Lipid Digestion Monogastric Digestion
Challenges Lipids are not water soluble Triglycerides too large to be absorbed Digestive solution Triglycerides mix with bile and pancreatic secretions Emulsification and digestion Bile
Produced in liver, stored in gallbladder Except horse Alkaline solution composed of: Bile salts Cholesterol Lecithin Bilirubin
Responsible for fat emulsification Detergent action Mixed micelle formed by bile salts, triacylglycerols and pancreatic lipase. Digestion of Lipid
Bile salts emulsify lipids Pancreatic lipase acts on triglycerides Triglycerides sn-2 monoglyceride + 2 fatty acids Pancreatic colipase Activated by trypsin Interacts with triglyceride and pancreatic lipase Displaces bile to allow recycling Improves activity of pancreatic lipase Pancreatic Colipase
Secreted from pancreas as procolipase Activated (cleaved) by trypsin Anchors lipase to the micelle One colipase to one lipase (i.e., 1:1 ratio) Bile Salts Dietary Fat Lipase 2-Monoglyceride (large TG droplet) + 2 FFA
Lipid emulsion Emulsification Produces small lipid spheres Greater surface area Lipases attack TG at 1 and 3 positions
G Fatty Acid1 G l l y y Lipase Fatty Acid1 c c Fatty Acid2 + e Fatty Acid2 2 H20 e Fatty Acid r r 3 o o l Fatty Acid3 l 2 Free Fatty Acids Triglyceride 2-Monoglyceride Digestion of Lipid
Phospholipase A1 and A2 Hydrolyzes fatty acids from phospholipids Cholesterol esterase Hydrolyzes fatty acids from cholesterol esters Micelle Formation
Complex of lipid materials soluble in water Contains bile salts, phospholipids & cholesterol Combines with 2-monoglycerides, free fatty acids and fat-soluble vitamins to form mixed micelles Micelle Formation Lipid Absorption
Mixed micelles move to intestinal mucosal cells (enterocytes) and release contents near cell The bile salts are re-absorbed further down the gastrointestinal tract (in the ileum), transported to the liver, and finally recycled and secreted back into the digestive tract Nutrient Absorption - Lipids
Fatty acids, 2-monoglycerides, cholesterol, and cholesterol esters move down concentration gradient (passive diffusion) Repackaged in intestinal cell for transport to liver Some is reformed into triglycerides Chylomicrons Lipid Absorption
Once in enterocyte Glycerol and short chain fatty acids directly enter mesenteric blood 2-monoglycerides and longer-chain free fatty acids reformed into triglycerides, and then packaged with protein to form chylomicrons Phospholipids hydrolyzed to free fatty acids Lipid Absorption
simple diffusion exocytosis
Short and medium chain fatty acids Lipid Absorption (Chylomicrons)
Chylomicrons absorbed from enterocytes into lacteals (lymph vessels) Ultimately enter blood via thoracic duct Most long chain fatty acids absorbed into lymphatic system Exception is poultry Blood lipids transported as lipoproteins Lipid Absorption (Direct Transfer)
Alternate route is for free fatty acids to enter circulation directly Free fatty acids (FFA) also called non- esterified fatty acids (NEFA) Mostly less than 12 C (short and medium chain fatty acids) Non-esterified fatty acids enter the liver via the portal vein Overview of Fatty Acid Uptake Short- and medium-chain fatty acids Enter portal blood directly from enterocytes Bound to albumin in blood Albumin–FFA complex Oxidized in liver or elongated and used for triglyceride formation Long-chain fatty acids Form chylomicrons Drain into the lymphatics via the lacteal in mammals (no lacteal in avian small intestinal villi) Enter bloodstream at the thoracic duct Upstream from liver Slow entry into the blood In the Enterocyte... Long-chain fatty acids (more than 10–12 carbons) are bound to fatty acid binding protein (FABP) Transport to the endoplasmatic reticulum (ER) In the ER Re-formation of triglycerides From two free fatty acids and one 2- monoacylglycerol In the Enterocyte...
Newly formed triglycerides accumulate as ‘lipid droplets’ at the endoplasmic reticulum Coated with a protein layer Stabilizes lipids for transport in lymph and blood (aqueous environment) At the Golgi apparatus, carbohydrates are attached to the protein coat The glycoproteins act as signaling molecules These protein-coated lipid droplets are called chylomicrons Overview of Lipid Digestion in Mammals Overview of Lipid Digestion and Absorption in Avians Portal blood* Fatty acid binding protein
*Lymph in mammals Repackaging in the Liver
Lipid is repackaged in the liver to VLDL or very low density lipoprotein Lipoproteins are classified by density Lipoproteins transport lipid to the rest of the body
TG TG VLDL LDL HDL Lipoproteins
Classified by density Protein:lipid ratio More protein, increased density More lipid, decreased density Four classes of lipoproteins Chylomicrons VLDL LDL Formed in liver HDL The Good, the Bad, and the Ugly
Lipoproteins are similar to chylomicrons They are lipid wrapped in proteins and cholesterol When cholesterol is measured VLDL = “bad” cholesterol HDL = “good” cholesterol Lipoproteins
Formed in enterocytes 1 st formed in liver – “bad” cholesterol
Transports cholesterol to cells Transports cholesterol from cells to liver Lipid Transport
Free fatty acids transported as complex with albumin in blood Lipids rapidly removed from blood Liver Fat depots Other tissue Lipid Digestion - Ruminants
Microbes rapidly modify lipids: Lipolysis Triglycerides Glycerol + 3 free fatty acids Biohydrogenation Addition of H to unsaturated fatty acids Saturation If carried to completion, all double bonds become single bonds Biohydrogenation
Weight percent of fatty acids Fatty acid Diet Abomasal digesta 16:0 (palmitic) 26 29 18:0 (stearic) 6 45 18:2 (linoleic) 17 4 18:3 (linolenic) 31 6 Sheep fed alfalfa hay Biohydrogenation
Reduction of double bonds Result: fatty acids that are more saturated with hydrogen
Unsaturated
Saturated Biohydrogenation of Linoleic Acid
Linoleic acid (18:2) isomerase cis-9, trans-11 CLA reductase trans-11 18:1 reductase Stearic acid (18:0) Lipid Digestion and Synthesis by Microbes
Rumen microbes Produce “trans” configured double bonds Alter chain length Change position of double bonds Produce odd-chain and branched-chain FA Rumen adipose tissue varies greatly from dietary fat Dietary fat must be rumen protected to affect animal Effect of Lipid on Rumen Fermentation
Excess amounts of unsaturated fatty acids and triglycerides Decrease methane production Impair fiber digestion Form soaps Alter rumen metabolism towards propionate production – less acetate Decrease milk fat Produce trans fatty acids Inhibit lipid synthesis in mammary gland Decrease milk fat Lipid Digestion - Ruminant
Digestion and absorption of lipids is similar to monogastrics except Fat enters small intestine in different form than was presented to animal in diet Lipids absorbed more slowly More transported as VLDL