Digestion and absorption of dietary fats in non-ruminants

1. Digestion and absorption of dietary fats in non-ruminants

2. Background and Review • Fatty acid nomenclature relevant to this advanced nutrition class • 14:0 myristic acid • 16:0 palmitic acid • 18:0 stearic acid • 18:1 cis Δ9 oleic acid • 18:2 cis Δ 9,12 linoleic acid • 18:3 cis Δ 9,12,15 linolenic acid • 20:4 cis Δ 5,8,11,14 arachidonic acid • 20:5 cis Δ 5,8,11,14,17 eicosapentaenoic acid (an omega-3 fatty acid because of double bond 3 C from distal end)

3. Nomenclature • Chain length • Medium chain: caproic (C6), caprylic (C8), capric (C10) and lauric acid (C12) • Long chain: C14…….. • Saturation • Saturated (C16:0) • Monounsaturated (C18:1) • Polyunsaturated (C20:4)

4. Nomenclature • Position of first double bond relative to carboxylic acid or methyl end • n−3 fatty acids: referred to as n-3 (ω−3 fatty acids or omega-3 fatty acids); a family of unsaturated fatty acids that have a double bond in the “n-3” position; that is, the third carbon from the methyl end of the fatty acid C20:5, n-3 (eicosapentaenoic acid) From the acid end, Δ 5,8,11,14,17

5. Nomenclature • Position of first double bond relative to carboxylic acid or methyl end • n−6 fatty acids: referred to as n-6 (ω−6 fatty acids or omega-6 fatty acids); a family of unsaturated fatty acids that have a double bond in the “n-6” position; that is, the sixth carbon from the methyl end of the fatty acid C18:2, n-6 (linoleic acid) If discuss with respect to the acid end, we would use: 18:2 cis Δ 9,12

6. Lipid Digestion-The Overview • Digestion to component parts • Absorption of component parts by enterocytes • Reassembly of complex lipids • Delivery to systemic circulation via blood (or lymphatics) • Uptake by recipient peripheral tissues (liver, adipose tissue, muscle) • Metabolism and utilization for energy or other processes

7. Fundamental Problem • Fatty acids are not stored in feeds or animal tissues as fatty acids, they are stored as triglycerides (triacylglycerol esters), phospholipids, etc. • Complexed lipids must be digested and the constituent parts absorbed by the enterocyte

8. Triacylglycerol ester

9. Solution to problem • Must hydrolyze to component parts before molecules can traverse the lumen of the intestine and be absorbed by the enterocyte • Must be able to accommodate hydrophobic molecules in an aqueous/hydrophilic environment

10. Digestion • Lipases

11. Triacylglycerol (TAG) digestion • Gastric/Lingual Lipase (Acid Lipase) • Sn-3 position→ 1,2 DAG + FA to help emulsify additional fat • Active to pH 6.5 (through upper duodenum) • Prefers triglycerides composed of medium chain FA (milks are rich in MCT) • Particularly important for newborns and suckling young due to slow development of pancreatic lipase • No activity on PL or cholesterol esters • Gastric lipase + mixing/motility → fine lipid droplets less than 0.5 mm diameter

12. Most fat digestion occurs in the small intestine • Pancreatic lipase • Sn-1 and 3 positions → FFA + 2-MAG • Requires co-lipase to function in presence of bile salts: pro-colipase + trypsin → co-lipase co-lipase + lipase → TAG hydrolysis

13. Not all fatty acids are equally absorbed by the enterocyte • Lard: saturated fatty acids esterified in the sn-2 position: lipase activity produces 2-MAG + free fatty acids, many of which are unsaturated • As the free fatty acid, unsaturated fatty acids are more readily absorbed than are saturated fatty acids

14. Not all fatty acids are equally absorbed by the enterocyte • Beef Tallow: saturated fatty acids esterified in the sn-1 and sn-3 positions: lipase activity produces 2-MAG + free fatty acids, many of which are saturated • As the free fatty acid, saturated fatty acids are less readily absorbed than are unsaturated fatty acids

15. Which has the higher metabolizable energy value, lard or beef tallow? • Lard: less energy lost due to lack of absorption, BECAUSE saturated fatty acids are more readily absorbed as the 2-MAG, and because unsaturated fatty acids are more readily absorbed than saturated fatty acids

