Lactose

Lactose is a sugar made of a pair of simple sugar molecules: glucose and galactose. It is found naturally in mammal milk. Unlike other sugars, it does not dissolve easily in water. 
Lactose is broken down in infants and some adults by an enzyme, which allows both of its constituents to be absorbed into the bloodstream through the intestines. In persons without the enzyme, lactose is broken down by gut bacteria.
Lactose at Wikipedia