Digestion and absorption of carbohydrates

1. DIGESTION AND
ABSORPTION OF
CARBOHYDRATES
SEMINAR BY ROLL NO. 174 AND 175

2. Index
1.Introduction to digestion
2.Digestion of carbohydrates
*Digestion in mouth
*Digestion in stomach
*Digestion in small intestine
3.Absorption of monosaccharides
4.Mechanism of absorption
5.Non digestible carbohydrates
6.Abnormalities of carbohydrate digestion
7.Lactose intolerance
8.Flatulence
9.Oral Rehydration Therapy

3. INTRODUCTION TO DIGESTION
• A process involving hydrolysis of large and complex
organic molecules into smaller molecules.
• These smaller and water soluble molecules can be
easily absorbed by gastrointestinal tract(GI tract).
• Cooking of food and mastication improve the
digestibility of foodstuffs by the enzymes.

4. DIGESTION OF CARBOHYDRATES
• Principal dietary carbohydrates-Polysaccharides(starch,
glycogen);Disaccharides(lactose,
sucrose);monosaccharides(glucose, fructose).
• Digestion of carbohydrates occurs briefly in mouth and
largely in the intestine.
• Hydrolysis of glycosidic bonds is carried out by a group of
enzymes called glycosidases.

5. 1. DIGESTION IN MOUTH- Salivary amylase(ptyalin) acts on starch randomly and cleaves alpha-1, 4-
glycosidic bonds. Products formed include alpha-limit dextrins, maltotriose and maltose.
2. DIGESTION IN STOMACH- Salivary amylase is inactivated by high acidity(low pH) in the stomach.
Consequently, carbohydrates are not digested in the stomach.
3. DIGESTION IN SMALL INTESTINE- Acidic dietary contents of the stomach are first neutralized by
bicarbonate produced by pancreas. The pancreatic alpha-amylase acts on starch and specifically
acts on alpha-1, 4-glycosidic bonds and not on alpha-1, 6-bonds.

6. • The resultant products are disaccharides (maltose, isomaltose) and
oligosaccharides.
• The final digestion of di- and oligosaccharides to monosaccharides
primarily occurs at the mucosal lining of the upper jejunum.
• This is carried out by oligosaccharidases (e.g. glucoamylase acting on
amylose) and disaccharidases(e.g. maltase, sucrase, lactase).
• The enzyme sucrase is capable of hydrolysing a large quantity of table
sugar (sucrose).
• In contrast, lactase (E-galactosidase) is the rate-limiting, and,
consequently, the utilization of milk sugar (lactose) is limited in
humans.

8. ABSORPTION OF MONOSACCHARIDES
• The principal monosaccharides produced by the digestion of carbohydrates are glucose, fructose and
galactose.
• Of these, glucose accounts for nearly 80% of the total monosaccharides.
• The absorption of sugars mostly takes place in the duodenum and upper jejunum of small intestine.
• The relative rates of absorption of important monosaccharides in comparison with glucose are
Glucose — 100 Galactose — 110
Fructose — 43 Mannose — 20
Xylose — 15 Arabinose — 9
• Insulin has no effect on absorption of sugar.

9. MECHANISM OF ABSORPTION
• Different sugars possess different mechanisms for their absorption.
• All the sugars are absorbed by simple diffusion to some extent.
1. Glucose absorption
• Glucose is transported into the intestinal mucosal cells by a carrier
mediated and energy requiring process.
• Glucose and Na+ share the same transport system (symport) which is
referred to as sodium-dependent glucose transporter.
• The concentration of Na+ is higher in the intestinal lumen compared
to mucosal cells.
• Na+, therefore,moves into the cells along its concentration gradient
and simultaneously glucose is transported into the intestinal cells.

10. • The intestinal Na+ gradient is the immediate energy source for glucose transport.
• The enzyme Na+-K+ ATPase is involved in the transport of Na+ in exchange of K+ against the
concentration gradient.
• GLUCOSE TRANSPORTERS(GLUT): These are 1to 7 in no.(GLUT-1 to GLUT-7).GLUT-2 is found in
kidney,intestine and pancreatic beta cells and operates at co- transport system. GLUT-4 is found in
heart, skeletal muscle and adipose tissue and is involved in insulin stimulated glucose uptake.
2. Fructose absorption
• Fructose absorption is relatively simple.
• It does not require energy and is independent of Na+ transport.
• Fructose is transported by facilitated diffusion mediated by a carrier.
• Inside the epithelial cell, most of the fructose is converted to glucose.
3. Galactose absorption
• The mechanism of absorption of galactose is similar to that of glucose.
4. Pentoses absorption
• Pentoses are absorbed by simple diffusion.
NOTE-INHIBITOR PHLORIZIN BLOCKS THE Na+ DEPENDENT TRANSPORT OF GLUCOSE AND
GALACTOSE.

12. NON DIGESTIBLE CARBOHYDRATES
• The plant foods are rich in fibrous material which cannot be digested either by
the human enzymes or intestinal bacteria.
• The fibers are chemically complex carbohydrates which include cellulose,
hemicellulose, pectins, lignin and gums.
• Fiber in nutrition is of special importance.

13. ABNORMALITIES OF CARBOHYDRATE DIGESTION
• Since only the monosaccharides are absorbed, any defect in the activities of
disaccharidases results in the passage of undigested disaccharides into the large
intestine.
• Any alteration in the mucosa of the small intestine caused by severe diarrhea,
malnutrition, intestinal diseases or drug therapy will lead to a temporary acquired
deficiency of disaccharidases.
• Hereditary disorders with deficiency of individual disaccharidases in infants and
children cause intolerance of specific disaccharides.

14. LACTOSE INTOLERANCE
• Lactose intolerance is the most common disorder of carbohydrate digestion in humans.
• This is due to a defect in the enzyme lactase (E-galactosidase).
• Continued consumption of lactose by lactose intolerant individuals causes flatulence.
1. Primary/congenital lactose intolerance
2. Secondary/acquired lactose intolerance: May occur due to sudden and high intake of milk-based
diets. If milk is withdrawn temporarily, diarrhoea will be limited.
• For lactose intolerant people, consumption of curd is beneficial, since lactobacilli present in curd
contain the enzyme lactase.
• Yeast rich in lactase can also be used for treatment of lactose intolerance.

15. FLATULENCE
• Flatulence is characterized by increased intestinal motility, cramps and irritation.
• Certain disaccharides (lactose in lactose intolerance) and oligosaccharides (Raffinose) are not digested
by humans. They can be degraded by bacteria present in ileum to liberate monosaccharides.
• As the monosaccharides are utilized by intestinal bacteria, gases such as hydrogen, methane and
carbon dioxide- besides lactate and short chain fatty acids-are released and they cause flatulence.
• Leguminous seeds are very common to cause flatulence as they contain several non-digestible
oligosaccharides by human intestinal enzymes.
• Raffinose(contains galactose, glucose and fructose) is a predominant oligosaccharide found in
leguminous seeds.

16. ORAL REHYDRATION THERAPY (ORT)
• ORT is the most common treatment of diarrhoea.
• The oral rehydration fluid contains glucose and sodium.
• Intestinal absorption of sodium is facilitated by the presence of
glucose.