Lactose is a disaccharide sugar derived fromgalactose and glucose that is found in milk. Lactose makes up around 2–8% ofmilk (by weight), although the amount varies among species and individuals, andmilk with a reduced amount of lactose also exists. It is extracted from sweetor 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 thehydrate formula C12H22O11·H2O, making it an isomer of sucrose.