RCSI Digestion and Absorption of Lipids PDF

RCSI Royal College of Surgeons in Ireland Coláiste Ríoga na Máinleá in Éirinn Digestion and Absorption of Lipids Class: DEM Y2 Course: GIHEP Lecturer: Dr. Sarah O’Neill ([email protected]) Date: September 16th 2024  LEARNING OUTCOMES At the end of...

RCSI Royal College of Surgeons in Ireland Coláiste Ríoga na Máinleá in Éirinn Digestion and Absorption of Lipids Class: DEM Y2 Course: GIHEP Lecturer: Dr. Sarah O’Neill ([email protected]) Date: September 16th 2024  LEARNING OUTCOMES At the end of this lecture, you will be able to: 1. Describe the digestion of lipids in the body 2. Describe the products of lipid digestion 3. Explain the mechanism of the absorption of lipids in the body 4. Explain the significance of malabsorption syndromes SITES OF LIPID DIGESTION Mouth Stomach Small Intestine WHAT FATS SHOULD WE CONSUME? Focus on eating beneficial ‘good’ fats Avoid harmful ‘bad’ fats ‘Good’ fats = Monounsaturated and polyunsaturated fats e.g. vegetable oils (such as olive, canola, sunflower, soy, and corn), nuts, seeds, and fish. ‘Bad’ fats = trans fats e.g. processed foods made with trans fat from partially hydrogenated oil. Saturated fats are best consumed in moderation, e.g. red meat, butter, cheese, and ice cream. Some plant-based fats like coconut oil and palm oil are also rich in saturated fat. A BIT OF REVISION Phospholipid Triglyceride LIPID DIGESTION IN THE MOUTH Mechanical digestion: – Starts with mastication: chewing your food – Also get emulsification: The breakdown of large droplets of lipid into tiny droplets Chemical digestion: – Saliva contains lingual lipase – Lingual lipase hydrolyse medium and long- Emulsification chain triglycerides – Products are diglycerides, monoglycerides and fatty acids – They are serine proteases LIPID DIGESTION IN THE STOMACH 2 enzymes act on lipids in the stomach: – Lingual lipase (comes through from the mouth) – Gastric lipase (secreted by the chief cells of the stomach) Both lipases are relatively acid stable They hydrolyse the ester bonds in medium and long chain triglycerides The products are diacylglycerides and fatty acids Gastric lipase belongs to the family of serine proteases LIPID DIGESTION IN THE SMALL INTESTINE Mechanical digestion Emulsification continue Increase surface area of lipid droplets so enzymes can work at lipid/aqueous interface Achieved by – Peristalsis: mechanical mixing – Bile salts: secreted by the gall bladder; they have detergent properties CHEMICAL DIGESTION IN THE SMALL INTESTINE Pancreas secretes pancreatic lipase and pancreatic co-liapse Pancreatic lipase binds to the surface of the droplet in the presence of pancreatic co-lipase The lipase co-lipase complex spreads over the surface of the droplet These enzymes hydrolyse to triglycerides to monoacylglycerol and fatty acids (FAs) = lipid digestion products LIPID DEGRADATION BY OTHER PANCREATIC ENZYMES Cholesterol esterase – Hydrolyses cholesteryl esters to cholesterol and free fatty acids – Activity greatly increased by bile salts Phospholipase A2 – Digests phospholipid to lyso-phospholipid by removing 1 fatty acid – Activated by trypsin – Requires bile salts for optimum activity Lysophospholipase – Removes remaining fatty acid at C1 – leaving glycerylphosphoryl base: excreted, further degraded or absorbed LIPID DIGESTION OVERVIEW Pancreatic enzymes digest triacylglycerol, cholesteryl esters, phospholipids Secretion of pancreatic enzymes is hormonally controlled Primary products are free fatty acids, free cholesterol, 2- monoacylglycerol LIPID DIGESTION SUMMARY THE PRODUCTS OF FAT DIGESTION FORM INTO MIXED MICELLES Mixed micelle: Bile salts Free fatty acids Free cholesterol 2-monoacylglycerol Phospholipid ABSORPTION OF LIPIDS BY ENTEROCYTES Hydrophilic surface of micelles facilitates transport of hydrophobic lipids across unstirred water layer to brush border membrane Short-medium chain FAs do not require mixed micelles for absorption CHYLOMICRON FORMATION IN THE ENTEROCYTE Fatty acids and monoglyceride are transported into the endoplasmic reticulum, where they are used to synthesise triglyceride Triglycerides are grouped with – Cholesterol esters – Free cholesterol – Phospholipids – Vitamins – Apolipoprotein B48 Form into chylomicrons and go to the Golgi apparatus CHYLOMICRON LIFE CYCLE IN THE ENTEROCYTE They are extruded from the Golgi into exocytotic vesicles Transported to the basolateral aspect of the enterocyte The vesicles fuse with the plasma membrane and undergo exocytosis, dumping the chylomicrons into the extracellular space and enter the lymph A REMINDER OF EXOCYTOSIS https:// https:// www.y www.y outube. outube. com/w com/w atch?v= atch?v= nXrU6A nXrU6A aPp88 aPp88 NOT EVERYTHING CAN BE DIGESTED! Digestion of most nutrients is incomplete 95% of dietary fat is utilised 70-90% of starch is digested efficiently Keratins and plant polymers such as cellulose and pectin are not digested Some vegetables contain indigestible carbohydrates