Lipid Absorption

Lipid Absorption in Intestinal Cells

Following digestion int the small intestine lumen, bile salts combine with FFAs, cholesterol, and 2-monoacylglycerol to form mixed micelles, which are disc shaped and 4-6 nm across. These micelles diffuse into the unstirred water layer and dissociate in the low pH found there. The lipids are then absorbed by the brush border of the intestinal mucosa. Short-and medium-chain FAs do not require mixed micelles to be absorbed.

Resynthesis of TAG and Cholesterol Esters in Intestinal Cells

TAG biosynthesis takes place in the sER, involving many different enzymes, and produces TAGs, cholesterol esters, and phospholipids. Short- and medium-chain FAs are released directly into the portal circulation.

Lipid Secretion from Enterocytes

Newly synthesized fats are very hydrophobic and aggregate. These aggregates, or chylomicrons, are surrounded by phospholipids, unesterified cholesterol, and apolipoprotein B-48. These particles give fat-rich lymph, or chyle, a milky appearance. Chylomicrons are released into lacteals and flow to the thoracic duct, into the left subclavian vein, and into the blood, where they transport TAG to peripheral tissues.

Lipid Absorption in the Periphery

Lipids are absorbed by various tissues in the periphery using recognition of specific proteins on different lipoproteins.

All tissues contain LDL receptor, recognizing Apo B-100 or Apo E on LDL for taking up cholesterol.

Liver, brain, and placenta also contain LRP (LDL-Receptor-related Protein) that recognizes Apo E on chylomicrons and VLDL remnants.

Scavenger receptors SR-A1 and -A2 on macrophages take up oxidized LDL.