Lactose is a disaccharide sugar derived from 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 is extracted from sweet or sour whey. The name comes from lac orlactis, the Latin word for milk, plus the -ose ending used to name sugars. It has a formula of C12H22O11.
Lactose is not added directly to many foods, because its solubility is less than other sugars commonly used in food. Infant formula is a notable exception, where the addition of lactose is necessary to match the composition of human milk.