Sugar is the generalized name for sweet, short-chain, soluble carbohydrates, many of which are used in food. They are carbohydrates, composed of carbon, hydrogen, and oxygen. There are various types of sugar derived from different sources. Simple sugars are called monosaccharides and include glucose (also known as dextrose), fructose and galactose. The table or granulated sugar most customarily used as food is sucrose, a disaccharide. (In the body, sucrose hydrolyses into fructose and glucose.) Other disaccharides include maltose and lactose. Longer chains of sugars are called oligosaccharides. Chemically-different substances may also have a sweet taste, but are not classified as sugars. Some are used as lower-calorie food substitutes for sugar described as artificial sweeteners. Sugars are found in the tissues of most plants, but are present in sufficient concentrations for efficient extraction only in sugarcane and sugar beet. Sugarcane refers to any of several species of giant grass in the genus Saccharum that have been cultivated in tropical climates in South Asia and Southeast Asia since ancient times. A great expansion in its production took place in the 18th century with the establishment of sugar plantations in the West Indies and Americas. This was the first time that sugar became available to the common people, who had previously had to rely on honey to sweeten foods. Sugar beet, a cultivated variety of Beta vulgaris, is grown as a root crop in cooler climates and became a major source of sugar in the 19th century when methods for extracting the sugar became available. Sugar production and trade have changed the course of human history in many ways. It influenced the formation of colonies, the perpetuation of slavery, the transition to indentured labour, the migration of peoples, wars between sugar trade-controlling nations in the 19th century, and the ethnic composition and political structure of the new world. The world produced about 168 million tonnes of sugar in 2011. The average person consumes about of sugar each year (33.1 kg in industrialised countries), equivalent to over 260 food calories per person, per day. Since the latter part of the twentieth century, it has been questioned whether a diet high in sugars, especially refined sugars, is good for human health. Sugar has been linked to obesity, and suspected of, or fully implicated as a cause in the occurrence of diabetes, cardiovascular disease, dementia, macular degeneration, and tooth decay. Numerous studies have been undertaken to try to clarify the position, but with varying results, mainly because of the difficulty of finding populations for use as controls that do not consume or are largely free of any sugar consumption.