Milk Sugar: Lactose

The only carbohydrate found in any quantity in milk is also peculiar to milk (and a handful of plants), and so was named lactose, or “milk sugar.” (Lact- is a prefix based on the Greek word for “milk”; we’ll encounter it again in the names of milk proteins, acids, and bacteria.) Lactose is a composite of the two simple sugars glucose and galactose, which are joined together in the secretory cell of the mammary gland, and nowhere else in the animal body. It provides nearly half of the calories in human milk, and 40% in cow’s milk, and gives milk its sweet taste.