All carbohydrates absorbed in the small intestine must be hydrolyzed to monosaccharides prior to absorption. Hydrolysis precedes transport of monosaccharides in hamster intestine. From sucrose, glucose is taken up much faster than fructose. Monosaccharide transport saturates with -glucose at 30 mM.
Digestion of starch begins with the action of salivary alpha-amylase/ptyalin, although its activity is slight in comparison with that of pancreatic amylase in the small intestine. Amylase hydrolyzes starch to alpha-dextrin, which are then digested by gluco-amylase (alpha-dextrinases) to maltose and maltotriose. The products of digestion of alpha-amylase and alpha-dextrinase, along with dietary disaccharides are hydrolyzed to their corresponding monosaccharides by enzymes (maltase, isomaltase, sucrase and lactase) present in the brush border of the small intestine. In the typical Western diet, digestion and absorption of carbohydrates is fast and takes place usually in the upper small intestine. However, when the diet contains carbohydrates not easily digestible, digestion and absorption take place mainly in the ileal portion of the intestine.
Digestion of food continues while simplest elements are absorbed. The absorption of most digested food occurs in the small intestine through the brush border of the epithelium covering the villi (small hair-like structure). It is not a simple diffusion of substances, but is active and requires energy use by the epithelial cells.
During the phase of carbohydrate absorption, fructose is transported into the intestinal cell's cytosol, glucose and galactose competes with other [|Na+ transporter required for operation. From the cytosol, monosaccharides pass into the capillaries by simple or facilitated diffusion.
Carbohydrates not digested in the small intestine, including resistant starch foods such as potato, bean, oat, wheat flour, and several monosaccharide oligosaccharides and starch, are digested in a variable when they reach the large intestine. The bacterial flora metabolize these compounds anaerobically in the absence of oxygen. This produces gases (hydrogen, carbon dioxide and methane) and short-chain fatty acids (acetate, propionate, butyrate). The gases are absorbed and excreted by breathing or through the anus (flatulence). Fatty acids are rapidly metabolized. Butyrate is used mainly by cells in the colon and acetate is absorbed into the blood and taken up by the liver, muscle and other tissue. Propionate is an important precursor of glucose in some animals, but not humans.