Malabsorption

Question 1

What are the three locations where carbohydrate and protein digestion occur?

Answer 1

1. Intraluminally
2. At the brush border
3. Transport digestion

Question 2

What are the components of sucrose?

Answer 2

glucose+fructose

Question 3

What are the components of lactose?

Answer 3

galactose+glucose

Question 4

What are the components of maltose?

Answer 4

glucose+glucose

Question 5

What are the four steps of carbohydrate digestion?

Answer 5

Step 1: Antral grinding
Step 2: Amylase breaks starch down into oligosaccharides
Step 3: Brush border enzymes hydrolyze disaccharides
Step 4: Carbohydrate absorption

Question 6

Describe the process by which amylase breaks down starches. Which amylases are at work? Where are they activated?

Answer 6

Gastric acid inactivates salivary amylase

–CCK+secretin stimulate release of pancreatic amylase into the duodenum

Question 7

What is the rate limiting step when brush border enzymes hydrolyze disaccharides?

Answer 7

Absorption of sugars is the rate limiting step, not the hydrolysis of disaccharides into monosaccharides.

Exception: Lactase is a rate limiting enzyme and reaches mas efficiency pretty quickly.

Question 8

Describe the important transporters involved in carbohydrate absorption, both apical and basolateral

Answer 8

Apical side: 
-GLUT5: facilitated fructose transporter
-SGLT1: Na/glucose symporter 
Basolateral
-GLUT2 symporter: fructose and glucose/galactose faciliated transporter

Question 9

Which one of the carbohydrate transporters is dependent on the Na/K ATPase generated gradients?

Answer 9

SGLT-1, the sodium/glucose symporter

Question 10

What part of the GI tract is the main site of carbohydrate digestion?

Answer 10

Jejunum. NO carb digestion occurs in the colon. There is a little absorption in the duodenum

Question 11

Describe what happens when there is a defect in carb absorption/digestion

Answer 11

These are metabolized to short chain fatty acids by bacteria. Gas is synthesized by bacteria feeding off of the carbs. SCFAs are absorbed by colonic enterocytes as a coping mechanism and produce calories

Question 12

What is a typical scenario resulting in a defect in carb assimilation?

Answer 12

resection

Question 13

Carb and protein digestion are similar in many ways. How are they different?

Answer 13

Small polymers and AAs are absorbable while ONLY disaccharides can be absorbed in the gut.

Proenzymes require activation unlike amylase.

Question 14

What activates pepsinogen? Is it essential to protein digestion?

Answer 14

the low gastric pH. Note that the low pH does NOT cleave the AA chain, it only denatures the proteins.

Pepsinogen is nonessential to protein digestion. You can have a total gastrectomy and still be able to absorb protein

Question 15

Where are most proteins digested?

Answer 15

In the duodenum. The pancreas releases trypsinogen which is activated by enterokinase. Trypsin then activates 4 other peptidases. Eventually trypsinogen undergoes autodigestion to prevent destruction of normal tissue.

Question 16

How are amino acids absorbed?

Answer 16

1. Facilitated transport. Depends on Na/K pump gradient. These pumps tend to carry similar AAs (neutral, charged, etc)
2. Some oligopeptides are absorbed WHOLE through paracellular absorption
3. Peptide hormones like T3/T4 can diffuse right through without being digested

Question 17

Describe the steps involved in the digestion/absorption of fats

Answer 17

1. Emulsification
2. Chief cells release gastric lipase while pancreas releases pancreatic lipase
3. Micelle formation
4. Packaging TGs into chylomicrons
5. Transport to the tissues

Question 18

What are the components involved in emulsification?

Answer 18

Mechanical emulsification (prevent aggregation of fat molecules)

• bile salts+phospholipids
• CCK stimulates gallbladder to release bile salts

Question 19

What helps with the activation and deactivation of gastric lipase?

Answer 19

Gastric lipase is activated by low PH in stomach, then inactivated by bicarb secretions into the duodenum. It is further digested by trypsin in the duodenum

Question 20

What does gastric lipase breakdown?

Answer 20

Gastric lipase converts triacylglycerol to diacylglycerol

Question 21

What does pancreatic lipase break down?

Answer 21

Triacylglycerol to monoacylglycerol

Question 22

What does pancreatic lipase need to work?

Answer 22

1. Pancreatic co-lipase
2. Alkaline pH
3. Bile salts

Question 23

How does pancreatic co-lipase work?

Answer 23

Pancreatic co-lipase prevents bile salts from inactivating pancreatic lipase

Question 24

Describe how micelles help with absorption of fatty acids

Answer 24

Now that fatty acids are “soubilized” they can move to the surface of the enterocyte and LPL releases the fatty acids to diffuse across the phospholipid bilayer

Question 25

Describe how fatty acids are transported to the peripheral tissues

Answer 25

Chylomicrons enter the lymphatics, bypassing the liver, and are circulated to peripheral tissues. LPL cleaves TAG to release fatty acids and glycerol from the lipoprotein

Question 26

Where is vitamin B12 absorbed? processed?

Answer 26

B12 is absorbed into the ileum then processed in the liver

Question 27

What are the different causes of B12 malabsorption?

Answer 27

1. Pernicious anemia
2. Increased R factor to B12 binding
3. Pancreatic insufficiency
4. Decreased ileal B12 absorption (crohn’s or resection)