Lipid absorption

The products of fat digestion e.g. monoglycerides, cholesterol and free fatty acids, mix with bile salts to form a range of mixed micelles. These are small enough to come into close contact with the lipophilic brush border of absorptive epithelial cells within the jejunum. There, they are passively absorbed into the cell with some facilitation by surface membrane receptors for individual components e.g. fatty acid binding protein.

Within the cytoplasm, metabolism of the fat particles occurs:

• short chain fatty acids:
  • relatively soluble and pass directly into blood in portal vein for binding with albumin
  • circulate for further metabolism by the liver, muscles and adipocytes
• long chain fatty acids and monoglycerides:
  • joined together enzymatically within the endoplasmic reticulum
  • passed into chylomicrons
• phospholipids:
  • may have fatty acids removed
  • pass into chylomicrons
• cholesterol: most incorporated into chylomicrons
• vitamins: fat soluble vitamins cannot mix with hydrophilic environment of cell and so become incorporated into chylomicron for transit

After stabilization by apolipoproteins, chylomicrons are exocytosed into the lymphatic system for passage to the blood stream.