Carbohydrate digestion begins in the oral cavity and continues into the intestine, where the various nutrients are absorbed. The purpose of this process is the hydrolysis of disaccharides, oligosaccharides and polysaccharides in the single monosaccharides that make them up, in order to make them absorbable by the intestinal mucosa. As mentioned, the sugars introduced with the diet, such as glucose and fructose, do not require any digestive process and are absorbed as such. Glucose, in particular, is absorbed by active transport, while fructose passes through the intestinal mucosa by facilitated diffusion; it follows that levulose is absorbed more slowly and this contributes to lowering the glycemic index.
Starch is the predominant part of complex carbohydrates taken in a balanced diet; it is made up of many glucose units linked together in a linear way (amylose) and branched (amylopectin), and is introduced mainly through potatoes, legumes, cereals and derived products, such as pasta and bread. Its digestion begins in the mouth, where it is attacked by salivary α-amylases, which release maltose and isomaltose (disaccharides formed by the association of two glucose units, united by α-1, 4 and α-1, 6 bonds), maltotriose (this time there are three glucose molecules) and dextrins (7-9 units of glucose, with the presence of a branching). At the level of the mouth, the digestion of carbohydrates is in any case limited, given the limited time the food remains in the oral cavity.
The activity of salivary α-amylases stops in the stomach due to the acidity that characterizes the gastric environment. The digestion of carbohydrates resumes and is completed in the small intestine, thanks to the combined action of pancreatic and intestinal juices. In the former, an α-amylase enzyme analogous to the salivary enzyme is present, which as such transforms the starch into maltose, and dextrins. These cannot be hydrolyzed by pancreatic amylases and undergo the action of specific debranching enzymes (α-1, 6 glycosidases, α-dextrinase or isomaltases) present in epithelial cells of the small intestine. At this level we find additional enzymes involved in the digestion of disaccharides; the sucrase, for example, leads to the formation of glucose and fructose starting from a sucrose molecule and provides for the hydrolysis of maltose and maltotriose in synergy with the enzyme maltase; finally, lactase digests milk sugar by breaking it down into glucose and galactose (the deficiency of this enzyme, very common in adulthood, particularly in color populations, is responsible for lactose intolerance).
Once the digestion of carbohydrates in the single monosaccharides that make them up is complete, the sugars are ready to be absorbed. As anticipated, this absorption can take place by facilitated diffusion (fructose) or by active transport (glucose, galactose).
Not all carbohydrates introduced with the diet are digestible and even the starch itself, especially if raw, can be difficult to digest. Some vegetables, such as legumes, contain for example indigestible oligosaccharides (raffinose, verbascose and stachiosio). The same applies to dietary fiber, including cellulose. The digestion of these carbohydrates is instead possible for other animals, such as ruminants, and for the bacteria present in our large intestine. These microorganisms ferment the dietary fiber producing fatty acids with a laxative effect, trophic for the mucous membrane of the colon and precious for the general health of the whole organism.
