Understanding Fat Digestion and Absorption

Sep 17, 2024

Lecture Notes: Fat Digestion and Absorption

Introduction

  • Importance of fats in diet
    • Protective role
    • Contribution to heart disease
  • Fats are primarily composed of triglycerides
    • Triglycerides = 3 fatty acids + glycerol backbone

Fat Breakdown Process

1. Mouth

  • Begins with lingual lipase and mastication
    • Lipase hydrolyzes and breaks down lipids

2. Stomach

  • Involves gastric lipase
  • Lipids are still in triglyceride chunks

3. Duodenum

  • Fat droplets and stomach acidity stimulate hormone production:
    • Cholecystokinin (CCK)
      • Stimulates bile production and secretion
      • Stimulates pancreatic enzyme release (pancreatic juice)
    • Secretin
      • Stimulates bicarbonate secretion from the pancreas
      • Raises intraluminal pH to ~6.5 for optimal fat digestion

Small Intestine Digestion

  • Lipids in small intestine exposed to bile and pancreatic enzymes
  • Bile salts and pancreatic lipase:
    • Bile salts emulsify lipids for digestion by pancreatic lipases
    • Pancreatic lipase breaks triglycerides into fatty acids and monoglycerides
    • Micelles formation
      • Composed of bile salts, fatty acids, monoglycerides, cholesterol, phospholipids

Absorption

  • Jejunum (middle part of the small intestine)
    • Monoglycerides and fatty acids absorbed across apical membrane of enterocytes
    • Bile salts remain in intestinal lumen

Enterohepatic Circulation

  • Bile salts reach terminal ileum
    • Actively reabsorbed into portal circulation to liver
    • Recycled and re-secreted into bile

Formation and Transport

  • Chylomicrons formation in cells
    • Fatty acids and monoglycerides are packaged
  • Transport:
    • Bind to basolateral membrane
    • Transported to intestinal lymphatics, then into general circulation

Additional Points

  • Fat-soluble vitamins (A, D, E, K) rely on fat for absorption in jejunum
  • Water-soluble vitamins (B, C) do not rely on fat digestion

Summary

  • Fat digestion starts in mouth, continues in stomach, and is completed in the small intestine
  • Involves enzymes and hormones for breakdown and absorption
  • Fats absorbed as fatty acids and monoglycerides, recycled via enterohepatic circulation
  • Packaged as chylomicrons and transported through the lymphatics to general circulation