Most people who follow a dairy-free diet are allergic to dairy products. However, a few people who are exquisitely sensitive to lactose—milk sugar—also need to avoid all dairy products.

People who react to lactose are called “lactose-intolerant.” They lack adequate amounts of an enzyme (called lactase) needed to digest milk sugar. Unlike people who are allergic to dairy products, most lactose-intolerant people can consume dairy products that naturally contain only traces of lactose (such as hard cheese) or those that have been treated to break down lactose (e.g., Lactaid™ milk). Many lactose-intolerant people can also eat yogurt without suffering, despite the high amount of lactose found in yogurt. The ability of many lactose-intolerant people to consume yogurt without getting sick used to baffle scientists. Now we know that the bacteria in most yogurt products (except frozen yogurt) consume most of the lactose as soon as the yogurt moves from the stomach to the intestines. These bacteria are so efficient that they often break down the lactose before the lactose-intolerant person has a chance to react to it.

Milk allergy is an immune system response to the presence of milk protein in the body. The body perceives the protein as “foreign” and proceeds to mount an attack against it, which results in the symptoms of an allergic reaction. Cow’s milk is the most common cause of food allergy in infants and young children. Cow’s milk proteins are potent allergens and around 2.5% of infants experience cow’s milk allergy in the first years of life. However, food allergies usually diminish with advancing age. Up to 85% of children will outgrow their allergy by the age of three; the majority will outgrow it by the time they reach school.