Lactose is a natural carbohydrate (sugar) present in milk. It is a disaccharide, a sugar whose molecules contain two monosaccharide residues. Monosaccharide cannot be hydrolyzed to give a simpler form. Therefore Lactose is the most simple carbohydrate whose molecules consist of glucose and galactose units. Some babies have trouble digesting lactose in their newborn tummies, this is known as a lactose intolerance.