Galactose is a naturally occurring monosaccharide that forms the disaccharide lactose when combined with glucose (another monosaccharide). Galactosemia means “galactose in the blood”. This inherited disorder prevents your body from breaking down the sugar galactose, causing it to build up to toxic levels in your blood. People with galactosemia have to avoid dairy products, breast milk and most baby formulas. Galactose is a simple sugar, which belongs to simple carbohydrates. Galactose is composed of the same elements as glucose, but has a different arrangement of atoms. Galactose is a simple monosaccharide sugar naturally occurring in milk. When it’s chemically linked with glucose, it forms lactose, the principal sugar in milk and dairy foods. Galactose is important in early human development and plays a key role in our metabolism, even as adults. Galactose exists in both open-chain and cyclic form. The open-chain form is an aldehyde (RCHO). Four isomers are cyclic, two of them with a pyranose (six-membered) ring and two with a furanose (five-membered) ring. Galactose, a member of a group of carbohydrates known as simple sugars (monosaccharides). It is usually found in nature combined with other sugars, as, for example, in lactose (milk sugar). Galactose is also found in complex carbohydrates (see polysaccharide) and in carbohydrate-containing lipids. Galactose is a fundamental simple sugar, chemically known as a monosaccharide. It is one of the three main dietary monosaccharides, alongside glucose and fructose, that the body absorbs directly for energy.