Malabsorption Flashcards
What are the three locations where carbohydrate and protein digestion occur?
- Intraluminally
- At the brush border
- Transport digestion
What are the components of sucrose?
glucose+fructose
What are the components of lactose?
galactose+glucose
What are the components of maltose?
glucose+glucose
What are the four steps of carbohydrate digestion?
Step 1: Antral grinding
Step 2: Amylase breaks starch down into oligosaccharides
Step 3: Brush border enzymes hydrolyze disaccharides
Step 4: Carbohydrate absorption
Describe the process by which amylase breaks down starches. Which amylases are at work? Where are they activated?
Gastric acid inactivates salivary amylase
–CCK+secretin stimulate release of pancreatic amylase into the duodenum
What is the rate limiting step when brush border enzymes hydrolyze disaccharides?
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.
Describe the important transporters involved in carbohydrate absorption, both apical and basolateral
Apical side: -GLUT5: facilitated fructose transporter -SGLT1: Na/glucose symporter Basolateral -GLUT2 symporter: fructose and glucose/galactose faciliated transporter
Which one of the carbohydrate transporters is dependent on the Na/K ATPase generated gradients?
SGLT-1, the sodium/glucose symporter
What part of the GI tract is the main site of carbohydrate digestion?
Jejunum. NO carb digestion occurs in the colon. There is a little absorption in the duodenum
Describe what happens when there is a defect in carb absorption/digestion
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
What is a typical scenario resulting in a defect in carb assimilation?
resection
Carb and protein digestion are similar in many ways. How are they different?
Small polymers and AAs are absorbable while ONLY disaccharides can be absorbed in the gut.
Proenzymes require activation unlike amylase.
What activates pepsinogen? Is it essential to protein digestion?
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
Where are most proteins digested?
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.
How are amino acids absorbed?
- Facilitated transport. Depends on Na/K pump gradient. These pumps tend to carry similar AAs (neutral, charged, etc)
- Some oligopeptides are absorbed WHOLE through paracellular absorption
- Peptide hormones like T3/T4 can diffuse right through without being digested
Describe the steps involved in the digestion/absorption of fats
- Emulsification
- Chief cells release gastric lipase while pancreas releases pancreatic lipase
- Micelle formation
- Packaging TGs into chylomicrons
- Transport to the tissues
What are the components involved in emulsification?
Mechanical emulsification (prevent aggregation of fat molecules)
- bile salts+phospholipids
- CCK stimulates gallbladder to release bile salts
What helps with the activation and deactivation of gastric lipase?
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
What does gastric lipase breakdown?
Gastric lipase converts triacylglycerol to diacylglycerol
What does pancreatic lipase break down?
Triacylglycerol to monoacylglycerol
What does pancreatic lipase need to work?
- Pancreatic co-lipase
- Alkaline pH
- Bile salts
How does pancreatic co-lipase work?
Pancreatic co-lipase prevents bile salts from inactivating pancreatic lipase
Describe how micelles help with absorption of fatty acids
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
Describe how fatty acids are transported to the peripheral tissues
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
Where is vitamin B12 absorbed? processed?
B12 is absorbed into the ileum then processed in the liver
What are the different causes of B12 malabsorption?
- Pernicious anemia
- Increased R factor to B12 binding
- Pancreatic insufficiency
- Decreased ileal B12 absorption (crohn’s or resection)
