Carbohydrates are the sugars, starches and fibers found in fruits, grains, vegetables and milk products, one of the basic food groups.
Sugar is the generalized name for sweet, short-chain, soluble carbohydrates, many of which are used in food. They are composed of carbon, hydrogen, and oxygen. Simple sugars are called monosaccharides and include glucose (or dextrose), fructose, and galactose. The table sugar most customarily used as food is sucrose - hydrolyses into fructose and glucose in the body. Other disaccharides include maltose and lactose. Longer chains of sugars are called oligosaccharides.
Starch is a polymeric carbohydrate consisting of a large number of glucose units joined by glycosidic bonds. This polysaccharide is produced by most green plants as an energy store. It is the most common carbohydrate in human diets and is contained in large amounts in staple foods such as potatoes, wheat, maize (corn), rice. Pure starch is a white, tasteless and odorless powder that is insoluble in cold water or alcohol.
Fiber is consists of non-starch polysaccharides such as arabinoxylans, cellulose, and many other plant components such as resistant starch, resistant dextrins, inulin, lignin, chitins, pectins, beta-glucans, and oligosaccharides, and are derived from plants. Dietary fibers are not digested by gastrointestinal enzymes. Soluble fiber dissolves in water, changes as it goes through the digestive tract, where it is fermented by bacteria, partially into physiologically active byproducts - healthful compounds. Insoluble fiber does not dissolve in water and stays intact as it moves through your digestive system, can be prebiotic and metabolically ferment in the large intestine. Dietary fibers can change absorption of other nutrients and chemicals. Some soluble plant fibers can modulate intestinal inflammation and are contrabiotic. Many types of so-called dietary fiber are not actually fibrous.