All carbohydrates absorbed in the small intestine must be hydrolyzed to monosaccharides prior to absorption. The digestion of starch begins with the action of salivary alpha-amylase, although their activity is slight in comparison with that made by 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 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.

Continue the digestion of food while their simplest elements are absorbed. The absorption of most of digested food occurs in the small intestine through the brush border of the epithelium covering the villi. It is not a simple diffusion of substances, but is active and requires energy use by the epithelial cells

In a phase of carbohydrate absorption, fructose is transported by a transporter of fructose into the intestinal cell cytosol, glucose and galactose competes with other Na + transporter required for operation. The cytosol, monosaccharides pass into the capillaries by simple diffusion or facilitated diffusion.

Carbohydrates that are not digested in the small intestine, including resistant starch foods such as potatoes, beans, oats, wheat flour, as well as several non-polisacacáridos oligosaccharides and starch, are digested in a variable when they reach the large intestine. The bacterial flora metabolize these compounds 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. Fatty acids are rapidly metabolized. Thus butyrate, used mainly in the colonic, is an important nutritional source for these cells and regulates their growth, aceteto into the blood and taken up by the liver, muscle and other tissues, and propionate, which is an important precursor of glucose in animals, it is not so in humans.