Lactose - milk sugar

Lactose is also sometimes called milk sugar. Lactose is a non-scientific description. The correct biochemical description is β-D-galactopyranosyl-(1-4)-D-glucose.
Lactose belongs to the group of disaccharides and consists of two molecules, D-galactose and D-glucose (also known as dextrose) which are linked by a β-1-4 glycosidic bond.

In order to be able to utilise the lactose it needs to be broken down into its two monosaccarides, galactose and glucose, in the small intestine. This is made possible with the aid of the enzyme lactase which can be produced naturally by the body. If the body is unable to produce sufficient amounts of lactase, and the milk sugar therefore reaches other parts of the intestine without having been digested, this is called lactose intolerance (see "Symptoms and function of lactose intolerance").

General information about lactose

The salt of lactic acid is called lactate. Lactulose is used as a synthetic milk sugar (used as a mild laxative). The word lactose derives from the Latin word lac, lactis (milk) and the ending –ose for sugar. Lactose plays an important part for the feeding of young mammals as a component of breast milk. The amount of lactose depends on the animal species. Human breast milk (5-6g/100ml), for example, contains much more lactose than cow’s milk (4,5g/100ml). Dolphin milk (0,9g/100ml) contains the least lactose.

Sources:
Stryer, L., Biochemie, Specktrum Akademischer Verlag, 4. Auflage
Beliz, H.D., Lehrbuch der Lebensmittelchemie, Springer Verlag, 2001