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Digestive and Renal Sciences > Digestion and absorption of fat > Flashcards

Digestion and absorption of fat Flashcards

1
Q

Why are fats important and stored evolutionary?

A

During the fasted state fat acts as an energy source.

Lipids are the most concentrated depot of energy storage because they give us twice the amount of energy per gram compared to proteins and carbs and therefore is more efficient.

Fat can also be stored in an anhydrous state.

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2
Q

Where does most of the bodies fat depots exist?

A

Exist in the subcutaneous adipose tissue layers however is also found in visceral depots in obese and older people.

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3
Q

Which fat is more metabolically active, visceral or subcutaneous?

A

Visceral fat is more metabolically active.

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4
Q

how much fat would a typical 70Kg human carry?

A

15Kg of fat which is enough to sustain the bodies entire resting metabolic requirement for in excess of 50 days.

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5
Q

how is ATP generated from fats?

A

Generated by beta-oxidation, where fatty acids are converted into fatty acyl CoA which can then enter Beta-oxidation.

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6
Q

How does Fatty acyl CoA vary?

A

Depending on the length of the fatty acid, the length of fatty acyl CoA will vary

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7
Q

Explain the process of Beta-oxidation

A
  1. Oxidation which generates ATP and energy
  2. Hydration step
  3. Oxidation step generative ATP and energy
  4. Cleavage step cleaving the fatty acid (cleaves 2 carbons) off the acyl CoA to form Acetyl CoA which can enter the TCA cycle.

Therefore the longer the fatty acid going into the cycle the more energy is released as the fatty acyl CoA can be recycled more times over.

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8
Q

Why are fats important?

A

Calories and energy deposits

absorption of Fat soluble vitamins

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9
Q

What are the fat soluble vitamins?

A

Vitamine A,D,E and K

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10
Q

What does Vitamin A deficiency cause?

A

Leads to night blindness, corneal drying (xerosis), corneal degeneration and blindness (xerophthalmia)

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11
Q

What can Vitamin A overdose cause?

A

Overdosing can cause hypervitaminosis A which can lead to hair loss, nausea, jaundice, irritability, vomiting, blurry vision, headaches and muscle pains.

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12
Q

What is Vitamin D?

A

In the skin this exists as Vitamin D3 (cholecalciferol) which can then be hydroxylated to form the active form of vitamin D (1,25-dihydroxycholecalciferaol)

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13
Q

What can vitamin D deficiency cause?

A

Impaired bone, mineralisation, rickets, osteoporosis.

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14
Q

What is Vitamin E?

A

Tocopherol which is most abundant in vegetable oils.

It is an antioxidant - role in protection against cardiovascular disease and cancer.

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15
Q

What can vitamin E deficiency cause?

A

neurological problems due to poor nerve conduction

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16
Q

What is Vitamin K?

A

found in various foods in the diet but also produced in intestinal bacteria.
Used in bone metabolism, and blood coagulation.

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17
Q

What are the essential fatty acids?

A

Linoleic, linolenic and arachidonic acid

Essential because the cannot be derived endogenously and therefore must be obtained from the diet.

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18
Q

Why are the essential fatty acids important?

A

formation of healthy cell membranes
Development and functioning of the brain and nervous system
Production of Eicosanoids (eg.PGs)
Responsible for regulating blood pressure, blood viscosity, vasoconstriction, immune and inflammatory responses

19
Q

what does fatty acid deficiency cause?

A 
Skin atrophy
dry skin
weakness
mood swings
hemorrhagic dermatitis
high blood pressure
impaired growth.
20
Q

how do most dietary fat exist in the diet?

A

Exist as triglycerides (90%)

21
Q

How is endogenous lipids in the GI tract derived?

A

Derived from bile

22
Q

What is the process of emulsification?

A
  1. Food preparation (i.e blending, and cooking)
  2. Chewing and gastric churning which allows mixing lingual and gastric juices
  3. Squirting of gastric contents into the duodenum
  4. Intestinal peristalsis mixes luminal contents with pancreatic and biliary secretions
23
Q

What is the purpose of emulsions?

A

These mechanical processes reduce the size of the lipid droplets and increases their ratio of surface area to volume.

24
Q

What prevents the lipid particles from coalescing?

A

Coating the emulsion droplets with membrane lipids.

The polar groups of the phospholipids project into water and thus prevent coalescence of the emulsion particles.

