Digestion of Food Stuffs: Lipid Digestion PDF

I. Digestion of dietary lipids Adult human ingests about 60-150 g of lipids/day of which >90% is triacylglyerol (TAG), the remainder is cholesterol, cholesteryl esters, phospholipids and free fatty acids (FFAs). Digestion of Triglycerides (TAG) Triglycerides are digested by a group of lipase e...

I. Digestion of dietary lipids Adult human ingests about 60-150 g of lipids/day of which >90% is triacylglyerol (TAG), the remainder is cholesterol, cholesteryl esters, phospholipids and free fatty acids (FFAs). Digestion of Triglycerides (TAG) Triglycerides are digested by a group of lipase enzymes: 1) Lingual Lipase: Secreted by Ebner’s glands on the dorsal surface of the tongue. Digestion of TAG by this enzyme is minimal, because foods remain for a short time in the mouth. 2) Gastric Lipase: Its optimum pH is 4-6, thus it cannot act in adult stomach due to high acidity. It may be of value in infants' stomach (pH:5), as it acts on mother milk fat that contains TAGs consisting of short or medium chain fatty acids. 3) Pancreatic Lipase: It is secreted into the intestine and prefers TAG with long chain FAs. It is the most important lipase in TAGs digestion. It digests the primary ester bonds (position 1 and 3), hydrolyzing them into two fatty acids and 2-monoacylglycerol. The presence of emulsifying agents as bile salts and phospholipids is important for action of pancreatic lipase. Note: Emulsification means breakdown of large globules into small ones. This increases the surface area of lipids exposed to the lipase enzyme.  Pancreatic Lipases attack TAG at 1 and 3 positions 1 2 3 The resulting 2-monoacylglycerols due to action of pancreatic lipase will undergo the following: i) 72% are absorbed as such. ii)28% are converted into 1-monoacylglycerols by isomerase enzyme which are then: 1) Absorbed as 1-monoacylglycerols (6%). 2) Hydrolyzed by intestinal Lipase into glycerol and fatty acids (22%) which are then absorbed. 4) Intestinal Lipase: It acts on 1-monoacylglycerols converting them into glycerol and FFAs. Thus the end products of triacylglycerols are: a) 2-monoacylglycerols (72%). b) 1-monoacylglycerols (6%). c) Glycerol and free fatty acids (22%). Pancreatic Lipase Pancreatic Lipase Intestinal Lipase Complete hydrolysis of triglyceride. In the intestines, generally fats are only partially hydrolyzed Digestion of Cholesterol: H2O Cholesterol itself undergoes no digestion and absorbed as such. Cholesterol ester are digested by cholesterol esterase secreted in the pancreatic juice into cholesterol and free fatty acids. Digestion of Phospholipids: Phospholipids may be absorbed as such or digested by phospholipase enzymes (A1, A2 (B), C and D). They act on phospholipid hydrolyzing them into FFAs, glycerol, phosphate and nitrogenous base. X = Alcohol = Ethanolamine, Choline, Serine, inositol  II. Absorption of dietary lipids At the intestinal lining, short chain fatty acids and glycerol are absorbed and pass via portal circulation to the liver, long chain fatty acids are converted back into triglycerides and combined with protein forming chylomicrons that travel in the lymph. Cholesterol is absorbed in the free form; it is then re-esterified in the intestinal mucosa and passes along with triglycerides in the form of chylomicrons to the lymph vessel then systemic circulation (milky appearance of serum after fatty meals) Phospholipids are resynthesized in the intestinal mucosa and form part of chylomicrons. Overview of lipid digestion Fate of absorbed lipid fragments: Glycerol and FFAs are taken up by different tissues for the following functions: 1. Formation of depot fats. 2. Oxidation for energy production 3. Synthesis of biologically active compounds III. Defects of lipids digestion and absorption: Steatorrhoea: It is a condition in which the fat content of the stool is abnormally increased (normally < 5 grams/day). Causes: It results from deficiency of any factor essential for digestion or absorption of lipids as pancreatic lipase, bile salts or healthy intestinal mucosa.