Carbohydrates such as starch, dextrin, glycogen (animal starch), sucrose (cane sugar), maltose (malt sugar), and lactose are broken down in the digestive tract into simple, six-carbon sugars that pass easily through the intestinal wall. Fructose (fruit sugar) and glucose are unchanged in the digestive tract and are absorbed as such. Cellulose, a common constituent of many foods, is an important nutritional element for some animals, notably cattle and termites, but has no value in human nutrition (see Nutrition, Human).
The digestion of carbohydrates is performed by various enzymes (see Enzyme). Amylase, found in saliva and in the intestine, breaks starch, dextrin, and glycogen into maltose, a 12-carbon sugar. Other sugar-converting enzymes in the small intestine break 12-carbon sugars into 6-carbon sugars. Maltase breaks maltose into glucose; sucrase, or invertase, breaks cane sugar into glucose and fructose; lactase breaks milk sugar into glucose and galactose.
The six-carbon sugars, which become the end products of carbohydrate digestion, pass through the wall of the small intestine into minute blood vessels and thence into the portal vein, which carries them to the liver. They are then converted into a single compound, glycogen (see Starch), which is stored there. This glycogen is available at all times and is converted to glucose and released into the bloodstream as required by the body. One of the end products of glucose metabolism in the muscles is lactic acid, which is carried by the bloodstream back to the liver and partly reconverted into glycogen.