Lecture 12 - Digestion and Absorption of Fat

Question 1

What is fatty acid beta-oxidation?

Answer 1

The process where fatty acids are broken down to produce energy. The transfer of electrons in the form of hydrogen from fatty acids to specific molecules, can then be used to generate ATP.

Question 2

Wha are the stages of beta-oxidation?

Answer 2

1 - Fatty acid chain initially is converted to fatty acyl-CoA. It utilises energy here, it consumes energy.
2 - Fatty acyl-CoA enters a cycle of reactions. Firstly, an oxidation step that liberates energy in the form of FAD –> FADH2 via acyl-CoA dehydrogenase
3 - Hydration step
4 - Oxidation step, liberating energy in form of NAD+ –> NADH via L-hydroxyacyl-CoA dehydrogenase
5 - Cleavage step, cleavage of acetyl-CoA from fatty acyl-CoA.
6 - Depending on how long the remnant fatty acyl-CoA is, it can then go back into this cyclical process and liberate more energy and acetyl-CoA

Question 3

What are the symptoms of vitamin A deficiency?

Answer 3

• Night blindness
• Corneal dying
• Corneal degeneration
• Blindness
• Impaired immunity
• Hypokeratosis
• Keratosis pilaris

Question 4

What are the symptoms of vitamin A overdosing?

Answer 4

• Hair loss
• Nausea
• Jaundice
• Irritability
• Vomiting
• Blurry vision
• Headaches
• Muscle and abdominal pain

Question 5

What are the symptoms of vitamin D deficiency?

Answer 5

• Impaired bone mineralisation
• Bone softening diseases
• Rickets in children
• Osteomalacia in adults
• Possibly contributes to osteoporosis
• May also be linked to cancer

Question 6

What is the main symptom of vitamin E deficiency?

Answer 6

Neurological problems due to poor nerve conduction

Question 7

What are the symptoms of essential fatty acid deficiency?

Answer 7

• hemorrhagic dermatitis
• Skin atrophy
• Scaly dermatitis
• Dry skin
• Weakness
• Impaired vision
• Tingling sensations
• Mood swings
• Edema
• High blood pressure
• High triglycerides
• hemorrhagic foliculitis
• Immune and Mental deficiencies
• Impaired growth

Question 8

What are the stages of emulsification?

Answer 8

1. Food preparation
2. Chewing and gastric churning which allows mixing lingual and gastric juices
3. Squirting gastric contents into the duodenum
4. Interstitial peristalsis mixes luminal contents with pancreatic and biliary secretions

Question 9

What prevents these lipid particles from coalescing?

Answer 9

• Coating the emulsion droplets with membrane lipids, denatured protein, dietary polysaccharides, production of digestion and biliary phospholipids and cholesterol.
• The polar groups of the phospholipids project into the water; and thus prevent coalescence of the emulsion particles
• The core of the emulsion particle is composed of triglycerides, which also contains cholesterol esters and other non-polar lipids

Question 10

What does lingual and gastric lipase initiate?

Answer 10

Lipid digestion

Question 11

What does CCK stimulate?

Answer 11

1. flow of bile into the duodenum by gall bladder contraction and relaxation of odd.
2. The secretion of pancreatic enzymes, including lipase and esterase’s.

Question 12

How is CCK released?

Answer 12

Once the fatty acids generated in the stomach reach the duodenum, they trigger the release of CCK from I cells in the duodenal mucosa.

Question 13

What does full lipolytic activity of pancreatic lipase require?

Answer 13

Colipase, alkaline pH, bile salts, fatty acids.

Question 14

What is the function of co-lipase?

Answer 14

• acting as an anchor for the binding of lipase

- first forms a collapse-pancreatic-lipase complex that can then bind to the lipase interface.

Question 15

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

Answer 15

1. Mucous gel layer that lines the intestinal epithelial surface.
2. Unstirred water membrane.
3. Apical membrane.

Question 16

How do short/medium chain fatty acids get into enterocytes?

Answer 16

Diffusion through the unstirred water layer and into the enterocyte is efficient.
As chain length increases, solubility in water decreases, whereas its partitioning into micelles increases.

Question 17

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

Answer 17

They encounter a low-pH generated by Na-H exchange at the brush-border membrane. The fatty acids are protonated and leave the mixed micelle to enter the enterocyte by either non-ionic diffusion, collision and incorporation into cell membrane, active carrier mediated processes.

Question 18

What happens to the remaining bile salts after entry into the enterocyte?

Answer 18

Absorbed at the terminal ileum and colon, where they are redirected to the liver in the portal blood. This is known as the enterohepatic circulation.

Question 19

Describe re-esterification?

Answer 19

1. Long chain fatty acids bind to fatty acid-binding proteins.
2. The re-esterification to form triglycerides occurs within the SER, using either absorbed 2-MG or glycerol-3-phosphate as a substrate.

Question 20

What happens to exported chylomicrons?

Answer 20

Find their way to the endothelial surface, where they encounter lipoprotein lipase, which hydrolyses triglycerides in the chylomicrons to fatty acids and glycerol-muscle and adipose tissue.

Question 21

What is the main symptom of fat malabsorption?

Answer 21

Steatorrhea - bulky stool

Question 22

What are some reasons for fat malabsorption?

Answer 22

• Impaired bile acid production.
• Lipase activity.
• Compromised small bowel.
• Impaired chylomicron formation.