Lipid digestion and absorption
Also known as: fat digestion, micelles, chylomicrons, emulsification, steatorrhoea
Fat is emulsified by bile salts, hydrolysed by pancreatic lipase and colipase, and carried to the enterocyte in micelles. Inside, it is re-esterified and packaged into chylomicrons, which enter the lacteals and reach the blood through the thoracic duct.
Overview
About 90% of dietary lipid is triglyceride. The remainder is phospholipid, cholesterol and cholesterol esters, and the fat-soluble vitamins. Because fat is insoluble in water, its digestion depends on detergents.
Emulsification. Gastric churning breaks fat into droplets. Lingual and gastric lipases release some fatty acids, which help stabilise the emulsion. These lipases are important in neonates and in pancreatic insufficiency. In the duodenum, bile salts and phospholipids coat the droplets, which keeps them small and greatly increases their surface area.
Hydrolysis. Pancreatic lipase cannot reach the triglyceride on its own because bile salts coat the droplet surface. Colipase (activated by trypsin) anchors the lipase to the bile-salt-coated surface. Lipase cleaves the ester bonds at positions 1 and 3, producing 2-monoacylglycerol and two free fatty acids. Phospholipase A2 produces lysophospholipids, and cholesterol esterase frees cholesterol.
Micelles. Above their critical micellar concentration, bile salts form mixed micelles. These are tiny discs with a hydrophilic surface and a lipid core containing monoacylglycerol, fatty acids, cholesterol, lysolecithin and vitamins A, D, E and K. Micelles ferry the lipids across the unstirred water layer to the brush border, where the lipids diffuse or are carried (CD36, FATP4; NPC1L1 for cholesterol) into the enterocyte. The bile salts stay in the lumen and are reabsorbed in the terminal ileum.
Re-esterification and export. In the smooth ER, monoacylglycerol and fatty acids are rebuilt into triglyceride, and cholesterol is esterified (ACAT). With apolipoprotein B-48, loaded by microsomal triglyceride transfer protein (MTP), these are assembled into chylomicrons. The chylomicrons are processed in the Golgi and exocytosed from the basolateral membrane. Chylomicrons are too large to enter blood capillaries, so they enter the central lacteals. They travel through the mesenteric lymph, cisterna chyli and thoracic duct to the left venous angle, so dietary fat reaches adipose tissue and muscle (lipoprotein lipase) before the liver. Short- and medium-chain fatty acids, under about 12 carbons, are water-soluble enough to enter the portal blood directly.
Parts & subdivisions
- Emulsify fat and form micelles.
- Main triglyceride-digesting enzymes.
- Smooth ER and Golgi assemble chylomicrons with apoB-48.
- Chylomicrons enter the lymph rather than the blood.
Function
- Absorbs dietary fat (about 95% efficient), cholesterol and the fat-soluble vitamins A, D, E and K.
Clinical significance
Steatorrhoea
Pale, bulky, greasy, foul-smelling stools that float. There are four groups of cause: lipase deficiency (chronic pancreatitis, cystic fibrosis), bile salt deficiency (cholestasis, ileal resection, bacterial overgrowth), mucosal disease (coeliac disease, Whipple disease), and lymphatic obstruction (lymphangiectasia, lymphoma). Deficiencies of the fat-soluble vitamins follow: A (night blindness), D (osteomalacia), E (neuropathy, haemolysis) and K (bleeding, prolonged prothrombin time).
Drugs acting on fat absorption
Orlistat inhibits gastric and pancreatic lipases and is used for weight loss; it causes oily stools. Ezetimibe blocks NPC1L1 cholesterol uptake and lowers LDL. Medium-chain triglyceride (MCT) oil is used in malabsorption because it bypasses the lymphatic route.
Abetalipoproteinaemia
Absent MTP prevents assembly of chylomicrons and VLDL. The result is fat malabsorption with lipid-laden enterocytes, acanthocytes, very low cholesterol, and deficiency of vitamin E (ataxia, retinopathy).
Test yourself
Dietary long-chain triglycerides leave the enterocyte mainly as:
- Free fatty acids bound to albumin in portal blood
- Chylomicrons entering the central lacteals
- Micelles entering villous capillaries
- VLDL secreted into the hepatic sinusoids
Answer: Chylomicrons entering the central lacteals
Inside the enterocyte, fatty acids and monoacylglycerol are re-esterified and packaged with apoB-48 into chylomicrons. These are too large for blood capillaries, so they enter the lacteals and reach the blood via the thoracic duct. Micelles never cross the epithelium; they only carry lipids to the brush border.
Compiled from standard anatomy and physiology references — see the Digestive System sources. For education only; not medical advice.