Digestion and Absorption of Fat

Question 1

when is fat an energy source?

Answer 1

in the fasted state

Question 2

What makes lipids good storage molecules?

Answer 2

non-polar and can be stored in the anhydrous state

2x amount of energy per gram compared to other sources

Question 3

What does Beta-oxidation produce?

Answer 3

acetyl-CoA which can go into the TCA cycle

Question 4

Why are fats important for some vitamins?

Answer 4

vitamins A, D, E and K are fat-soluble and depend on solubilisation within bile micelles for intestinal absorption

Question 5

What diseases can vitamin D deficiency lead to?

Answer 5

• impaired bone mineralisation so rickets and osteomalacia

- also can be linked to osteoporosis and cancer

Question 6

What are some ways you can get vit D?

Answer 6

sunlight

egg yolk

Question 7

Ways to get vit E?

Answer 7

veg oils
palm oil
olive oil

Question 8

What is vit E important for?

Answer 8

antioxidant protecting against cardiovascular disease and cancer

Question 9

What are the symptoms of vit E deficiency?

Answer 9

neurological problems due to poor nerve conduction

Question 10

What is vit K important for?

Answer 10

blood coagulation
bone metabolism
vascular biology or deposition of insoluble calcium stars in the arterial vessel walls

Question 11

What are some symptoms of essential fatty acid deficiency?

Answer 11

skin atrophy 
dry skin
impaired vision
mood swings
impaired growth

Question 12

What form of fat is 90% of dietary fat?

Answer 12

triglycerides

Question 13

Why is not all fat derived from the diet?

Answer 13

-endogenous lipids in the GI lumen are predominately from bile which contains: phospholipids, unesterified cholesterol, membrane lipids from desquamated cells, lipids derived from dead colonic bacteria

Question 14

extra

Answer 14

!!!

Question 15

What are the ways that emulsification can take place

Answer 15

1. chewing and gastric churning allows mixing of lingual and gastric juices
2. squirting of gastric contents in the duodenum
3. food prep
4. intestinal peristalsis mixes luminal contents with pancreatic and biliary secretions

Question 16

Why is emulsification required?

Answer 16

to reduce the size of the lipid droplets and increase their surface area to ratio volume

Question 17

What prevents lipid particles from coalescing?

Answer 17

• coating of emulsification droplets with membrane lipids, denatured protein, dietary polysaccharides etc
• polar groups of phospholipid project into the water
• core of emulsion parcel composed of triglyceride which contains cholesteryl esters and other non-polar lipids

Question 18

Give a summary of lipid digestion

Answer 18

1. lipid digestion begins in the mouth mediated by lingual lipase
2. in the stomach, both lingual lipase and gastric lipase digest large amounts of lipid
3. lingual and gastric lipase release a single fatty acid from triglycerides, leaving behind intact diglycerides

Question 19

What happens to the released long chain fatty acids?

Answer 19

they are not absorbed in the stomach as they re insoluble at acidic PH and so remain in the core of the triglyceride droplets

Question 20

What happens to the medium and short chain fatty acids?

Answer 20

mainly ionised at the acidic gastric PH, remain in solution and are passively absorbed across the gastric mucosa into portal blood

Question 21

What happens once the fatty acids reach the duodenum?

Answer 21

they trigger the release of CCK from I cells in the duodenum mucosa

Question 22

What does CCK stimulate?

Answer 22

1. the flow of bile into the duodenum by gall bladder contraction and relaxation of the sphincter of Oddi
2. The secretion of pancreatic enzymes including lipase and esterases
3. stimulate production of zymogen

Question 23

What is the major lipolytic enzyme?

Answer 23

pancreatic lipase (digests all dietary triglycerides not hydrolysed in the stomach)

Question 24

What is required for full lipolytic activity of pancreatic lipase?

Answer 24

1. colipase (for ligand binding conformation)
2. alkaline PH
3. bile salts
4. fatty acids

Question 25

What is colipase important for?

Answer 25

1. acting as the anchor for the binding of the lipase | 2. and or by first forming a colipase-pancreatic-lipase complex that can then bind to the lipid interface

Question 26

What is the product to pancreatic lipase?

Answer 26

- fatty acids | - 2-monoacylglyceride

Question 27

How does lipid absorption occur?

Answer 27

- as surface triglycerides are hydrolysed they re replaces by triglycerides from the core of the emulsion particle resulting in an ever decreasing emulsion droplet - this leads to the budding off from the main droplet to form multi-lamellar vesicle (several lipid bilayers). This occurs due to fatty acids, monoglycerides, bile salts etc building up on the surface of the emulsion particle - addition of more bile salts to the multilamellar vesicles thins out the lips coating and coverts it to unilamellar vesicles, which are then converted into mixed micelles composed of bile salts and mixed lipids

Question 28

What happens as the fatty acid chain length gets longer?

Answer 28

solubility decreases and partitioning into micelles increases

Question 29

What are the barriers that lipolytic products need to overcome to get into the enterocytes?

Answer 29

- mucus gel layer lining the intestinal epithelial surface - the unstirred water layer - the apical membrane

Question 30

Describe short/medium chain fatty acid absorption ease into the enterocyte

Answer 30

-readily soluble in water, so diffusion through unstirred layer into the enterocyte is efficient

Question 31

What happens when the fatty acid/bile mixed micelles reach the enterocyte surface?

Answer 31

-they encounter a low-PH generated by the Na-H exchange at the brush border membrane -fatty acids are protonated and leave the mixed micelle to enter the enterocyte by: non-ionic diffusion, collision and incorporation of the fatty acid into the cell membrane or an active carrier mediated process into the lymphatic system

Question 32

What happens to bile salts after entry?

Answer 32

absorbed at the terminal ileum and colon where they are redirected to the liver and portal blood (enterohepatic circulation)

Question 33

What happens when the long chain fatty acids reach the enterocyte?

Answer 33

1. long chain fatty acids and other products of lipid digestion are converted back into triglycerides, phospholipids and esters of cholesterol in the ER 2. Fat droplets form cisternae of the SER 3. Apoproteins are synthesised in the RER and then transferred to the SER where apoproteins associate with lipid dropletes 4. Nascent chylomicrons arrive at the cis face of the Golgi apparatus. Here, apoproteins are glycosylated 5. Vesicles carrying chylomicrons or VLDLs bud off from the trans golgi apparatus and move to the basolateral membrane in the transport vesicles 6. transport vesicles fuse with the basolateral membrane, releasing chylomicrons or VLDRs 7. Chylomicrons and VLDLs pass through large internclothelial channels of lymphatic capillaries and enter the lymph 8. Glycerol, short chain and medium chain fatty acids pass through the enterocyte and enter the blood capillary