Digestion & Absorption of Lipids Flashcards
What is bile?
Bile is a digestive fluid produced by the liver
Bile is composed of water, electrolytes, bile acids/salts, cholesterol, phospholipids (such as lecithin), bilirubin (a waste product of red blood cell breakdown), and various other organic molecules
What is the function of bile?
Digestion & absorption of fats
Excretion of water-insoluble substances = cholesterol & bilirubin
Bile acids act as detergents to emulsify large fat globules into smaller droplets. Emulsification increases the surface area of fats, making them more accessible to digestive enzymes for breakdown and absorption.
Where is bile stored?
Gall bladder
Bile flows from the common hepatic duct into the cystic duct, which connects to the gallbladder. In the gallbladder, bile is stored and concentrated. Between meals, the gallbladder concentrates bile by removing water and electrolytes, making it more potent for fat digestion when released into the small intestine during digestion.
What triggers bile release?
When food/FA enters the duodenum (1st part of small intestine)
CCK release
Gallbladder contracts and releases stored bile through the common bile duct into the duodenum.
Bile enters the duodenum to aid in the digestion and absorption of fats and fat-soluble nutrients from ingested food
What is bile copmosed of?
Primary and secondary bile acids
Bile pigments
Phospholipids
Electrolytes
Name primary bile acids and how they are formed
Cholic and chenodeoxycholid acids
Synthesized from cholesterol = more water soluble than cholesterol
What happens to primary bile acids in ileum?
Actively reabsored in ileum
Name secondary bile acids and how they are formed
Deoxycholic and lithocholic acids
Deconjugations and dehydroxlation fo primary bile acids by intestinal bacteria
Name bile pigments
Biliirubin & biliverdin = metabolites of haemoglobin
For excretion = yellow colour of bile
What causes brown colour of stool?
Bacteria convert bilirubin to urobilin
Describe the phospholipids that make up bile acids
Most lecithins = 2nd most abundant organic bile compound
Amphipathic
What is the role of phospholipids?
Increase cholesterol solubilization in bile micelles
Stabilizing the crude triglyceride emulsion
What is added to bile acids to make bile salts?
Bile acids conjugated with glycine or taurine = bile salts
What is the difference between bile acids and bile salts?
Bile salts are my hydrophilic because of the glycine or taurine conjugated
What is the more hydrophilic conjugate for bile acids?
Taurine is more hydrophilic than glycine
What properties does bile acid conjugation alter?
More amphiphatic
Resistant to hydrolysis by pancratic enzymes
Define amphiphatic
Refers to a molecule or structure that contains both hydrophilic (water-attracting) and hydrophobic (water-repelling) regions within the same molecule or structure.
What does being amphiphatic alter?
Easier to form micelles = aid in fat absorption
Reabsorb poorly = stay in gut longer
What does the enterohepatic circulation do?
Recycle bile from small intestine to liver and back again
What dictates the rate of bile synthesis?
Depends on how much is returned to the liver
Where is bile reabsorbed?
Terminal ileum = 80-90% reabsorbed
Remaining are excreted into faeces
What amount of bile is needed per meal and what happens to the pool of bile?
Need 4-8g of bile in each meal
Bile pool limited to 3.6g
So total pool recirculates twice per meal
What are the pahses of lipid assimilation and where do they occur?
Digestive phase = luminal
Absorptive phase = mucosal
Post-absorptive phase = delivery???
What enzyme digests fats and where?
Gastric lipase
Pancreatic lipase
What % of lipid digestion happens intragastrically and when does this increase?
Intragastric lipolysis = 20-30% of total lipid digestion
Increases to over 90% if pancreatic lipase secretion decreases
What are lingual lipases?
Lingual lipase is an enzyme produced by specialized cells in the tongue called lingual glands. It plays a role in the digestion of fats and is secreted into the oral cavity (mouth) where it begins the process of fat digestion
What is needed for optimal intra-intestinal digestion of fats?
Alkaline pH
Adequate CALCIUM
Bile salts
Lecithin
Lipolytic enzymes
What is lecithin?
Lecithin, also known as phosphatidylcholine, is a type of phospholipid found abundantly in bile and cell membranes
Lecithin increase detergent power of
bile
What does the pancreas secrete for lipid digestion?
3 water soluble lipases
Glycerol ester hydrolase = main lipase
Cholesterol esterase (ester hydrolase)
Phospholipase A2
Reaction of glycerol ester hydrolase
Triglyceride»_space;> 2-monoglyceride + 2 free FA
Reaction of cholesterol ester hydrolase
Cholesterol ester»_space;> cholesterol + FA
Reaction of phospholipase A2
Lecithin»_space;> lyselecithin + FA
How are lipids emulsified?
Bile salts act as detergents
Allowing fat to be soluble in water
What is affects cholesterol solubility in bile?
What is the composition of micelles?
5nm in diameter
20-30 molecules of lipids & bile salts
What is the role of micelles?
Transport extremely hydrophobic meterials = cholesterol & fat-soluble vitamins
What do micelles have minimum amounts of?
Bile salts
Where are lipids absorbed and from what?
Lipids are absorbed from micelles once in contact with microvilli
What is the rate limiting step of lipid absorption?
Migration of micelle from chyme to microvilli surface
How much of lipids are absorbed?
Almost all ingested lipids = absorbed
Fat in stool is from colonic bacteria & desquamated intestinal cells
What lipid form is absorbed most slowly?
Choelsterol
How is cholesterol absorption decreased?
Increased plant steroids = statins
What is the post-absorptive phase?
Postprandial phase or the fasting state, R
Refers to the period following the absorption of nutrients from a meal. It begins once the nutrients from the ingested food have been absorbed by the gastrointestinal tract and entered the bloodstream for distribution to cells and tissues throughout the body.
What happens to lipids in the post-absorptive phase?
Re-esterification
Formation of chylomicros for re-esterified lipids
Lymphatic transport
No re-esterification for MCFA & SCFA
What lengths are MCFA & SCFA?
Medium = 6-12 C
Short = less than 6 C
What is re-esterification?
Synthesis of triglycerides (TGs) from fatty acids (FAs) and glycerol within adipose tissue.
This process allows for the storage of excess energy in the form of fat for later use during periods of fasting or energy deficit.
Why are MCFAs and SCFAs not re-esterified as much?
More used for energy metabolism, providing a readily available source of fuel for cellular processes
They are more water soluble so can be transported more easily in the bloodstream and readily taken up by tissues for immediate use as energy substrates.
Once absorbed from the intestine, MCFAs and SCFAs are quickly transported to the liver via the portal vein.
In the liver, they undergo rapid oxidation (beta-oxidation) to generate energy through the tricarboxylic acid (TCA) cycle.
Because of their shorter carbon chain length, MCFAs and SCFAs can enter mitochondria more efficiently than LCFAs and are metabolized more rapidly for energy production.
