Vitamin C is simple sugar that is also called ascorbic acid. Vitamin C is essential in humans because the enzyme required for its synthesis, gulonolactone oxidase, is absent from the liver. However, residual activity to produce vitamin C is present in some humans. Most other animals are able to synthesise their requirements for vitamin C hepatically and so dietary intake is not required. Deficiency of vitamin C causes scurvy in humans, a disease characterised by a disintegration of the collagen in the body, which results in excessive bleeding from gums, skin and blood vessels. Vitamin C is required in the diet in order to provide reducing power to the aqueous environment of cells. Here it interacts with urate, glutathione and vitamin E to maintain a favourable oxidation to reduction potential and this limit free radical damage. In addition, vitamin C is also required to provide reducing power in a number of synthetic reactions including some hormones and neurotransmitters, as well as collagen and carnitine. Its ability to reduce metal ions also allows vitamin C to increase the absorption of iron by maintaining it in its reduced Fe2+ form, which is more easily absorbed.