16. Animals not consuming just TAG • Phospholipids

17. Phospholipid digestion • Phospholipase A2 • - secreted by pancreas, some activity intrinsic in intestinal mucosa depending on species • - activated by trypsin • - targets sn-2 postion (FFA + Lyso- phosphatidyl choline)

18. Digestion of Cholesteryl Esters The R (fatty acid group) varies across plants and across animals

19. Digestion of Cholesteryl Esters • Cholesterol esterase (carboxyl ester hydrolase, bile salt-stimulated lipase, nonspecific esterase) – Secreted by pancreas, no activation required – Broad esterase activity (TAG, CE, phosphoglycerides, spingolipids, A, D, MAG) – Bile salt micelles (sodium taurocholate); self aggregation to polymeric form (dimer) to protect against proteolytic degradation

20. How do we get lipid digestion products into the blood for distribution to recipient tissues? • Lipids have little solubility in water (minimal polarity) • “Unstirred water layer”, presents a barrier even with vigorous intestinal motility and mixing of intestinal contents

22. Bile is crucial for absorption of lipids • Bile is produced by hepatocytes in the liver, and drains out through the many bile ducts that penetrate the liver • The common bile duct in turn joins with the pancreatic duct to empty into the duodenum; If the sphincter of Oddi is closed, bile is prevented from draining into the intestine and instead flows into the gall bladder, where it is stored and concentrated • Cholesterol is released with the bile, dissolved in the acids and fats found in the concentrated bile solution • When food is released by the stomach into the duodenum in the form of chyme, the gallbladder releases the concentrated bile to provide bile salts to aid in digestion

23. Mixed Micelles at CMC (1-2 mM) • Micelles form from bile salts (acids) + lipid moieties (cholesterol, etc.), engulf hydrophobic products of fat digestion, and provide the polarity that enables these molecules to penetrate the unstirred water barrier • Increase the concentration of lipid digestion products (100-1000X) • Diffusion is thus toward the enterocyte

24. Micelles

25. Entering the Enterocyte • Passive • Lipid-rich brush border • TAG, phospholipids, cholesterol esters reformed to sustain [gradient] • Glycerol, short chain FA, readily diffuse through unstirred water barrier and into enterocyte due to [gradient] Initial diffusion followed by re-esterification at the endoplasmic reticulum

26. Carrier-mediated active transport Fatty acids • High concentrations of FA in the lumen: • diffusion is likely major mechanism of • uptake • Importance of FATP-4 increases as the • concentration of FA decreases FATP-4 Fatty acids

27. Intracellular Metabolism • Must traverse an aqueous cytosol to get to ER • FABP (Villi vs. crypt; jejunum vs. ileum; high fat diet vs. low fat diet) • I-FABP (fatty acids only) • L-FABP (lysophosphatidyl choline, retinoids,…, MAG) • SCP-1 • SCP-2 (cholesterol)

28. Re-esterification • Triacylglycerol complex on cytosolic surface of the ER • Then TAG must penetrate the ER; aided by a transport protein • Abetalioproteinemia- chylomicrons not formed, despite presence of apoB

30. Phospholipids • Lysophatidyl choline, etc. Acylated Phosphatidyl choline Hydrolyzed glycerol 3-phosporyl choline + liver fatty acids MAG TAG • 2 lysophatidyl choline phosphatidyl choline + Glycerol 3-phosporyl choline

31. Cholesterol Cholesterol (diet and endogenous) Free Pool • chylomicrons LYMPH

32. Chylomicrons • Apo A-1, apo A-II, apo B-48 apoE and C added in circulation • Fatty acid composition ~ diet (unlike phospholipids) • ER Golgi prechylomicrons Exocytosis intracellular space

33. Avian vs. Mammalian Species • Mammals: chylomicrons, enter circulation via the lymphatics at the thoracic duct • Avian: portomicrons, transported to the liver via the portal vein, then delivered to systemic circulation

34. Summary • Digestion: lipases • Micelle formation • Uptake of component parts by diffusion and carrier mediated (FATP) processes • Reassembly of triglyceride via MAG or glycerol phosphate pathways • Incorporation of TAG, etc. and apo proteins into lipoproteins called chylomicrons (or portomicrons) in enterocyte • Secretion via exocytosis and entry in systemic circulation via lymphatics or portal circulation via liver