Lactose is a disaccharide sugar composed of galactose and glucose that is found in milk. Lactose makes up around 2–8% of milk (by weight), although the amount varies among species and individuals, and milk with a reduced amount of lactose also exists. It can be extracted from sweet or sour whey. The name comes from lac (gen. lactis), the Latin word for milk, plus the -ose ending used to name sugars. It has a formula of C12H22O11 and the hydrate formula C12H22O11·H2O, making it an isomer of sucrose.