Carbohydrates- digestion and absorption Flashcards
Gives examples of Digestible dietary carbohydrates and non-digestible carbohydrates
Digestible dietary carbohydrates:
Polysaccharides: Starch, glycogen
Disaccharides: sucrose, maltose, lactose, trehalose
Non-digestible carbohydrates
Fiber: eg: Cellulose
Name a list of enzymes for carbohydrate digestion
•α- Amylase – Pancreatic/Salivary (secreted enzymes)
- Maltase-glucoamylase
- Sucrase-Isomaltase
- Lactase
- Trehalase
—–> Brush-border enzymes (membrane proteins in the microvillous brush border)
What is starch and what is it made up of? What are the two components of of starch? What is glycogen and how it is different ?
•Starch is a plant polysaccharide made up of glucose units
It has two components
— Amylose – linear glucose polymer. Individual glucose units are linked with each other by α,1-4 glycosidic bonds, unbranched
— Amylopectin – Branched glucose polymer. Has both α,1-4 & α,1-6 glycosidic bonds
•Glycogen (animal sources) resembles amylopectin portion of starch, but has more & shorter branches than amylopectin
Explain the digestion of starch & glycogen in the following areas:oral cavity. stomach, small intestines. What is starch finally broken down to?
Oral cavity:
Salivary a-amylase, requires Cl- ( as a cofactor)
Cleaves internal a-1,4 glycosidic bonds in starch & glycogen (endoglucosidase); does not cleave a-1,6 bonds
Stomach:
Salivary amylase inactivated by acidic pH, no digestion of carbohydrates
Small intestine:
•Pancreatic a-amylase digests most of the polysaccharides; activity similar to salivary amylase (cleaves internal a-1,4 bonds), requires Cl-
starch, glycogen
* * Cl- /a-amylase**
* * Maltose, maltotriose, a-limit dextrin, isomaltose**
Once Starch is broken down, it forms maltose, maltotriose and a-dextrins and isomaltase. Describe each of these
maltose: has alpha 1-4 bonds
isomaltose: has alpha 1-6 bonds
Maltotriose: Trisaccharide
A-Dextrin is a oligosaccride with alpha 1-6 branches
Brush border dissacharides :
What are the four enzyme complexes that are used to break down sugars?
a-limit dextrin, isomaltose-> glucose
(a-1,6 link)
Isomaltase (cleaves a-1,6 bond)
3. Lactase: acts on b-1,4 bond in lactose
lactose -> glucose + galactose
4. Trehalase: acts on a-1,1 bond in trehalose
trehalose (found in mushrooms) -> 2 glucoses
•Deficiency of which enzyme causes lactose intolerance? What are the symptoms of lactose intolerance? Compare primary lactose intolerance with secondary lactose intolerance and congentical lactose intolerance What are the symtoms of lactose intolerance?
- Common condition, due to deficiency of lactase
- Abdominal pain, discomfort, flatulence & diarrhea after consumption of lactose containing foods (dairy products)
1. Primary lactose intolerance: (adult hypolactasia)
- Gradual loss of lactase activity in adults
- Seen in non-white populations
- Decreased synthesis of lactase
2. Secondary lactose intolerance:
Develops secondary to
- Gastroenteritis
- Colitis
- Kwashiorkor
- Tropical sprue- inflammation of small intestine
3.Congenital lactose intolerance:
- Complete absence of lactase from birth (autosomal recessive)
- Symptoms develop soon after infant is fed breast milk or lactose containing formulas
Symptoms of lactose intolerance:
->After intake of milk,
•Abdominal cramps
•Bloating & flatulence
•Diarrhea & dehydration
-?Occur due to fermentation of undigested lactose by intestinal bacteria
Explain how lactose causes in bloating , flatulence, diarrhea and dehydration in a diagram. What type of test can be done to diagnose lactasr deficiency?
•Breath hydrogen test can be done to diagnose lactase deficiency after ingesting a test dose of lactose.
What can be done to manage lactose intolerance?
- Avoid consumption of milk products
- Lactose-free milk preparations can be fed to infants
- Live-culture yogurt can be given, contains b-galactosidase, helps digest lactose
- Lactase pills
•What is sucrase deficiency? What type of inheritance is it? What is the treatment? How can it be diagnosed?
- Reported in about 10% of Inuit group of Canada, Alaska & Greenland
- Autosomal recessive inheritance
- Clinical manifestations are almost similar to lactose intolerance
- Treatment: Withdrawal of sucrose containing diet. Enzyme therapy may be attempted
- About 2% of North Americans are heterozygous (unaffected carriers) for sucrase deficiency
- Diagnosis: Measurement of H2 gas in breath after ingestion of sucrose
Which are the two major mechanisms why which glucose can be absorbed into the small intestine? Explain each . Explain the clinical correlation.
TWO major mechanisms
- Facilitated diffusion
- Na+ dependent active transport
1. Facilitated diffusion:
- Mediated by glucose transporter-5 (GLUT-5), a membrane protein (also transports galactose, fructose)
- glucose moves ‘down’ the gradient
- Energy-independent
Na+ dependent active transport
•Energy-dependent
•Na+ & glucose co-transported into mucosal cells by SGLT-1 (sodium dependent glucose transporter-1)
•Na+ transported ‘down’ its gradient
•Glucose transported against the gradient
•Low intracellular Na+ levels maintained by Na+-K+-ATPase pump
•‘secondary’ active transport
•GLUT-2 and GLUT-5 are uniport systems
•SGLT-1 is a co-transport (symport) system
•Na-K ATPase is a counter transport (antiport) system
- Severe diarrhea leads to loss of water & electrolytes, may cause hyponatremia
- Oral rehydration solution (ORS) used in management of severe diarrhea and dehydration
- Contains glucose, NaCl, KCl & sodium citrate
- Glucose & Na+ co-transported by SGLT-1
- Replenishes electrolytes
- Helps in absorption of water
What are the types of glucose transporters?
GLUT 1: RBC & Blood brain barrier
GLUT 2: Liver, intestine, pancreatic islets β cells, renal
tubular cells
GLUT 3: Neuronal cells
GLUT 4: Muscle and fat (adipose) cells (the only insulin-sensitive glucose transporter)
GLUT 5 : Intestine (preferably a fructose transporter)
GLUT 7: Endoplasmic reticulum
SGLT: Intestine, renal tubular cells
- About 14 types has been reported:
- GLUT 1, 2, 3 and so on.
- SGLT , mainly functioning in intestine (major role in glucose absorption)
- SGLT is also present in Kidneys, required for reabsorption of filtered glucose from renal tubular fluid – Defect may lead to **Renal glucosuria **
