1. Lipid Digestion

2. 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

3. Bile Produced in liver, stored in gallbladder Except horse Alkaline solution composed of: Bile salts Cholesterol Lecithin Bilirubin Responsible for fat emulsification Detergent action

4. Mixed micelle formed by bile salts, triacylglycerols and pancreatic lipase.

5. 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

6. 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)

7. Dietary Fat (large TG droplet ) Bile Salts Lipid emulsion Lipase 2-Monoglyceride + 2 FFA

8. Emulsification Produces small lipid spheres Greater surface area Lipases attack TG at 1 and 3 positions GlycerolGlycerol Fatty Acid 1 Fatty Acid 2 Fatty Acid 3 Lipase GlycerolGlycerol Fatty Acid 3 Fatty Acid 1 Fatty Acid 2 Triglyceride 2-Monoglyceride + 2 Free Fatty Acids 2 H 2 0

9. Digestion of Lipid Phospholipase A 1 and A 2 Hydrolyzes fatty acids from phospholipids Cholesterol esterase Hydrolyzes fatty acids from cholesterol esters

10. 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

11. Micelle Formation

12. 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

13. 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

14. 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

15. Lipid Absorption Short and medium chain fatty acids simple diffusionexocytosis

16. 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

17. 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

18. 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

19. 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

20. 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

21. Overview of Lipid Digestion in Mammals

22. Overview of Lipid Digestion and Absorption in Avians Portal blood * *Lymph in mammals Fatty acid binding protein

23. 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 VLDL TG LDL TG HDL

24. Lipoproteins Classified by density Protein:lipid ratio More protein, increased density More lipid, decreased density Four classes of lipoproteins Chylomicrons VLDL LDL HDL Formed in liver

25. 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

26. Lipoproteins Transports cholesterol to cellsTransports cholesterol from cells to liver Formed in enterocytes 1 st formed in liver – “bad” cholesterol

27. Lipid Transport Free fatty acids transported as complex with albumin in blood Lipids rapidly removed from blood Liver Fat depots Other tissue

28. Lipid Digestion - Ruminants Microbes rapidly modify lipids: Lipolysis TriglyceridesGlycerol + 3 free fatty acids Biohydrogenation Addition of H to unsaturated fatty acids Saturation If carried to completion, all double bonds become single bonds

29. Biohydrogenation Weight percent of fatty acids Fatty acidDietAbomasal digesta 16:0 (palmitic) 18:0 (stearic) 18:2 (linoleic) 18:3 (linolenic) 26 6 17 31 29 45 4 6 Sheep fed alfalfa hay

30. Biohydrogenation Reduction of double bonds Result: fatty acids that are more saturated with hydrogen Saturated Unsaturated

31. Biohydrogenation of Linoleic Acid Linoleic acid (18:2) cis-9, trans-11 CLA trans-11 18:1 Stearic acid (18:0) isomerase reductase

32. 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

33. 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

34. 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