Carbohydrate digestion is necessary because the gastrointestinal tract can only absorb monosaccharides.
It is dependent on several types of enzyme:
- alpha-amylases:
- hydrolyze alpha-1,4 bonds to convert polysaccharides with alpha-1,4 bonds e.g. glycogen and starch, into oligo- and disaccharides such as maltose and alpha-limit dextrins
- the first amylase that food encounters is oral ptyalin which works until inactivated by stomach acidity
- pancreatic amylase carries out the same reaction from the duodenum distally
- disaccharidases:
- found in association with the small intestinal brush border
- sucrase: hydrolyzes sucrose to glucose and fructose
- maltase: hydrolyzes maltose to 2 glucose residues or maltose triose to 3 glucose residues
- lactase: converts lactose into glucose and galactose
- alpha-limit dextrinase: hydrolyzes dextrins into short chains of glucose residues
- 1,6-glucosidase:
- secreted by the small intestine
- hydrolyzes branched dextrins
Therefore ultimately, dietary carbohydrate is digested into glucose, fructose and galactose.