A polyhydric alcohol, CH2OH(CHOH)4CH2OH, derived from mannose or fructose. It is the main soluble sugar in fungi and an important carbohydrate reserve in brown algae. Mannitol is used as a sweetener in certain foodstuffs and as a diuretic in medicine to relieve fluid retention, e.g. in acute kidney failure, or to treat swelling inside the eye or around the brain.