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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
Bile Produced in liver, stored in gallbladder
Except horse Alkaline solution composed of:
Bile salts Cholesterol Lecithin Bilirubin
Responsible for fat emulsification Detergent action
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
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)
Emulsification Produces small lipid spheres
Greater surface area Lipases attack TG at 1 and 3
positionsGlycerol
Fatty Acid1
Fatty Acid2
Fatty Acid3
Lipase
Glycerol
Fatty Acid3
Fatty Acid1Fatty
Acid2
Triglyceride 2-Monoglyceride
+
2 Free Fatty Acids
2 H20
Digestion of Lipid
Phospholipase A1 and A2
Hydrolyzes fatty acids from phospholipids
Cholesterol esterase Hydrolyzes fatty acids from
cholesterol esters
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
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
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
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
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
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
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
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
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 moleculesThese protein-coated lipid droplets are called chylomicrons
Overview of Lipid Digestion and Absorption in Avians Portal
blood*
*Lymph in mammals
Fat
ty a
cid
bind
ing
prot
ein
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
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
The Good, the Bad, and the Ugly
Lipoproteins are similar to chylomicrons
They are lipid wrapped in proteins and cholesterol
When cholesterol is measuredVLDL = “bad” cholesterolHDL = “good” cholesterol
Lipoproteins
Transports cholesterol to cells
Transports cholesterol from cells to liver
Formed in enterocytes 1st formed in liver – “bad” cholesterol
Lipid Transport
Free fatty acids transported as complex with albumin in blood
Lipids rapidly removed from blood Liver Fat depots Other tissue
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
become single bonds
Biohydrogenation
Weight percent of fatty acids
Fatty acid Diet Abomasal digesta
16:0 (palmitic)18:0 (stearic)18:2 (linoleic)18:3 (linolenic)
2661731
294546
Sheep fed alfalfa hay
Biohydrogenation Reduction of double bonds Result: fatty acids that are more
saturated with hydrogen
Saturated
Unsaturated
Biohydrogenation of Linoleic Acid
Linoleic acid (18:2)
cis-9, trans-11 CLA
trans-11 18:1
Stearic acid (18:0)
isomerase
reductase
reductase
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
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