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Lipid Digestion

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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

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Bile Produced in liver, stored in gallbladder

Except horse

Alkaline solution composed of: Bile salts Cholesterol Lecithin Bilirubin

Responsible for fat emulsification Detergent action

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

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Digestion of Lipid Bile salts emulsify lipids

Pancreatic lipase acts on triglycerides

Triglycerides sn-2 monoglyceride + 2 fattyacids

Pancreatic colipase

Activated by trypsin

Interacts with triglyceride and pancreatic lipase

Displaces bile to allow recycling

Improves activity of pancreatic lipase

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Pancreatic Colipase Secreted from pancreas as

procolipase

Activated (cleaved) bytrypsin

Anchors lipase to the

micelle One colipase to one lipase

(i.e., 1:1 ratio)

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Dietary Fat

(large TG droplet)

Bile Salts

Lipid emulsion

Lipase 2-Monoglyceride

+ 2 FFA

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Emulsification Produces small lipid spheres

Greater surface area

Lipases attack TG at 1 and 3 positionsGly

cerol

Fatty Acid1

Fatty Acid2

Fatty Acid3

Lipase

Gly

cerol

Fatty Acid3

Fatty Acid1

Fatty Acid2

Triglyceride 2-Monoglyceride

+

2 Free Fatty Acids

2 H20

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Digestion of Lipid Phospholipase A1 and A2

Hydrolyzes fatty acids from phospholipids

Cholesterol esterase

Hydrolyzes fatty acids from cholesterolesters

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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 toform mixed micelles

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Micelle Formation

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Nutrient Absorption - Lipids Fatty acids, 2-monoglycerides,

cholesterol, and cholesterol esters move

down concentration gradient (passivediffusion)

Repackaged in intestinal cell fortransport to liver Some is reformed into triglycerides

Chylomicrons

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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)

Glycerol and short chain fatty acids directlyenter mesenteric blood

These protein-coated lipid droplets are called chylomicrons

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Lipid Absorption

Short andmedium

chain fattyacids

simple diffusion exocytosis

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Lipid Absorption (Chylomicrons)

Chylomicrons absorbed fromenterocytes 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

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Overview of Fatty Acid Uptake Short- and medium-chain fatty acids

Enter portal blood directly from enterocytes

Bound to albumin in blood AlbuminFFA complex

Oxidized in liver or elongated and used fortriglyceride 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

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Overview ofLipid

Digestion inMammals

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Overview of Lipid Digestionand Absorption in Avians

Portal

blood

*

*Lymph in mammals

Fatty

acid

binding

protein

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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 becomesingle bonds

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Biohydrogenation

Weight percent of fattyacids

Fatty acid Diet Abomasaldigesta

16:0 (palmitic)

18:0 (stearic)18:2 (linoleic)

18:3 (linolenic)

26

617

31

29

454

6

Sheep fed alfalfa hay

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Biohydrogenation

Reduction of double bonds

Result: fatty acids that are more

saturated with hydrogen

Saturated

Unsaturated

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Biohydrogenation of Linoleic Acid

Linoleic acid (18:2)

cis-9, trans-11 CLA

trans-11 18:1

Stearic acid (18:0)

isomerase

reductase

reductase

Intermediate fat ty acids are conjug ated l in oleic acids

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Lipid Digestion and Synthesisby 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

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Effect of Lipid on RumenFermentation

Excess amounts of unsaturated fatty acidsand 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

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Lipid Digestion - Ruminant

Digestion and absorption of lipids issimilar to monogastrics except

Fat enters small intestine in different formthan was presented to animal in diet

Lipids absorbed more slowly