Digestion and Absorption of Carbohydrate



The carbohydrate diet mainly consists of polysaccharides (starch and glycogen) and disaccharides (sucrose and milk lactose). It also contains indigestible cellulose, hemicelluloses and pentosans etc.

Digestion of Carbohydrate:

Mouth:
Salivary amylase (ptyalin) starts the digestion of cooked starch in the mouth. But very little digestion takes places in the mouth since the food remains in the mouth for a very short period of time.
Stomach:
Since the food gets mixed with the gastric juice the action of amylase ceases due to high acidity. Some of the sucrose present in the food get hydrolysed by the action of HCl in the stomach.
Small intestine:
The pancreatic amylase in the small intestine converts starch and glycogen into a mixture of maltose and isomaltose:
Then maltose and isomaltose along with su­crose, lactose present in the diet are digested by the different disaccharidases present in the intestinal mucosa into their corresponding monosaccharides as shown:
Cellulose is not digested in human G.I. tract due to the absence of cellulase.

Absorption of Carbohydrate:

The comparative rates of absorption of monosaccharides taking glucose as 100 may be indicated as follows: galactose (110), glucose (100), fructose (43), mannose (39), xylose (15) and arabinose (9).
Galactose and glucose are absorbed at a faster rate than fructose. Pentose’s are slowly absorbed. This is due to the fact that glucose and galactose are actively transported while fructose, mannose and pentose’s are absorbed by simple dif­fusion.
The monosaccharides are absorbed into the mucosal cells of small intestine and pass into cir­culation via portal vein. A very small amount may be absorbed by the lymph. The microvilli (brush border) lining the mucosa cells greatly help the absorption by increasing the surface area.
The rate of absorption of monosaccharides is independent of blood sugar concentration. Glucose and galactose for absorption follow the active transport against a concentration gradi­ent; because they have the same chemical char­acteristics which are necessary for active transport mechanism.
The chemical characteristics are
1. The OH on carbon 2 should have the same configuration as in glucose.
2. A pyranose ring should be present.
3. A methyl or a substituted methyl group should be present on carbon 5.
Active Transport of Glucose:
1. A mobile carrier which binds both glu­cose and Na+ at separate sites and which transports them through the plasma mem­brane of the intestinal cell is required.
2. Both the glucose and Na are released into the cytosol, allowing the carrier to return for more “cargo”.
3. The Na+ is transported down its concen­tration gradient and at the same time causes the carrier to transport glucose against its concentration gradient.
4. The free energy required for this active transport is obtained from the hydrolysis of ATP linked to sodium pump which ex­pels Na+ from the cell.
Since polysaccharides and oligosaccharides are quickly hydrolysed the absorptive mechanism for glucose-galactose and fructose are quickly satu­rated. But the hydrolysis of lactose proceeds at half the rate for sucrose. This slower rate of hydrolysis of lactose shows that the digestion of lactose does not lead to saturation of the transport mechanism for glucose and galactose.
Active Transport of Glucose
Factors Controlling Active Transport of Glucose:
1. Active transport of sugar is depressed by agents (cyanide, malonate, fluoroacetate) that inhibit respiration and by dinitrophenol which uncouples oxidative phospho­rylation.
2. Phlorihizin which interacts with the mem­brane site at which the sugar enters, inhib­its intestinal absorption of glucose and ga­lactose.
3. Ouabain (cardiac glycoside), an inhibitor of the sodium pump, inhibits active trans­port of gucose.
4. The absorption of glucose is interfered by various intoxications, prolonged under-nutrition and vitamin deficiencies, espe­cially of thiamine, pantothenic acid and pyridoxine.
5. Absorption may be decreased in the pres­ence of abnormalities (structural or func­tional) of the mucous membrane, as in in­flammation (enteritis), edema and celiac disease.
6. The absorption is retarded in hypothy­roidism and accelerated in hyperthy­roidism.
7. Diminished absorption in adrenal corti­cal insufficiency is dependent upon the decreased concentration of sodium in the body fluid.
Disacchariduria:
The intestine normally is virtually impermeable to disaccharides. If it is absorbed, it is not metabolised. An increase in the excretion of intact disaccharides are found in some patients with disaccharidase deficiencies. The pa­tients with intestinal damage also excrete more disaccharides. This condition is said to be disacchariduria.

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