It is a glycid found in milk and has chemical formula C12H22O11.
It is a disaccharide resulting from the condensation of a glucose molecule with a galactose molecule.
Lactose is produced only in mammalian mammary glands, being unique to milk and the sole carbohydrate of this food.
In human milk, it represents about 7.2% and in cow's milk about 4.7%.
Lactose is responsible for that slightly sweet taste of milk. When we drink a glass of milk or eat one of its derivatives, like cheese, lactose becomes energy to fuel our cells.