digestion and absorption of dietary fats in non ruminants l.
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Digestion and absorption of dietary fats in non-ruminants. 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 Δ 9 oleic acid 18:2 cis Δ 9,12 linoleic acid

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background and review
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)
nomenclature
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)
nomenclature4
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

nomenclature5
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

lipid digestion the overview
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
fundamental problem
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
solution to problem
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
digestion
Digestion
  • Lipases
triacylglycerol tag digestion
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
most fat digestion occurs in the small intestine
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

not all fatty acids are equally absorbed by the enterocyte
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
not all fatty acids are equally absorbed by the enterocyte14
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
which has the higher metabolizable energy value lard or beef tallow
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
phospholipid digestion
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)
digestion of cholesteryl esters
Digestion of Cholesteryl Esters

The R (fatty acid group) varies across plants and across animals

digestion of cholesteryl esters19
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

how do we get lipid digestion products into the blood for distribution to recipient tissues
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
bile is crucial for absorption of lipids
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
mixed micelles at cmc 1 2 mm
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

entering the enterocyte
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

carrier mediated active transport
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

intracellular metabolism
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)
re esterification
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
phospholipids
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

cholesterol
Cholesterol

Cholesterol (diet and endogenous)

Free

Pool

  • chylomicrons

LYMPH

chylomicrons
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

avian vs mammalian species
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
summary
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