Overview
This lecture explains the process of fat digestion and absorption in the human body, detailing the key enzymes, hormones, and transport mechanisms involved.
Structure and Breakdown of Fats
- Fats in the diet consist mainly of triglycerides, which are three fatty acids attached to a glycerol backbone.
- To be absorbed, triglycerides must be broken down into their building blocks.
Stages of Fat Digestion
- Fat digestion starts in the mouth with lingual lipase and mastication.
- In the stomach, gastric lipase continues breaking down triglycerides into smaller components.
- Partially digested fats enter the duodenum along with stomach acidity.
Hormonal Regulation
- Acidity and fat droplets in the duodenum stimulate the release of cholecystokinin (CCK) and secretin.
- CCK stimulates bile production/secretion and pancreatic enzyme (pancreatic juice) release.
- Secretin prompts the pancreas to release bicarbonate, raising the intestinal pH for optimal fat digestion.
Fat Emulsification and Absorption
- Bile salts coat and emulsify fat droplets, making them accessible to pancreatic lipases.
- Pancreatic lipase breaks triglycerides into fatty acids and monoglycerides, forming micelles.
- Micelles contain bile salts, fatty acids, monoglycerides, cholesterol, and are coated by phospholipids.
Uptake and Transport
- Fatty acids and monoglycerides in micelles are absorbed across enterocytes in the proximal jejunum.
- Bile salts remain in the intestinal lumen and are reabsorbed in the terminal ileum, returning to the liver via enterohepatic circulation.
- Absorbed fats are repackaged as chylomicrons, which enter lymphatics and then the general circulation.
Fat-Soluble and Water-Soluble Vitamins
- Vitamins A, D, E, and K require fat for absorption in the jejunum.
- Vitamins B and C are water-soluble and do not depend on fat digestion or absorption.
Key Terms & Definitions
- Triglyceride — fat molecule with three fatty acids and a glycerol backbone.
- Lipase — enzyme that breaks down lipids (fats).
- Cholecystokinin (CCK) — hormone stimulating bile and pancreatic enzyme secretion.
- Secretin — hormone prompting pancreatic bicarbonate release.
- Bile salts — molecules aiding fat emulsification and digestion.
- Micelle — structure of emulsified fat components ready for absorption.
- Enterohepatic circulation — recycling pathway of bile salts from intestine to liver.
- Chylomicron — fat-transport particle formed in enterocytes.
Action Items / Next Steps
- Review fat digestion steps and related hormones for upcoming quiz.
- Study the absorption mechanisms of fat-soluble vs. water-soluble vitamins.