Monosaccharides
Monosaccharides are the simplest form of carbohydrates, made up of just one sugar unit, and they are directly absorbed into the bloodstream from the small intestine. The most important monosaccharide is glucose, also known as blood sugar or dextrose. It is used for energy in both plants and animals and is the exclusive fuel for the central nervous system (CNS), including the brain and spinal cord. Fructose, another monosaccharide, is the sweet sugar found in fruits and honey. It is absorbed from the gut into the blood, but the liver quickly converts it into glucose for energy use, or stores it as glycogen or fat. Galactose has the same number of carbon, hydrogen, and oxygen atoms as glucose and fructose, but with a different structure. It does not appear in foods on its own; instead, it is part of lactose, the main sugar in milk, where it is bound with glucose. In the liver, galactose is either converted into glucose, broken into smaller units, or turned into fat. These monosaccharides—glucose, fructose, and galactose—are naturally present in fruits, vegetables, berries, honey, and syrups like glucose-fructose syrup. Sucrose, a disaccharide of glucose and fructose, is found in sugar beet, sugar cane, and fruits, while lactose (glucose + galactose) is found in milk, and maltose (glucose + glucose) appears in malt and starch-based syrups. Polyols are sugar alcohols such as sorbitol, xylitol, and isomalt, which are sweet like sugars and used in chewing gum and confectionery. Most polyols are made from sugars and, although safe in small amounts, can cause a laxative effect if consumed in large quantities.