Carbohydrates have traditionally supplied the bulk of the energy in most human diets. They are the biggest source of energy for most people especially those in developing countries whose diets are based upon cereals or starchy roots. Carbohydrates are monomers or polymers of simple sugar units or saccharides. Carbohydrates may be described according to the number of saccharide units they contain the monosaccharides, the disaccharides, the oligosaccharides and the polysaccharides. Lactic acid is the end product of anaerobic metabolism in mammalian cells and anaerobic energy production is only possible from carbohydrate substrate. Lactose is a disaccharide found exclusively in milk and is the least sweet of the major dietary sugars. Sucrose is a disaccharide composed of one unit of glucose and a unit of fructose and it is digested by the enzyme sucrase, which is located on the absorptive surface of the small intestine.