DIGESTION AND ABSORPTION OF CARBOHYDRATES.

Being a good source of energy, carbohydrates are important constituent of food. In diet, they are mainly taken in the form of starch (rich, wheat, potato and other cereals), lactose (milk) and sucrose (used for sweetening food). Monosaccharide content is negligible. These dietary polysaccharides and disaccharides must be hydrolysed to monosaccharides before they can be absorbed by intestinal tract.

DIGESTION OF CARBOHYDRATES

DIGESTION IN MOUTH

Digestion of carbohydrates begins after the food is masticated and mixed with saliva in the mouth. Saltva is a colourless, viscous fluid secreted mainly by parotid, submaxillary and sublingual salivary glands. It has a pH of 6.4 to 6.9 and contains a glycoprotein, mucin and a polysaccharide hydrolysing enzyme, salivary amylase or ptyalin, in addition to several inorganic ions. Salivary amylase catalyses the hydrolysis of a-1, 4-glucosidic bond in starch, glycogen and dextrin, and converts them to oligosaccha rides and maltose. It is activated by chloride ions and has an optimum pH 5.5-6.5.

DIGESTION IN STOMACH

The partially digested carbohydrates are pushed down the oesophagus into the stomach. Due to peristalsis the ptyalin continues acting upon the carbohydrates and its immediate products, till the pH of the medium is lowered due to the presence of gastric HCl in the stomach. Apart from limited hydrolysis of sucrose no significant digestion of carbohydrate occurs in stomach due to the lack of specific enzymes.

DIGESTION IN INTESTINE

The chyme later enters the small intestine, where the majority of carbohydrates are digested. As the partially digested food passes through duodenum, it is mixed with pancreatic juice, which contains a carbohydrase, pancreatic amylase (amylopsin). The pancreatic juice which is alkaline (pH 7.5-8.2) due to its high bicarbonate content, neutralizes the acidity of chyme and raises the pH of intestinal content. This change in pH is most favourable for the action of pancreatic amylase which acts best at pH 7.1. Similar to salivary amylase, this also catalyses the hydrolysis of a -1, 4-glucosidic bonds and converts partially digested polysaccharides to maltose and small oligo saccharides containing a-1, 6-glucosidic linkages (limit dextrins).

The only functional difference between the pancreatic amylase and salivary amylase lies in the fact that while pancreatic amylase acts on the native starch, the salivary amylase act on the cooked starch only. However, both are activated by chloride ions.

In addition to pancreatic secretions, the mucosal cells of intestine secrete a mixture of fluids called Succus entericus (intestinal juice), which contains maltase, lactase, sucrase and isomaltase (oligo-1, 6-glucosidase).

Maltase catalyzes the hydrolysis of maltose and maltotriose to glucose and functions optimally at pH 5.8-6.2.

Lactase converts lactose to glucose and galactose and functions maximally at pH 5.4-6.0.

Sucrase also called as invertase converts sucrose to fructose and glucose; acts optimally at pH 5.0-7.0.

Isomaltase (Oligo-1, 6-glucosidase), catalyses the hydrolysis of 1-6 glycosidic linkage in the limit dextrins, and is therefore, also called as Dextrinase. Limit Dextrinase are connected to maltose and glucose by this enzyme.

ABSORPTION OF CARBOHYDRATE.

The monosaccharides resulting from digestion are easily absorbed into the mucosal cells of small intestine and pass into circulation via portal vein. A very small amount may also be absorbed by the lymph. The microvilli (brush border) lining the mucosal cells greatly help in absorption by increasing the surface area. The present concept regarding the absorption of mono saccharides rules out the involvement of glucose phosphorylation. The absorption of glucose and galactose is now believed to take place by an active transport rather than by simple diffusion. The transfer is mediated by a specific macromolecule of protein nature called "carrier protein" which is present on the external surface of the brush border membrane of intestine. Na" has been found to be essential for accomplishing such an active transport. The carrier bears the specific binding sites for both glu & gal and also for Na*. Thus Na enhances the binding of these monosaccharides to the carrier, for their transportation across the brush border cell membrane. This is followed by their release into the cytoplasm of the mucosal cells. From the cytoplasm glucose/galactose diffuses into the blood capillaries. Such a mechanism also facilitates the transport of Nat.

This active transport of gal/glu requires energy, provided by the hydrolysis of ATP.

However, fructose is absorbed by the process of simple diffusion, believed to be facilitated again by some kind of a carrier molecule. The rate of absorption of monosaccharides has been found to be independent of blood sugar concentration. There is, however, considerable disparity in the absorption rates of different sugars. The highest rates of absorption are found in case of galactose and glucose, whereas fructose is absorbed at about half of this rate. Mannose and Xylose are absorbed much more slowly.