Carbohydrates (from 'hydrates of carbon') or saccharides (Greek σάκχαρον, sákcharon, meaning "sugar") are the most abundant of the four major classes of biomolecules. They fill numerous roles in living things, such as the storage and transport of energy (starch, glycogen) and structural components (cellulose in plants, chitin in animals). Additionally, carbohydrates and their derivatives play major roles in the working process of the immune system, fertilization, pathogenesis, blood clotting, and development.
Chemically, carbohydrates are simple organic compounds that are aldehydes or ketones with many hydroxyl groups added, usually one on each carbon atom that is not part of the aldehyde or ketone functional group. The basic carbohydrate units are called monosaccharides, such as glucose, galactose, and fructose. The general stoichiometric formula of an unmodified monosaccharide is (C·H2O)n, where n is any number of three or greater; however, the use of this word does not follow this exact definition and many molecules with formulae that differ slightly from this are still called carbohydrates, and others that possess formulae agreeing with this general rule are not called carbohydrates (eg formaldehyde).
Monosaccharides can be linked together into what are called polysaccharides (or oligosaccharides) in almost limitless ways. Many carbohydrates contain one or more modified monosaccharide units that have had one or more groups replaced or removed. For example, deoxyribose, a component of DNA, is a modified version of ribose; chitin is composed of repeating units of N-acetylglucosamine, a nitrogen-containing form of glucose. The names of carbohydrates often end in the suffix -ose.
Nutrition
Carbohydrates require less water to digest than proteins or fats and are the most common source of energy in living things. Proteins and fat are necessary building components for body tissue and cells and are also a source of energy for most organism.Carbohydrates are not essential nutrients in humans: the body can obtain all its energy from protein and fats. However, the brain and neurons generally cannot burn fat and need glucose for energy; the body can make some glucose from a few of the amino acids in protein and also from the glycerol backbone in triglycerides. Carbohydrate contains 3.75 and proteins 4 kilocalories per gram, respectively, while fats contain 9 kilocalories per gram. In the case of protein, this is somewhat misleading as only some amino acids are usable for fuel. Likewise, in humans, only some carbohydrates are usable for fuel; many monosaccharides and some disaccharides. Other carbohydrate types can be used, but only with the assistance of gut bacteria. Other animals, such as the ruminants and termites, can even process cellulose, while most animals cannot.
Foods high in carbohydrates include breads, pastas, beans, potatoes, bran, rice and cereals. Most such foods are high in starch.
Based on the effects on risk of heart disease and obesity, the Institute of Medicine recommends that American and Canadian adults get between 40-65% of dietary energy from carbohydrates.[6] The Food and Agriculture Organization and World Health Organization jointly recommend that national dietary guidelines set a goal of 55-75% of total energy from carbohydrates, but only 10% directly from sugars (their term for simple carbohydrates).
