids and glycerol
• Nearby cells pick up fatty acids and glycerol molecules to use for energy
• The liver clears cholesterol-rich remnants from the bloodstream and uses their contents to synthesize new lipids and lipoproteins

Recycling Bile Salts

• Most bile salts are absorbed in the ileum and reused by the liver to make new bile
• Enterohepatic circulation is the process of recycling bile salts
• Interfering with enterohepatic circulation can reduce blood cholesterol levels

Using Triglycerides for Energy

• Most cells can metabolize fatty acids for energy
• Adipose cells store triglycerides for energy and reassemble them into fatty acids and glycerol when needed
• The liver converts excess glucose and certain amino acids into fatty acids, contributing to triglyceride synthesis

Learn about the complex process of lipid digestion, including the role of lipases, salivary and gastric lipase, and the importance of the small intestine. Understand how Cholecystokinin (CCK) and bile salts facilitate lipid digestion.