Lipid Digestion and Absorption

Lingual lipase exists in

rats and some other rodent species - not in humans

lipase from human breast milk (bile salt-stimulated lipase) is take up by

infants and activated in the small intestine

majority of lipid digestion starts in the stomach:

-contraactions and churning results in fat/lipid droplets

-gastric lipase

-chyme 

contractions and churning results in fat/lipid droplets causes 

physical reduction in size 

gatric lipase

responsible for up to 25% of TAG hydrolysis, pH optimum ~4.0

chyme (composed of fat/lipid droplets) is allowed into the

small intesting via the pyloric sphincter

microorganisms synthesize lipases and

secrete them into the rumen 

Extracellular lipases completely hydrolyze

TAGs and other dietary lipids 

unsaturated fatty acids are extensively hydrogenated by other species of microorganisms into

stearic acid, cis-9, trans-11 CLA, trans-10, cis-12 CLA, and trans—vaccenic acid (18:1 trans-11)

only small amounts of TAGs and phospholipids enter the small intestine in ruminants, free fatty acids primarily

enter the small intestine

lipid digestion in the small intestine occurs

in the duodenum

challanges of lipid digestion in the small intestine are 

lipids are hydrophobic, TAGs can not be absorbed due to their size 

solutions to the challanges faced by lipid digestionin the small intestine: 

bile salts and pancreatic enzymes (lipase) are mixed with TAGs (TAGs become emulsified and become mixed micelles) 

secretion of cholecystokinin from the intestinal mucosal cells (stimulated by the presence of fat in SI) ccauses: 

gall bladder contractions, secretion of pancreatic digestive enzymes (~1200mL/day) 

gall bladder contractions

holds ~40 to 70 mL of bile salts and bile acids and releases ~700 mL/day (extensive recycling)

secretion of pancreatic digestive enzymes (~1200 mL/day) 

pancreatic lipase (very small amounts in ruminants), colipase 

colipase 

activates lipase in the presence of bile salts 

bile acid functions:

emulsify fats, activation of pancreatic lipase (at low concentration), formation of mixed micelles

bile salt emulsification

bile acids and glycine or taurine conjugates serve as detergents, cause the formation of TAG particles of 1 microm or less, other emulsifiers 

emulsifiers involved in lipid digestion other than bile salt

phospholipids (esp. lysolecithin), 2-monoacylglycerols

hydrolysis of dietary fat by pancreatic lipase wors at lipid-water interface and is activated by 

bile salts at low concentrations 

hydrolysis of dietary fat by pancreatic lipase is inhibited by 

bile salts at high concentration, hydrolyzes TAG to fatty acids and 2-monoacylglyerol (2-MAG)

pancreatic colipase is secreted by

the pancreas with lipase

pancreatic colipase binds to bile salt emulsion droplets and reduces

inhibitory action of bile salts on pancreatic lipase

absorption in the small intestin involves the formation of

mixed micelles

mixed micells are 4-6nm in diameter and are formed when

bile salts and fatty acids reach a critical micellar concentration spontaneously form

mixed micelles incorporate 2-MAGs, lysolecithin, free cholesterol, and long-chain fatty acids essentially 

detergents in themselves 

absorption in the the enterocytes (intestinal mucosal cells) by 2-MAG, lyso-phospholipids, fatty acis, and cholesterol dissociate from 

micelles at the surface of mucosal cells and produce locally high concentrations of 2-MAG, lyso-phospholipids and fatty acids 

the production from absorption into the enterocytes are then absorbed by the entrocytes of the 

duodenum and proximal jejunum 

synthesis of TAG and phospholipids in encytes produce 

triglycerols 

75% of TAGs are synthesized via the 2-MAG pathway located on the smooth endoplasmic reticulum, 25% of TAGs are synthesized via the 

standard TAG biosynthetic pathway located on the rough endoplasmic reticulum 

triaclyglycerols include

phopholipids (especially lecithin) and cholesterol esters 

synthesis of TAG and phopholipids in enterocytes cause the formation of

chylomicrons

chylomicrons synthesis starts when

lipid droplets form within the ER and goligi apparatus, these droplets contain TAG, phospholipids, cholesterol, cholesterol ester and apolipoprotein (with carbohydrates) complexes

apolipoproteins and phospholipids are formed in the rER and then TAG are formed in the

sER and transferred to apolipoprotein B to form a core for the chylomicron

after accumulating TAG and cholesterol ester, golgi vesciles form and CHO moietes are 

added to the apolipoproteins

golgi vescles fuse with the cell membrane and are extruded into the lacteals, chylomicrons are then transported 

via the lymphatics 

composition of chylomicrons

70-90% TAG, 4-8% phospholids, 3% cholesterol, 4% cholesterol ester, 2% protein (apolipoprotein B) 

apolipoprotein

protein that binds lipid for transport through circulatory and lymphatic systems 

ruminants do not synthesize true chylomicrons, too little TAG because

they consume little dietary fat, very low density lipoprotein (similar to chylomicrons)

dietary cholesteral absorption in humans

~0.4-0.5 g/day

dietary cholesteral digestion in herbivores

virtually no cholesterol ingested

biliary

20-30 g bile salts enter the SI daily, cholesterol and cholesterol ester enter the sI via bile 

a minor contributor to total cholesterol intake is 

intestinal mucosa

a minor contributor to total cholesterol intake

ruminal microflora

mechanism of cholesterol absorption in the small intestine starts when cholesterol esters are 

hydrolyzed by pancreatic cholesterol esterases 

free cholesterol is incorporated into mixed micelles (bile acid presence is needed); free cholesterol is absorbed into 

the intestional mucosal cells and re-esterfied to form cholesterol esters 

sources of cholesterol loss from GI tract

bile salts, poorly absorbed, plant sterols

0.8 g/day bile salts are lost in feces while the rest is 

taken up in the ileum for enterohepatic circulation 

cholesterol is porly absorbed: ~0.4 g/day lost in feces and 

only 30-40% dietary and biliary cholesterol is absorbed 

200-300 mg/day of plant sterols are ingested and 

absorbed in trace amounts 

in large amounts, plant sterols

inhibit cholesterol absorption

0.2-0.4 g/day of cholesterol is lost as

sloughed skin

cholesterol ester is

cholesterol with long chain fatty acids linked to the hydroxyl group

cholesterol ester is much less polar when

compared to normal cholesterol

cholesterol ester preferred form of transport is

in the blood