Carbohydrates

Carbohydrates 
Carbohydrate (starch) digestion begins in the mouth by the action of salivary amylases, which are capable of degrading starch to maltose and a    limit dextrins. The significance of salivar: Digestion is limited by the usually short stay and incomplete mastication of food in the mouth. By the time the food is well mLxed with the gastric juice in the stomach, the action of salivary amylase is inhibited by the low pH of the medium.
The small intestine is the major site of carbohydrate digestion and absorption. In the luminal phase pancreatic amylases continue the degradation of starch where the salivary action ends, yielding maltose, maltotriose and other oligosaccharides (up to 9 glucose units), and a-limit de.xtrins (1,6 linkages at the branching points). No free glucose or isomaltose is formed in the lumen.
The final stages of carbohydra te digestion take place by membrane-bound enzymes on the luminal side of the lipoprotein membrane of the mucosal cell (the brush­border phase). The brushborder exhibi ts multiple enzyme activity, which results in the hydrolytic breakdown of di­and oligosaccharides. As a result, the digestion of starch is completed by production of glucose from a-limit dextrins by a-dextrinase (previously called isomaltase) and from maltose and oligosaccharides by gIucoamylase (maltase). The common dietary disaccharides, sucrose and lactose, are also hydrolyzed by the mucosal disaccharidases, sucrase and lactase, respectively. Sucrase activity is actu­ally part of a hybrid molecule, which also hydrolyzes the 1,6 linkages of a-limit dextrins and, for this reason, is referred to as sucrase-a-dextrinase. Sucrase activity is known to respond to changes in the intake of sucrose.Only monosaccharides can enter the mucosal cell and, subsequently, the blood. Deficient mucosal disaccha­ridase activity is the cause of a group of malabsorption syndromes.
The membrane digestion is closely integrated "vith the absorption of the resulting monosaccharides. Only lactase activity limits the rate of the absorption oflactose. From other substrates, monosaccharides are produced at a rate greater than the mucosal uptake, and some diffuse to the intestinal contents, from which they are absorbed later at a site distal to the site of their production.
Glucose and galactose are believed to share a com­mon carrier system for the uptake into the mucosal cell. Although some ofthis transport is passive during the peak period of their production, the bulk of these sugars is absorbed by a process linked to the transport ofNa+. The exact nature of this process is still unsettled despite the many models that have been proposed for the system.
The most widely accepted theory!! suggests that glucose and galactose are transported together with so­dium by a carrier, which facilitates the diffusion of so­dium into the cell along the concentration gradient. The monosaccharide is bound to the same carrier and, because of this coupling, can be transported into the cell against its concentration gradient. The monosaccharides can then exit by diffusion downhill, and enter the portal vein. According to this theory, the step that requires en­ergy in the absorption of glucose and galactose is the expulsion of sodium from the cell.
Fructose absorption proceeds at a slower rate than tha t of glucose and galactose and has been assumed to be by a passive mechanism, although the rate is relatively high when compared to passive transport of other sugars. Existence of a separate specific mechanism for fructose transport is suggested by studies on sugar absorption in a patient ,vith the rare defect known as glucose-galactose malabsorption. An adult diagnosed as having this condition showed only minimal absorption of glucose and galactose (believed to be by passive diffu­sion). The rate of fructose absorption was normal, about four times that of glucose and galactose, suggesting a mechanism of facilitated diffusion.
After leaving the mucosal cell all monosaccharides enter the capillaries of the portal venous system and are carried to the liver, where fructose and galactose are readily converted to glucose.