Lactose is a disaccharide that consists of galactose and glucose. Lactose is contained mainly in milk and dairy products.

In order for lactose to be absorbed, it has to be split in the digestive process into the two sugars galactose and glucose, for which the body's enzyme lactase is necessary.

As infants, this enzyme is produced in sufficient quantity in order to digest breast milk however this production decreases with age. The rate of decline is slower for Europeans than for Africans or Asians; however, from the age of about 60 years most people are lactose intolerant.