Digestion of Dietary Carbohydrates

Main Carbohydrates of Diet

1- Monosaccharides: mainly glucose & fructose ABORBED with NO DIGESTION

2- Disaccharides: Sucrose, lactose & maltoseDIGESTED into monosaccharides

3-Polysaccharides: Starch (plant source e.g. rice, potato, flour) & glycogen (animal source)DIGESTED into monosaccharaides Cellulose (fibers of vegetables & fruits)NOT DIGESTED

Digestion of Carbohydrates of Diet

1- In the MouthThe major dietary polysaccharides are of plant (starch, composed of amylose & amylopectin) & animal (glycogen) origin.

During mastication, the enzyme salivary a-amylase acts on dietary starch and glycogen hydrolysing some a(1-4) bonds to give dexrtin

N.B. humans do not have b-amylase, so they can not digest cellulose, which is a carbohydate

of plant origin containing b(1-4) glycosidic bonds between glucose molecules.

Salivary a-amylase action on carbohydrates stops in the stomach as the media is strongly

acidic.

2- In the Lumen of small intestine

Pancreatic bicarbonate neutralizes the acidic juice of stomach.

pancreatic a-amylase continues the process of starch and glycogen digestion

Dextrin is cleaved into oligosachharides and disaccharides.

Digestion of Carbohydrates of Diet

3- At the Mucosal Lining of Small Intestine

Mucosa of small intestine secretes two types of enzymes:

1- Intestinal oligosaccharidases: digest oligosaccharides into disaccharides & monosaccharaides. 2- intestinal disaccharidases : digest disaccharides into monosaccharides.

Disaccharidases are:Sucrase: cleaves sucrose into glucose and fructoseMaltase: cleaves maltose into glucose and glucoseLactase: cleaves lactose into glucose and galactose

Finally, most carbohydrates of diet (polysaccharides & disaccharides) are hydrolysed (digested) into monosaccarides (mainly glucose, fructose & galactose).Only monosaccharides are absorbed ---- to blood

Cellulose of diet (in fibers of vegetables & fruits) are not digested.

Digestion of Carbohydrates of Diet

Overview of Digestion of Carbohydrates of Diet

1- From lumen to inside cells:Only monosaccharaides are absorbed in the small intestine

- The duodenum and upper jejunum absorb the bulk of the dietary sugars- Insulin hormone is not required for uptake of glucose by intestinal cells.

i- Galactose & glucose are transported into the mucosal cells by an active energy requiring process that requires the concurrent uptake of sodium ions.

The transport protein is the sodium dependent glucose cotransporter-1

Ii- Frucose absorption requires a sodium-independent transporter (GLUG-5) for its absorption (energy is not required)

2- From inside cells into blood:Glucose, galactose & fructose are transported from intestinal mucosal cells into the portal circulation (blood) by GLUT-2 transporter.(energy is not required)

Absorption of Monosaccharaides

Disaccharidase Deficiencies

Deficiency of a disaccharidase of the intestinal mucosa causes:

1- The disaccharide is not digested to monosaccharaide.2- The undigested disaccharide passes into the large intestine 3- In large intestine, disaccharides which are osmotically active draw water from the mucosa into the large intestine lumen causing osmotic diarrhea.4- Diarrhea is increased by the fermentation of the remaining carbohydrates to two- and three-carbon compounds which are also, osmotically active. 5- Large volumes of CO2 and H2 gases cause abdominal cramps and flatulence.

Disaccharides Deficiencies cont.

Causes of deficiency of enzymes:

1- Hereditary deficiencies of a disaccharidase deficiencyFor example:Lactose intolerance:Inability to digest lactose of milk due to deficiency of lactase enzymeSo, diarrhea will occur on ingestion of milk or milk productsFor infants (up to two years old): are treated by lactose-free milk

2- Intestinal diseases or drugs that injure the mucosa of the small intestine

Digestion of Dietary Proteins

Proteins must be digested to yield amino acids which can be absorbed

1- In the StomachDigestion by gastric secretion

The gastric juice contains hydrochloric acid (HCL) & pepsinogen

• Hydrochloric acid: - Denatures proteins to make them more susceptible to hydrolysis by enzymes - kills some bacteria.

• Pepsin: - An acid-stable endopeptidase secreted by stomach cells as inactive pepsinogen - Pepsinogen is activated to pepsin either by HCL or by other pepsin molecules - Action of pepsin: digests polypeptides into smaller polypeptides.

Pepsin

HCL

Secreted inactive (Zymogen) =

pepsinogen & needs HCL for starting

activation

Attacks peptide bonds formed by

pH 2

2- In the Lumen of Small IntestineDigestion by pancreatic enzymes

On entering the small intestine, • Polypeptides produced in the stomach by the action of pepsin are further cleaved to oligopeptides by

pancreatic proteases :

Trypsin, chymotrypsin, elastase & carboxypeptidase A&B

• These proteases are released from the pancreas as zymogens (inactive forms).

• Release of zymogens is mediated by cholecystokinin & secretin (hormones of GIT)

• Activation of zymogens is mediated by the enzyme enteropeptidase i.e. Trypsinogen (zymogen) is converted to trypsin (active enzyme).

Digestion of Dietary Proteins cont.

cholecystokinin & Secretin(Hormones from intestinal Mucosa)

Stimulate Pancreatic Secretions of Zymogens

Activation of Zymogens by enteropeptidase secreted from

intestinal mucosa

TrypsinSecreted inactive

(zymogen) & needs enterokinase

for activation

Attacks peptide bonds formed by

PH 7-8 Enterokinase

Chemotrypsin

Secreted as inactive (zymogen) & needs

trypsin for activation

Attacks peptide bonds formed by

PH 7-8

Elastase Is an endopeptidase which acts on elastic fibers .

It is secreted as an inactive form (proelastase) & activated by trypsin.

CarboxypeptidaseIt is an exopeptidase attacking the peptide chain at its carboxylic

end liberating amino acids. secreted as an inactive procarboxypeptidase activated by trypsin .

3- On the mucosa of Small Intestine digestion by intestinal aminopeptidases

Aminopeptidase is available on the luminal surface of the intestine cleaves the N-terminal amino acids from oligopeptides to produce

smaller oligopeptide & free amino acids

Digestion of Dietary Proteins cont.

Absorption of amino acids

• Free amino acids are taken up into the small intestinal cells by sodium-dependent transport system.

• Amino acids are taken via blood to the liver

Abnormalities in Protein Digestion

In individuals with a deficiency in pancreatic secretion (for example, due to chronic pancreatitis, cystic fibrosis or surgical removal of the pancreas

abnormal increase of lipids (steatorrhea) and undigested protein in the feces

Incomplete digestion & absorption of fat & protein