It is present in the milk of all mammals, but is not found anywhere else in nature. Manufacturers sometimes use lactose as a filler in products containing artificial sweeteners and also in some tablets. Lactose is normally broken down in the small intestine into its component sugars, glucose and galactose, under the action of an enzyme called lactase.
Humans are unique mammals because they continue to produce lactase throughout life.
Most people of Anglo-Saxon origin keep producing lactase and rarely develop lactase intolerance. But, from many parts of Asia, Africa, the Middle East and some Mediterranean countries and the Australian aboriginal people don't. Many gradually produce less lactase and develop lactose intolerance. Without sufficient lactase enzyme, the lactose sugar is not digested normally in the small intestine. Instead, it passes to the large intestine where it is fermented into bacteria.