25
Q

Where is lingual lipase produced?

A

Produced in the mouth and in the stomach.

26
Q

What cells in the stomach produce lingual and gastric liapses?

A

Chief cells in response to gastrin/ACh digests large amounts of lipids.

27
Q

What do the lingual and gastric lipases do?

A

breaks down triglycerides by release a single fatty acid from triglycerides leaving behind intact diglycerides.
The free fatty acid if small can be absorbed.

28
Q

What happens when fatty acids generated in the stomach reach the duodenum?

A

They trigger the release of CCK from I cells in the duodenal mucosa.

The CCK then stimulates:

  • the flow of bile into the duodenum by the gallbladder contraction and relaxation of the sphincter of oddi
  • Pancreatic acinar cells to release zymogens such as pancreatic lipase
29
Q

What is required for the full lipolytic activity of pancreatic lipases to occur?

A
  1. Colipase (essential for ligand binding conformation)
  2. Alkaline pH
  3. Bile salts
  4. Fatty acids
30
Q

What is the function of the co-lipase?

A

Binds onto the emulsified fat droplet acting as an anchor for lipase/pancreatic lipase to bind and start digesting the fat by converting diglyceride into 2-monoacyl glyceride

31
Q

how are lipids absorbed?

A
  1. The fat is present in an emulsion droplet with a multilamellar membrane surrounding it.
  2. Co lipase binds onto the droplet and allows pancreatic lipse to bind and slowly digest the fat droplet in the centre.
  3. As the fat is digested, part of the emulsion droplet buds off to form the multilamellar vesicle
  4. As more bile salts are added, there is thinning of the multilamellar membranes into a unilamellar vesicle
  5. This repeats until a single membrane surrounds the fat to form a mixed micelle.
32
Q

What barriers are need to be overcome for lipolytic products to get into enterocytes?

A
  1. The mucous gel layer that lines the intestinal epithelial surface
  2. The unstirred water layer
  3. The apical membrane.
33
Q

How do lipids get into enterocytes?

A

Short chain acids - readily soluble in water and can diffuse into the enterocyte

Longer Chain fatty acids - Length increases so solubility decreases but partitioning into micelles increases.

34
Q

What is enterohepatic circulation?

A

After entry, the remaining bile salts are absorbed at the terminal ileum and colon where they are redirected to the liver in the portal blood

35
Q

What happens to long chain fatty acids once they are in the enterocyte?

A
  1. Fats bind to a fatty acid binding protein and direct them to the ER
  2. at the ER there are turned back into fat droplets (re-esterification) whilst the RER is synthesising Apolipoproteins.
  3. Apolipoproteins associate with the fat droplets forming Chylomicrons
  4. Chylomicrons effluxed out of enterocyte into the lymph.
36
Q

What happens to the exported chylomicrons?

A

They circulate until they find lipoprotein lipases which hydrolyses triglycerides in the chylomicrons
to fatty acids and glycerol
where FAs will return to the adipose tissue for storage
Glycerol will go to hepatocytes for gluconeogenesis

37
Q

What is Orlistat?

A

A partial pancreatic lipase inhibitor used to help those who are obese.
This causes oily stool which means that fat is not being absorbed to reduce fat deposit increase.

38
Q

What happens to the larger products of lipolysis at the enterocyte brush border?

A

At the enterocyte brush border, they encounter a low pH generated by the Na-H exchange.

The fatty acids are protonated and leave the mixed micelle to enter the enterocyte by:

  1. Non-ionic diffusion
  2. Collision and incorporation of the fatty acid into the cell membrane
  3. Novel evidence of active carrier mediated processes
39
Q

What happens to the micelles and bile salts after the fatty acids have entered the enterocyte?

A

Micelles and bile salts are reabsorbed down in the terminal ileum to be returned to the liver to be repackaged.

40
Q

Why do fatty acids bind to fatty acid binding proteins in the enterocyte?

A

Prevent leakage back out of the cell
prevent lipid toxicity
ensure transfer of FAs to the SER

41
Q

What are the 2 pathways of re-esterification?

A

Monoglyceride pathway

Phosphatidic acid pathway

42
Q

Other than chylomicrons, What other types of lipoproteins are there?

A

VLDL
LDL
HDL

43
Q

What happens with malabsorption of fats?

A

Steatorrhea

Fat present leading to bulky stools

Digestive and Renal Sciences (26 decks)

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