The major carbohydrates present in our diet is glycogen, starch, sucrose, lactose, maltose, fructose, and pentoses. Except for monosaccharides, all other carbohydrates need digestion before absorption. Carbohydrates digestion mainly takes place in the mouth and small intestine.
Digestion in Mouth
Digestion of carbohydrates begins at the mouth, where they come in contact with saliva. During mastication salivary glands secrets an enzyme called salivary amylase. It is also known as ptyalin. it is an endoglycosidase that means it can cleave randomly internal α-1,4 glycosidic linkages of polysaccharides like starch and glycogen. In the mouth as a result of the action of salivary amylase polysaccharides are converted to dextrins and a small amount of maltotriose or maltose.
Digestion in Stomach
There is no digestion of carbohydrates in the stomach. This is mainly because of the absence of glycolytic enzymes and acidic pH which denatures salivary amylase that is coming from the mouth along with food.
Digestion in Small Intestine
In the small intestine there are 2 phases of carbohydrate digestion
- Digestion by pancreatic amylase
- Digestion by intestinal brush border enzymes
Digestion by pancreatic amylase
The product of the digestion from the stomach passes into the small intestine and it is called chyme which is acidic. Acidic chyme stimulates duodenal mucosal cells to release 2 local hormones called secretin and cholecystokinin. Secretin stimulates the pancreas to release bicarbonate, which in the small intestine neutralizes the gastric HCl and restores the pH to 7. Cholecystokinin stimulates the pancreas to release an enzyme pancreatic α-amylase. This enzyme further hydrolyzes dextrin to smaller molecules cleaving randomly internal α-1,4 glycosidic linkages. The products are known as limit dextrins which are oligosaccharides with branch formed by α-1,6 bond intact and a small amount of maltotriose and maltose.
Digestion by intestinal brush border enzymes
Intestinal brush border cells secrete enzymes for the final digestion of carbohydrates. Initially limit dextrinase or α-1,6 glycosidase hydrolyzes limit dextrins by cleaving the branch at α-1,6 to form maltotriose, isomaltase, and maltose. Later on lactase, maltase and sucrase hydrolyze disaccharides. Now we have all monosaccharides in the small intestinal lumen which must be absorbed into intestinal mucosal cells for transportation, utilization, and storage.
Absorption of carbohydrates
Products of digestion of dietary carbohydrates are absorbed from the small intestine. There are 2 mechanisms of absorption namely: secondary active transport and facilitative transport.
The secondary active transport mechanism
Glucose and galactose are absorbed actively and hence they require energy in the form of hydrolysis of ATP. The concentration of glucose less as compared to that of intestinal mucosal cells. Therefore glucose must be absorbed against the concentration gradient with the help of a transport called sodium-dependent glucose transporter-1 [SGLUT-1]. This transporter is a cotransport as glucose absorption coupled with sodium absorption in same direction. Since the hydrolysis is not at the same site of absorption but takes place in the basolateral surface of mucosal cells, this transport is called secondary active transport.
Facilitative transport mechanism
Although the concentration of fructose and mannose are higher in the intestinal lumen but their absorption is not by simple diffusion. A special transporter called glucose transporter-5 [GLUT-5] is required for absorption of these sugars.
Transport of carbohydrates
Once all monosaccharides are absorbed into the intestinal mucosal cells they are transported to portal circulation by glucose transporter -2 [GLUT-2]. Now, this glucose either transported to the liver and muscles for the storage as glycogen or uptake by the cells for utilization to generate ATP.
PowerPoint presentation of digestion and absorption of carbohydrates