Digestion and absorption of fat Flashcards
What is dietary fat?
90% are triglycerides
Mostly long chains
Acyl esters of glycerol
What is the structure of dietary fat?
Vary in chain length
Double bonds
Saturated - no double bonds
What are lipids?
5% come from cell membranes - phospholipids and sphingolipids
Phospholipid is a triglyceride with 2 fatty acid chains, and 1 phosphate.
Phosphatidylcholine, phosphatidylethanolamine, phosphatidylserine.
What are sphingolipids?
Membrane lipids
Serine not glycerol backbone
e.g. ceramide and skin
What are essential fatty acids?
Cannot be made endogenously.
Linoleic acid (omega 6) and linolenic acid (omega-3).
In the absence of linoleic acid also arachidonic acid
What are the functions of essential fatty acids?
Cell membranes
Erythropoiesis - red blood cells
Eicosanoids - signalling pathways
Blood pressure / viscosity
What are the lipases in digestion?
Lingual lipase - from the tongue, from Von Ebner glands, starts the process, optimum at low pH - less effective in mouth, more in stomach.
Gastric lipase - from chief cells in stomach.
Pancreatic lipase - from acinar cells
What is the process of lipid digestion in the stomach?
Releases a single fatty acid to form a diglyceride.
Long chains stay with triglyceride droplets (insoluble at acidic pH).
Shorter chains remain in solution, cross the gastric mucosa and enter the portal blood.
15% of digestion occurs.
What do fatty acids do in the intestine?
Fatty acids go from the stomach to the duodenum and stimulate enteroendocrine cells to stimulate CCK release.
What does CCK do in lipid digestion?
Stimulates contraction of the gall bladder which contains bile.
Stimulates contraction and dilation of the Sphincter of Oddi, bile released into duodenum.
Stimulates pancreas to release pancreatic juices, which contains pancreatic lipase.
What does secretin do in lipid digestion?
Stimulates contraction of the gall bladder which contains bile.
Stimulates contraction and dilation of the Sphincter of Oddi, bile released into duodenum.
Stimulates pancreas to release pancreatic juices, which contains pancreatic lipase.
What does secretin do in lipid digestion?
Secretin released from enteroendocrine cells.
Increase in HCO3-, helps achieve optimum pH for pancreatic lipase in small intestine - alkaline.
Neutralises the chyme from the stomach.
What does bile do?
Neutralises gastric pH.
Emulsifies fats.
Produces a large surface area for lipase.
Bile surrounds the triglycerides, so colipase is required for lipase to act.
What is colipase?
Comes from the pancreas.
Facilitates attachment of lipase to the droplet.
For efficient enzyme activity.
Prevents inhibitory effect of bile.
Produces 2-monoacylglycerol and fatty acid chain.
What is digestion of droplets?
Surface triglycerides are hydrolysed.
Replaced by a core lipid.
Produces a smaller lipid, which is made smaller by pancreatic lipase.
Vesicles bud off.
What is budding off of vesicles?
Multi-lamellar vesicle becomes Uni-lamellar vesicle
Gets smaller by lipase in bile salts, which removes the covering on triglycerides.
Unilamellar vesicle can be absorbed, it contains medium and short chain fatty acids.
The unilamellar vesicles then become mixed micelles, which contains long fatty acid chains.
How do short fatty acids enter the enterocyte?
Short chain fatty acids are in unilamellar, so there is only a small covering.
So they can be absorbed directly into the enterocytes in the jejunum, as they are water soluble.
Bile is reabsorbed then enters enterohepatic recirculation in the terminal ileum.
How do long fatty acids enter the enterocyte?
These are in mixed micelles, which have a large covering, so cannot be absorbed through the brush border.
Antiport proton attaches to the fatty acid and protonates it, which brings the fatty acid into the enterocyte,
Bile salts are reabsorbed and go into enterohepatic recirculation.
What do fatty acids do in the enterocyte?
Glycerol is added to long chain fatty acids and converted back to triglycerides in the SER.
Fat droplets form in the cisternae of the SER.
Apoproteins are synthesised in the RER and then transferred to the SER, and associate with lipid droplets.
What do fatty acids do in the golgi of the enterocyte?
New chylomicrons and Very Low Density Lipoproteins (VLDLs) arrive at the cis face of the Golgi apparatus, apoproteins are glycosylated.
Vesicles carrying chylomicrons and VLDLs bud off from the trans-Golgi, and move to the basolateral membrane.
Transport vesicles fuse with the basolateral membrane, releasing chylomicrons and VLDLs.
How do fatty acids leave the enterocytes?
Chylomicrons and VLDLs pass through large interendothelial channels of lymphatic capillaries and enter the lymph.
Chylomicrons then enter the subclavian vein via the thoracic duct.
How do short fatty acids leave the enterocyte?
Glycerol and short fatty acid chains pass through the enterocyte and enter the blood capillary.
What happens to chylomicrons in the blood?
Lipoprotein lipase breaks up chylomicrons into fatty acid and glycerol.
Remnant chylomicrons and glycerol go to the liver.
What do the fatty acids do from the blood?
Forms adipocytes and goes to muscles.
Fat storage.
Beta oxidation
What are the functions of lipids?
Major energy source in the fasted state.
Most concentrated source of energy.
Non-polar can be stored in anhydrous state.
What is the location of lipids?
Stored all over the body
Most in sub-cutaneous tissue in adipose - can be used as extra source of energy.
Obesity - visceral deposits around the organs.
Which vitamins are fat soluble?
ADEK
These are solubilised with micelles and adsorbed.
What is vitamin A?
Called Retinol in animals.
Beta carotene is from plants, and converted to retinol in the body.
Deficiency causes night blindness as vit A is needed for rhodopsin.
What is vitamin D?
From sunlight
Needed to absorb other things - e.g. Calcium
What is vitamin D deficiency?
Poor diet/lack of sun:
Bone softening
Demineralisation
Rickets in children
Osteomalacia and osteoporosis in adults
What is vitamin E?
a-tocopherol
Found majorly in oily foods.
Antioxidant - protection against cardiovascular disease.
What is a deficiency in vitamin E?
Effects are related to not absorbing enough antioxidants.
The immune system creates ROI and RNI, which need inactivating by antioxidants.
Rare.
What is vitamin K?
a-tocopherol
Majorly found in green leafy veg.
Produced by intestinal microbiota.
Needed for carboxylation of glutamate residues in proteins to form y-carboxyglutamate residues.
What is vitamin K deficiency?
From genetics or diet can cause:
Poor blood coagulation
Bone metabolism - malformation of developing bone
Vascular biology - prevents deposition of Ca salts in arterial vessel walls.
What is activation of vitamin K?
Reduced by Quinone reductase to vitamin KH2.
This then forms GGCX, which converts glutamate residues to y-carboxyglutamate.
GGCX is converted to vitamin KO, then reduced by VKOR (inhibited by Warfarin) to Vitamin K.
What is fat malabsorption?
Many of the vitamin deficiencies are due to poor nutrition or intestinal malabsorption.
Could be due to liver/gallbladder disease, pancreatic disease, or intestinal disease.
What are the symptoms of fat malabsorption?
Steatorrhea - fat in faeces.
Essential fatty acid deficiency and or vitamin deficiency.
How is fat malabsorption diagnosed?
Faecal fat test is the diagnostic test to evaluate the amount of fat digested and absorbed.
