carbohydrates
Carbohydrate Digestion The average American diet is about 50 percent carbohydrates, which may be classified according to the number of monomers they contain of simple sugars monosaccharides and disaccharides and/or complex sugars polysaccharides . Glucose, galactose, and fructose are the three monosaccharides that are commonly consumed and are readily absorbed. Your digestive system is also able to break down the disaccharide sucrose regular table sugar: glucose + fructose , lactose milk sugar: glucose + galactose , and maltose grain sugar: glucose + glucose , and the polysaccharides glycogen and starch chains of monosaccharides . Your bodies do not produce enzymes that can break down most fibrous polysaccharides, such as cellulose. While indigestible polysaccharides do not provide any nutritional value, they do provide dietary fiber, which helps propel food through the alimentary canal. The chemical digestion of starches begins in the mouth and has been reviewed above. In the small intestine, pancreatic amylase does the heavy lifting for starch and carbohydrate digestion Figure 23.29 . After amylases break down starch into smaller fragments, the brush border enzyme -dextrinase starts working on dextrin, breaking off one glucose unit at a time. Three brush border enzymes hydrolyze sucrose, lactose, and maltose into monosaccharides. Sucrase splits sucrose into one molecule of fructose and one molecule of glucose; maltase breaks down maltose and maltotriose into two and three glucose molecules, respectively; and lactase breaks down lactose into one molecule of glucose and one molecule of galactose. Insufficient lactase can lead to lactose intolerance.