Well, lactose is the sugar found in whole milk. When you drink milk, the villi in the small intestine naturally secrete an enzyme called lactase to help split the lactose into smaller units, glucose and galactose. These smaller units may be more readily absorbed by the villi into the blood stream to be carried to the cells of the body to be used in the process of cellular respiration. Lactaid is the offical brand name of a line of lactose-free milk products that are of benefit to people who have lactose-intolerance. This means their bodies either do not produce enough lactase to process whole milk that contains lactose, or do not produce the enzyme at all. So it is not so much that lactaid affects lactose, it is more accurate to say it replaces it as a dietary supplement for whole milk for those individuals who can not digest the lactose in whole milk.