Lactose intolerance and milk allergy differ in two important ways: (1) by the component of milk that people react to, and (2) by the cause of the reaction. These differences have implications for whether a person can consume any dairy at all, as well as what types of dairy may trigger a reaction.
Lactose intolerance is a reaction to the natural form of sugar present in milk and dairy products, called lactose. Milk allergy is a reaction to one or more of the proteins present in milk, namely whey and casein. People with a milk allergy must therefore avoid all dairy products, because even trace amounts of milk protein can trigger a reaction. People with lactose intolerance need only to minimize their intake of natural milk sugar (lactose), which they can do by: choosing naturally low lactose foods like aged cheese; choosing dairy foods with added lactase enzymes like Green Valley Lactose-Free products; or taking a lactase enzyme supplement pill with their first bite of a higher lactose foods, such as ricotta cheese or ice cream.
Lactose intolerance is caused by a deficiency in the enzyme responsible for breaking down lactose into simpler sugars, called lactase. It results in unpleasant digestive reactions after consuming dairy, including diarrhea, bloating and flatulence, but this reaction is not harmful to one’s health. Lactose intolerance is a common condition, affecting 60-70% of the world’s adult population.
Milk allergy is caused by an immune system reaction to the proteins found in milk, which triggers potentially life-threatening system-wide symptoms like hives, swelling and anaphylaxis. Some people may also experience gastrointestinal symptoms like vomiting and diarrhea. Milk allergy is far less common than lactose intolerance: Only 2.5% of young children have milk allergy, and most of them outgrow it; less than 1% of American adults are allergic to milk.