Digestion and absorption of carbohydrates.pptx

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. 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. DIGESTION IN STOMACH- Salivary amylase is inactivated by high acidity(low pH) in the stomach. Consequently, carbohydrates are not digested in the stomach. 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- an d 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.

7. 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.

8. Mechanism of absorption Different sugars possess different mechanisms for their absorption. All the sugars are absorbed by simple diffusion to some extent. 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.

9. The intestinal Na+ gradient is the immediate energy source for g lucose 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 liver, 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. Galactose absorption The mechanism of absorption of galactose is similar to that of glucose. Pentoses absorption Pentoses are absorbed by simple diffusion. NOTE-INHIBITOR PHLORIZIN BLOCKS THE Na+ DEPENDENT TRANSPORT OF GLUCOSE AND GALACTOSE.

10. 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.

11. 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.

12. 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. Primary/congenital lactose intolerance 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.

13. 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.

14. Oral rehydration therapy (ort) ORT is the most common treatment of diarrh o ea. The oral rehydration fluid contains glucose and sodium. Intestinal absorption of sodium is facilitated by the presence of glucose.