Lipid Digestion
Monogastric Digestion
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Challenges
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Lipids are not water soluble
Triglycerides too large to be absorbed
Digestive solution

Triglycerides mix with bile and pancreatic
secretions
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Emulsification and digestion
Bile

Produced in liver, stored in gallbladder

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Alkaline solution composed of:

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Except horse
Bile salts
Cholesterol
Lecithin
Bilirubin
Responsible for fat emulsification

Detergent action
Mixed micelle formed by bile salts, triacylglycerols and pancreatic lipase.
Digestion of Lipid

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Bile salts emulsify lipids
Pancreatic lipase acts on triglycerides
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Triglycerides
acids
sn-2 monoglyceride + 2 fatty
Pancreatic colipase
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Activated by trypsin
Interacts with triglyceride and pancreatic lipase

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Displaces bile to allow recycling
Improves activity of pancreatic lipase
Pancreatic Colipase

Secreted from pancreas as
procolipase

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Activated (cleaved) by
trypsin
Anchors lipase to the
micelle

One colipase to one lipase
(i.e., 1:1 ratio)
Bile Salts
Dietary Fat
(large TG droplet)
Lipase
2-Monoglyceride
+ 2 FFA
Lipid emulsion
Emulsification

Produces small lipid spheres

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Greater surface area
Lipases attack TG at 1 and 3 positions
G Fatty Acid1
l
y
c
e Fatty Acid2
r
o
l Fatty Acid3
Triglyceride
Lipase
2 H20
G
l
y
c Fatty Acid2
e
r
o
l
2-Monoglyceride
+
Fatty Acid1
Fatty Acid3
2 Free Fatty Acids
Digestion of Lipid

Phospholipase A1 and A2

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Hydrolyzes fatty acids from phospholipids
Cholesterol esterase

Hydrolyzes fatty acids from cholesterol
esters
Micelle Formation
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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
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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
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Fatty acids, 2-monoglycerides,
cholesterol, and cholesterol esters move
down concentration gradient (passive
diffusion)
Repackaged in intestinal cell for
transport to liver
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Some is reformed into triglycerides
Chylomicrons
Lipid Absorption
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Once in enterocyte
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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)
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Chylomicrons absorbed from
enterocytes into lacteals (lymph
vessels)
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Ultimately enter blood via thoracic duct
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Most long chain fatty acids absorbed into
lymphatic system
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Exception is poultry
Blood lipids transported as lipoproteins
Lipid Absorption (Direct Transfer)
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Alternate route is for free fatty acids to
enter circulation directly
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Free fatty acids (FFA) also called nonesterified 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
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Short- and medium-chain fatty acids
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Enter portal blood directly from enterocytes
Bound to albumin in blood
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Albumin–FFA complex
Oxidized in liver or elongated and used for
triglyceride formation
Long-chain fatty acids
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Form chylomicrons
Drain into the lymphatics via the lacteal in
mammals (no lacteal in avian small intestinal villi)
Enter bloodstream at the thoracic duct
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Upstream from liver
Slow entry into the blood
In the
Enterocyte...
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Long-chain fatty acids (more than 10–12
carbons) are bound to fatty acid binding
protein (FABP)
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Transport to the endoplasmatic reticulum
(ER)
In the ER
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Re-formation of triglycerides
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From two free fatty acids and one 2monoacylglycerol
In the Enterocyte...
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Newly formed triglycerides accumulate
as ‘lipid droplets’ at the endoplasmic
reticulum
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Coated with a protein layer
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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
Portal
blood*
Fatty acid
binding protein
Overview of Lipid Digestion
and Absorption in Avians
*Lymph in mammals
Repackaging in the Liver
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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
VLDL
TG
LDL
HDL
Lipoproteins
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Classified by density
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Protein:lipid ratio
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More protein, increased density
More lipid, decreased density
Four classes of lipoproteins
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Chylomicrons
VLDL
Formed in
LDL
liver
HDL
The Good, the Bad, and the Ugly
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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
1st formed in liver – “bad” cholesterol
Transports cholesterol to cells
Transports cholesterol from cells to liver
Lipid Transport
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Free fatty acids transported as complex
with albumin in blood
Lipids rapidly removed from blood
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Liver
Fat depots
Other tissue
Lipid Digestion - Ruminants
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Microbes rapidly modify lipids:
Lipolysis
Triglycerides

Glycerol + 3 free fatty acids
Biohydrogenation
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Addition of H to unsaturated fatty acids
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Saturation
If carried to completion, all double bonds become
single bonds
Biohydrogenation
Fatty acid
16:0
18:0
18:2
18:3
(palmitic)
(stearic)
(linoleic)
(linolenic)
Sheep fed alfalfa hay
Weight percent of fatty
acids
Diet
Abomasal
digesta
26
6
17
31
29
45
4
6
Biohydrogenation
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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
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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
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Decrease methane production
Impair fiber digestion
Form soaps
Alter rumen metabolism towards propionate
production – less acetate
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Decrease milk fat
Produce trans fatty acids
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Inhibit lipid synthesis in mammary gland
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Decrease milk fat
Lipid Digestion - Ruminant
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Digestion and absorption of lipids is
similar to monogastrics except
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Fat enters small intestine in different form
than was presented to animal in diet
Lipids absorbed more slowly
More transported as VLDL