2.3 Lipids: Digestion and Absorption of Dietary Lipids

Question 1

Dietary lipids consists mostly of what?

Answer 1

Triglycerols/triacylglycerols – concentrated source of energy

Question 2

What are alternative energy source if there are no triacylglycerols?

Answer 2

proteins and carbohydrates but deliver less than 1/2 the energy density of fats and require bulky meals to cover high-energy requirements.

Question 3

What are some purposes of having lipids in your diet?

Answer 3

• energy source
• increase texture and palatability of foods
• solubilization and dispersion of flaviors
• slow gastric emptying
• essential fatty acids and fat-soluble vitamins.

Question 4

Lipid Lipase
( Where is it released)
(What does it do)

Answer 4

• released in mouth

- begins emulsification with dietary phospholipids

Question 5

Acid-stable (lingual) lipase:
(Where is it released)
(What does it do)

Answer 5

• Stomach.
• hydrolysis slow (fats not yet emulsified)
• this enzyme is for babies only, in adults, little or no lipid hydrolysis in mouth or stomach.

Question 6

Gastric Lipase:
(Where is it released)
(What does it do)

Answer 6

Stomach.
pH 7 optima, only useful for short and medium chain fatty acids. Newborn babies have a pH of 7.
-stomach churning helps to physically separate fats.

Question 7

What must lipids be done before they can be digested?

Answer 7

must be emulsified (via bile) before they can be digested by water soluble enzymes in the intestine.

Question 8

Role of bile salts?
(Where is it synthesized)
(Stored)

Answer 8

make insoluble macroscopic fat particles into finely dispersed microscopic micelles.

• Bile acids (salts) are cholesterol derivatives (essentially a cholesterol with 1 or 2 aa on it).
• Synthesized in the liver and secreted as glycine and taurine conjugates.
• Stored in the gallbladder
• bile is an amphipathic cmpd that acts as biological detergents.

Question 9

What is the enterohepatic cycle?

Answer 9

Most bile acids get recycled: They are reabsorbed in the small intestine, returned to the liver via the portal vein, and may be re-secreted.
- Interrupting this cycle will lower cholesterol.

Question 10

Role of Cholecystokinin (CCK)? (3 roles)
(Where is it released)
(What does it do)

Answer 10

Small peptide hormone released from jejunum in response to chyme in duodenum.

• • So it recognizes the food coming from the stomach into the small intestine and then it:
    1) stimulates gall bladder bile release
    2) Release of pancreatic digestive enzyme
    3) Slows down gastric emptying.

Question 11

Secretin:
(Where is it released)
(What does it do)

Answer 11

Released in the duodenum in response to low pH of chyme entering small intestine from the stomach.
- Induces the pancreas to release a bicarbonate solution to neutrolize the acidic pH of the chyme.

-First hormone identified in 1902.

Question 12

What are the 3 pancreatic digestive enzymes?

Answer 12

1) Pancreatic lipase
2) Cholesterol esterase
3 Phospholipase A2

Question 13

Function of pancreatic lipase?

Answer 13

• Converts TAG into 2-monoacylglycerol and free fatty acids
• Lipase removes fatty acids from C1 and C3. Stabilized at lipid water interface by Colipase
• Colipase is released in inactive form, pancreatic lipase must be present in order for colipase to be active. Together= triglyceride digestion.

Question 14

What is the Interfacial Activation?

Answer 14

TAG digestion takes place at lipid-water interfaces. The rate of digestion is dependent of the surface after of the interface, which is increased by peristalsis and the introduction of bile.

Question 15

What are the two functions of Colipase?

Enterostatin

Answer 15

Colipase comes in as an inactive pro form. Trypsin cleaves this into the active form.
-The peptide portion that is cleaved off of colipase is called Enterostation.
-So it splits, the 1st 5 aa becomes enterostation and the 6-101 are Colipase.
So the 2 functions:
1) stabilize pancreatic lipase and in its formation
2) turn into enterostatin to as as a satiety signal (hunger regulation)

Question 16

Cholesterol esterase?

Answer 16

Degrades cholesterol esters to free fatty acids and cholesterol

Question 17

Phospholipase A2?

What activates it?

Answer 17

removes fatty acids from phospholipids

• Trypsin activates phospholipase
• Phospholipase removes the fatty acid at C2 leaving a lysophospholipid
• (The remaining fatty acid at c1 can be removed by lysophospholipase, leaving a glycerylphosphorylipase).

-Snake venom: so snake venom has phospholipase. So that’s why your phospholipid membrane breaks down when you get bitten by a snake.

Question 18

Sterol digestion? Easy or not?

Answer 18

• Cholesterol is made in adequate quantities by the liver and it not an essential nutrient.
• absorption is relatively poor. If ingested with dietary fiber or plant sterols, cholesterol absorption further decreased.

Question 19

What is cholesterol (sterol)esterase?

Answer 19

it’s a pancreatic glycoplrotein that hydrolyzes sterol esters into sterol plus fatty acids. (this allows for easier digestion of cholesterol).

Question 20

FABP2 (Fatty acid binding protein 2) aka I-FABP (Intestinal-type fatty acid-binding protein)?

Answer 20

These proteins bind onto intracellular fatty acids and then are being sequestered (removed). (intracellular metabolism)

• more specific: bound in the “beta-clam” structure.
• -aka fatty acid binding protein

Question 21

What are chylomicrons?

Answer 21

They transport dietary lipids from the intestines to other locations in the body. Chylomicrons are one of the five major groups of lipoproteins that enable fats and cholesterol to move within the water-based solution of the bloodstream.
- The proteins sitting outside of the chylomicrons will dictate where the fatty acids will end up.

Question 22

How does chylomicron formation occur in the intestinal cell?

Answer 22

• So short and medium chain fatty acids don’t need to be broken down to micelles for absorption.
• They travel to the liver from the small intestine via the hepatic portal blood system attached to albumin.
• Inside the mucosal cells, re-esterification of the fatty acids into triacylglycerol and cholestoral to choesterol to cholesteryl ester occues (SO PRETTY MUCH ESTERIFICATION)
• Then it gets incorporated into chylomicrons and these cells transport the fatty acids to necessary tissue sites throughout body that needs fat.

Question 23

What are the primary breakdown products of lipid digestion?

Answer 23

Free fatty acids, monoacylglycerol, cholesterol, and some random phospholipid products.

Question 24

What happens to a chylomicron once its secreted from the mucosal cells?

Answer 24

the chylomicrons circulate through the lymphatic system until it reaches the superior vena cava. During their time in the lymph and continuing in their plasma, chylomicrons undergo a number of physical-chemical and metabolic changes:

1) Gain apoC-II: Activates lipoprotein lipase. Which intitiates core triacylglycerol hydrolysis.

Question 25

What do chylomicrons consist of?

Answer 25

1) reformed triacylglrcerol (TAG)
2) Cholesterol
3) protein rich shell: phospholipids.

• glycerol and short and medium chain fatty acids pass directly into bloodstream without lipoprotein assistance.