Chapter 11: Lipid and Amino Acid Metabolism

Lipid Digestion and Absorption

Digestion

• Dietary fat consists mainly of triacylglycerols

  • The remainder comprised cholesterol, cholesteryl esters, phospholipids and free fatty acids

• Emulsification occurs upon entry into the duodenum

  • Emulsification - mixing of two normally immiscible liquids, i.e. fat and water

Emulsification INCREASES lipid surface area

• Emulsification is aided by bile; bile salts, pigments, cholesterol

  • Bile is secreted by the liver and stored in the gallbladder

• The pancreas secretes pancreatic lipase, colipase and cholesterol esterase

2-monacylglycerol, free fatty acids and cholesterol

Micelle Formation

Lipid digestion → Emulsification → Fat absorption

• Fats are absorbed by the intestinal wall

Micelle - formed from free fatty acids, cholesterol, 2-monoacylglycerol and bile salts

• clusters of amphipathic lipids soluble in the aqueous environment of the intestinal lumen

• Water soluble spheres with a lipid-soluble interior

• Digestion, transport and absorption of lipid-soluble substances

Duodenum → Ileum

• The ileum actively reabsorbs and recycles bile salts

  • remaining fat will pass into the colon and excrete as waste

Absorption

• Digested lipids pass through the brush border and are reabsorbed into the mucosa and re-esterified

  • Form triacylglycerols and cholesteryl esters, apoproteins, fat-soluble vitamins and other lipids → Chylomicrons

Chylimicrons → Lacteals → Thoracic duct