Digestion & Absorption of Lipids

Question 1

What is bile?

Answer 1

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

Question 2

What is the function of bile?

Answer 2

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.

Question 3

Where is bile stored?

Answer 3

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.

Question 4

What triggers bile release?

Answer 4

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

Question 5

What is bile copmosed of?

Answer 5

Primary and secondary bile acids
Bile pigments
Phospholipids
Electrolytes

Question 6

Name primary bile acids and how they are formed

Answer 6

Cholic and chenodeoxycholid acids

Synthesized from cholesterol = more water soluble than cholesterol

Question 7

What happens to primary bile acids in ileum?

Answer 7

Actively reabsored in ileum

Question 8

Name secondary bile acids and how they are formed

Answer 8

Deoxycholic and lithocholic acids

Deconjugations and dehydroxlation fo primary bile acids by intestinal bacteria

Question 9

Name bile pigments

Answer 9

Biliirubin & biliverdin = metabolites of haemoglobin

For excretion = yellow colour of bile

Question 10

What causes brown colour of stool?

Answer 10

Bacteria convert bilirubin to urobilin

Question 11

Describe the phospholipids that make up bile acids

Answer 11

Most lecithins = 2nd most abundant organic bile compound

Amphipathic

Question 12

What is the role of phospholipids?

Answer 12

Increase cholesterol solubilization in bile micelles
Stabilizing the crude triglyceride emulsion

Question 13

What is added to bile acids to make bile salts?

Answer 13

Bile acids conjugated with glycine or taurine = bile salts

Question 14

What is the difference between bile acids and bile salts?

Answer 14

Bile salts are my hydrophilic because of the glycine or taurine conjugated

Question 15

What is the more hydrophilic conjugate for bile acids?

Answer 15

Taurine is more hydrophilic than glycine

Question 16

What properties does bile acid conjugation alter?

Answer 16

More amphiphatic
Resistant to hydrolysis by pancratic enzymes

Question 17

Define amphiphatic

Answer 17

Refers to a molecule or structure that contains both hydrophilic (water-attracting) and hydrophobic (water-repelling) regions within the same molecule or structure.

Question 18

What does being amphiphatic alter?

Answer 18

Easier to form micelles = aid in fat absorption
Reabsorb poorly = stay in gut longer

Question 19

What does the enterohepatic circulation do?

Answer 19

Recycle bile from small intestine to liver and back again

Question 20

What dictates the rate of bile synthesis?

Answer 20

Depends on how much is returned to the liver

Question 21

Where is bile reabsorbed?

Answer 21

Terminal ileum = 80-90% reabsorbed
Remaining are excreted into faeces

Question 22

What amount of bile is needed per meal and what happens to the pool of bile?

Answer 22

Need 4-8g of bile in each meal

Bile pool limited to 3.6g
So total pool recirculates twice per meal

Question 23

What are the pahses of lipid assimilation and where do they occur?

Answer 23

Digestive phase = luminal
Absorptive phase = mucosal
Post-absorptive phase = delivery???

Question 24

What enzyme digests fats and where?

Answer 24

Gastric lipase
Pancreatic lipase

Question 25

What % of lipid digestion happens intragastrically and when does this increase?

Answer 25

Intragastric lipolysis = 20-30% of total lipid digestion Increases to over 90% if pancreatic lipase secretion decreases

Question 26

What are lingual lipases?

Answer 26

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

Question 27

What is needed for optimal intra-intestinal digestion of fats?

Answer 27

Alkaline pH Adequate CALCIUM Bile salts Lecithin Lipolytic enzymes

Question 28

What is lecithin?

Answer 28

Lecithin, also known as phosphatidylcholine, is a type of phospholipid found abundantly in bile and cell membranes Lecithin increase detergent power of bile

Question 29

What does the pancreas secrete for lipid digestion?

Answer 29

3 water soluble lipases Glycerol ester hydrolase = main lipase Cholesterol esterase (ester hydrolase) Phospholipase A2

Question 30

Reaction of glycerol ester hydrolase

Answer 30

Triglyceride >>> 2-monoglyceride + 2 free FA

Question 31

Reaction of cholesterol ester hydrolase

Answer 31

Cholesterol ester >>> cholesterol + FA

Question 32

Reaction of phospholipase A2

Answer 32

Lecithin >>> lyselecithin + FA

Question 33

How are lipids emulsified?

Answer 33

Bile salts act as detergents Allowing fat to be soluble in water

Question 34

What is affects cholesterol solubility in bile?

Question 35

What is the composition of micelles?

Answer 35

5nm in diameter 20-30 molecules of lipids & bile salts

Question 36

What is the role of micelles?

Answer 36

Transport extremely hydrophobic meterials = cholesterol & fat-soluble vitamins

Question 37

What do micelles have minimum amounts of?

Answer 37

Bile salts

Question 38

Where are lipids absorbed and from what?

Answer 38

Lipids are absorbed from micelles once in contact with microvilli

Question 39

What is the rate limiting step of lipid absorption?

Answer 39

Migration of micelle from chyme to microvilli surface

Question 40

How much of lipids are absorbed?

Answer 40

Almost all ingested lipids = absorbed Fat in stool is from colonic bacteria & desquamated intestinal cells

Question 41

What lipid form is absorbed most slowly?

Answer 41

Choelsterol

Question 42

How is cholesterol absorption decreased?

Answer 42

Increased plant steroids = statins

Question 43

What is the post-absorptive phase?

Answer 43

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.

Question 44

What happens to lipids in the post-absorptive phase?

Answer 44

Re-esterification Formation of chylomicros for re-esterified lipids Lymphatic transport No re-esterification for MCFA & SCFA

Question 45

What lengths are MCFA & SCFA?

Answer 45

Medium = 6-12 C Short = less than 6 C

Question 46

What is re-esterification?

Answer 46

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.

Question 47

Why are MCFAs and SCFAs not re-esterified as much?

Answer 47

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.