L8: digestion and absorption of fat

Question 1

What are dietary lipids useful for?

Answer 1

• energy source - stored energy
• cell membrane production
• eicosanoid production - regulate HR/BP, vessel constriction
• carries fat-soluble vitamins from food into body
• maintains healthy hair and skin
• protects vital organs
• keeps body insulated
• provides sense of fullness after meals

Question 2

What do dietary lipids consist of?

Answer 2

95%: triglycerides

• source of energy
• chemical form in which fat exists
• 3 fatty acids and glycerol

5%:

• phospholipids
• cholesterol
• essential lipids: fat soluble vitamins - A, D, E, K; essential fatty acids e.g. linolenic acid

Question 3

Importance of fat soluble vitamins

Answer 3

Vitamin A - visual system
Vitamin D - calcium metabolism
Vitamin E - nervous system
Vitamin K - blood clotting

Question 4

Importance of essential fatty acids

Answer 4

• structural components of cell membranes
• keep cholesterol level stable
• nervous system

Question 5

Why do lipids need a separate system for digestion?

Answer 5

Lipids are insoluble in water.
When aided in digestion, they have highly efficient absorption - only 5% lost.
Unlimited storage capacity.

Question 6

Lipid emulsification

Answer 6

Detergent/bile salts will break up the fat into small particles = lipid emulsification
Just physical breakup, not chemical breakup
Increases surface area

Question 7

Bile salts

Answer 7

Bile salts are made by the liver and transported within bile to the gallbladder, where they are stored until fat arrives in the duodenum and signals their release via CCK.
Will solubilise the lipid in the chyme and is therefore essential in transporting the products of triglyceride digestion to the walls of the small intestine for absorption.

Question 8

How do bile salts stop fats from coalescing?

Answer 8

• many large fat droplets suspended in aqueous chyme
• bile salts increase the surface area for enzyme attack and increase the lipid-water interface
• micelles keep fat suspended in solution
• prevents droplets coalescing
  (This is not digestion, just emulsification)

Question 9

How do lipids actually get digested?

Answer 9

Pancreatic and intestinal lipases attack the emulsified droplets of fat and digest it

• triglycerides too large for absorption
• lipolysis of TGs into 2 fatty acids and one monoglyceride

(Monoglyceride = 1 fatty acid connected to glycerol)

Question 10

Summary of how fats get to the duodenum

Answer 10

1. Chyme with fat enters the duodenum from the stomach
2. Cells of duodenum respond by secreting CCK into the bloodstream
3. CCK stimulates gallbladder to contract and release bile containing bile salts into cystic dcut
4. Bile passes down bile duct into duodenum via ampulla of Vater
5. CCK also stimulates pancreas to send digestive enzymes including lipase to duodenum
6. Digestive enzymes pass in pancreatic duct to duodenum via ampulla of Vater

Question 11

Mixed micelle transport

Answer 11

Lipids are incorporated into biliary micelles for transport

• micelles consist of 20-50 bile salt molecules
• if no micelles form, very few MG and FAs would reach the glycocalyx/brush border of absorptive enterocytes
• solubilised lipids
• shell = bile salt heads
• core = FAs, MG, cholesterol, fat soluble vitamins

Question 12

Absorption and re-esterification

Answer 12

Diffusion of hydrophobic FA and MG into enterocyte:

• micelle drops lipids off at brush border
• absorbed without a need for carriers - by the lipid membrane
• within the cell 2FA and 1MG form a TG - re-esterification
• TG combines with cholesterol, vitamins etc
• packed with protein = chylomicron
• chylomicron is huge - cannot leave enterocyte via capillary

Question 13

Lymphatic uptake

Answer 13

Mucosal cell chylomicrons enter the intestinal lymphatic system:

• since it’s too big to pass into the villous capillary, it passes into the villous lacteal
• lacteal drains into the thoracic duct which drains into the subclavian vein

Question 14

How does chylomicron enter the blood circulation?

Answer 14

Nascent chylomicron particles enter the lymphatic system, bypass the liver and go straight to peripheral tissues for use before entering blood circulation via the left subclavian vein and visiting the liver.

Question 15

What is seen when there are a large number of chylomicrons being absorbed?

Answer 15

The lymph from the small intestine appear milky and the lymphatics are easy to see.

Question 16

Summary of lipid absorption

Answer 16

1. Bile salts from liver emulsify TGs so they are solubilised
2. Pancreatic lipases and collapse break TGs stored in micelles to MGs/FAs
3. a) MG’s/FAs move out of micelles and enter absorptive intestinal cells by diffusion - micelles absorbed later in SI/colon
4. b) Cholesterol is transported into cells by a special membrane transporter
5. Absorbed MG’s/FAs rejoin to form TGs and combine with cholesterol and proteins to form chylomicrons
6. Chylomicrons released into lymph

Question 17

Sites and mechanisms of fat and bile salt absorption

Answer 17

Fat is absorbed in the upper third of the SI

Bile salts absorbed in lower half of SI - some in colon

Question 18

What is fat malabsorption?

Answer 18

Inability to digest or absorb fat

Question 19

Causes of fat malabsorption

Answer 19

• liver and/or gallbladder disease - gallstones, cholecystectomy
• pancreatic disease - cancer, pancreatitis, cystic fibrosis
• intestinal disease - crohn’s, coeliac, short bowel

Question 20

What can fat malabsorption cause?

Answer 20

• steatorrhea - fat in faeces
• greasy, frothy, foul smelling
• essential fatty acid deficiency
• fat soluble vitamin deficiency

Question 21

Dental relevance of vitamin D deficiency

Answer 21

Vitamin D plays an important role in calcium absorption. Deficiency can lead to enamel defects, unusually shaped teeth, sensitivity due to exposed dentine, caries, underdeveloped brittle teeth, gingivitis, periodontitis. Vitamin D can suppress inflammatory cytokines that lead to periodontal disease.

Question 22

Dental relevance of vitamin K deficiency

Answer 22

Vitamin L needed to make blood clotting/coagulation proteins. Deficiency can lead to gingival bleeding with brushing, easy bruising, and a excessive blood loss during extractions.