Digestion and Absorption of Carbohydrates

Carbohydrates form the largest single component of a typical diet, chiefly as starch (in cereals, pulses and root vegetables) together with smaller amounts of the disaccharides sucrose (cane sugar) and lactose (milk sugar), and their digestion begins, uniquely among the three major classes of food, in the mouth itself.

As food is chewed and mixed with saliva, salivary amylase (ptyalin) begins to act on any cooked or boiled starch present, hydrolysing the long, branched glucose chains of starch into the disaccharide maltose and shorter dextrin fragments; because starch is a large molecule and the time food actually spends in the mouth before being swallowed is short, this first pass of digestion is only ever partial. Once the bolus is swallowed and reaches the stomach, this enzymatic action effectively comes to a halt: salivary amylase requires a near-neutral pH to function, and the strongly acidic gastric juice (pH roughly 1.5 to 2) with which the bolus is churned denatures the enzyme almost immediately, so essentially no further carbohydrate digestion takes place while food remains in the stomach.

Digestion of the remaining, undigested starch resumes only once the chyme passes into the duodenum, where the incoming pancreatic juice's bicarbonate first neutralises the gastric acid, restoring a pH at which enzymes can once again act. Pancreatic amylase then continues the breakdown of any remaining starch and dextrins, again yielding maltose as the principal product. This maltose, together with the disaccharides sucrose and lactose that were present in the diet from the start and have not yet been touched by any enzyme so far, is finally acted on by a set of disaccharidases bound to the brush border of the intestinal epithelial cells themselves: maltase splits maltose into two molecules of glucose, sucrase splits sucrose into glucose and fructose, and lactase splits lactose into glucose and galactose. This brush-border digestion is the final chemical step, and because it occurs directly at the absorptive surface, the monosaccharides it yields are available for absorption the moment they are formed, with essentially no free monosaccharide pool accumulating in the lumen of the gut. …