Carbohydrates

The principal dietary carbohydrates are starches, sucrose and lactose. Starch is a glucose-containing polysaccharide with a molecular weight which varies from 100,000 to more than 1 million. The two major polysaccharides of starch are amylose and amylopectin. Indigestible carbohydrates e.g. cellulose are the main constituents of dietary fibre.

Salivary and pancreatic amylases initiate the hydrolysis of starch and exhibit their optimal activity near a neutral pH. The salivary amylase is inactivated once it reaches the acid in the stomach. The intraluminal digestion of carbohydrates occurs rapidly in the duodenum due to the large amount of amylase secreted by the pancreas. The final oligosaccharide products of luminal digestion are formed before the chyme reaches the jejunum. The major products of starch digestion are maltose and maltotriose. Carbohydrates are absorbed in the proximal part of the small intestine and they have completely disappeared from the lumen by the time the meal reaches the ileum.

The disaccharides are further digested to monosaccharides by the brush border enzymes lactase, sucrase, maltase and isomaltase during their transfer across the epithelium. It is likely that the enzymes and carriers are so orientated spatially that hydrolysis and and subsequent absorption are sequential events. Both passive diffusion and active transport absorb glucose rapidly and completely. The brush border possesses a sodium-dependent carrier which transports sugars across the membrane in either direction.

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