Carbohydrate digestion begins in the mouth with the enzyme salivary amylase and maltase which are produced by the pancreas. In humans, chyme produced by gastric lipases in the stomach cannot break down dietary starches and sugars into absorbable simple sugars like glucose and maltose. Chyme contains mainly disaccharides and monosaccharides such as sucrose (table sugar) from which the excess glucose is absorbed across the gut wall. In the small intestine, some monosaccharide absorption occurs but most disaccharide absorption does not begin until the disaccharide has been hydrolyzed by bacterial enzymes in the large intestine. In the large intestine, most disaccharides and their monosaccharide components are absorbed and absorbed water is lost. The remaining products are then absorbed across the small intestine wall into the bloodstream. Elemental calcium, phosphate, and bicarbonate ions also help to transport disaccharides in the gut lumen.
The mucus in the gastrointestinal (GI) tract plays a vital role in carbohydrate absorption as well. The mucus acts as a physical barrier to prevent carbohydrates from being washed away by gastric secretions or pancreatic juices that can escape into the intestinal lumen from beyond where they are produced. The mucus also traps and protects the carbohydrates while they are being digested. Fibres in the mucus also bind to any carbohydrate-protein complexes that have formed during digestion and are transported up into the lumen where they can be absorbed by specialised cells.
The first step of carbohydrate digestion is a so-called “fasting state” during which hepatocytes, intestinal mucus-producing cells, and enzymes in the stomach and intestines are reduced to a semi-starved state that prevents them from producing too much of whatever they may be digesting at any given time. In an effort to reduce the amount of food being broken down, the body produces less enzymes during this time. Enzymes produced in abundance by these organs, while fasted, include amylase and maltase which are produced by the pancreas and lactase which is produced by intestinal cells. Finally, if carbohydrates are not absorbed into the bloodstream during this phase of digestion they will be excreted from the body as waste material regardless of whether or not they were digested into absorbable monosaccharides.
Carbohydrate digestion begins in the mouth when amylase secreted from salivary glands hydrolyses starch in carbohydrates. The resulting products are maltose and disaccharides made of glucose, fructose, or sucrose. (Note that maltose and sucrose are forms of sugar that the body can convert into glucose.) The enzymes produced in saliva hydrolyse starch into maltose and disaccharides.
After being digested in the mouth, molecules of salivary amylase break down the starch molecules into smaller units consisting of glucose monomers: two glucoamylase units per disaccharide molecule. Amylases produced in this way are cleaved by other digestive enzymes in the small intestine (i.e., pancreatic amylase), becoming maltase which is also secreted from salivary glands. Digestion of starch in the mouth is a very important step in the digestion of carbohydrates because it leads to the formation of maltose and other simple sugars which can then be absorbed into the bloodstream.
The enzyme amylase secreted by salivary glands is also responsible for breaking down dietary starch into monosaccharides that are directly absorbed across the gut wall. In contrast, another pancreatic enzyme, chymotrypsinogen (which may be produced in the stomach) cleaves more complex nutritive starch molecules (i.e., polysaccharides like dextrins) into monosaccharides that are broken down further by maltase produced in salivary glands. These monosaccharide sugars are then absorbed across the gut wall.
Monosaccharides that have been digested in salivary glands or in the pancreas are transported to the small intestine by diffusion via a protein-lipid transmembrane transporter protein known as GLUT2, which is expressed primarily in the enterocytes of the small intestine. This protein was discovered by N. Papazian and colleagues and is named after Martin Luther King Jr., who was assassinated on April 4, 1968. Many years later (in 2008) GLUT2 has been identified as albumin-binding protein 2, which occurs in many tissues.