Why are some people "lactose intolerant"?

People who are lactose intolerant have difficulty digesting dairy products that contain the milk sugar lactose. When lactose intolerant people ingest milk products, they can experience very unpleasant gastrointestinal symptoms. Lactose intolerant people are deficient in the enzyme that helps digest lactose, called "lactase."  Humans actually have two different versions of lactase.  The infant version is made when we are babies and toddlers, and allows us to digest our mother's milk or formula.  It is almost unheard of for an infant to be lactose intolerant. Later, as we get older, our bodies phase out the infant lactase version and start producing the adult lactase. Adult lactase is less efficient, and not as good at breaking down lactose as the infant lactase. These two versions of lactase are made from two different genes in our genome.  Some people have a faulty adult lactase, it can be mutated so it doesn't work right, or it can be genetically deleted or mutated so that the person doesn't produce adult lactase at all or they produce far less than normal. Some people suddenly become lactose intolerant in adulthood because their body stops making adult lactase. Lactose intolerance is more common in people from African and Asian descent, however anyone can be lactose intolerant.  People of European descent are least likely to be lactose intolerant, however, it does happen.  This information was gleaned from my human genetics class at the University of Colorado.  Milk allergies or dairy allergies are entirely different.  These arise from an over-reaction of your immune system to a component of milk or dairy.  Milk allergies can result in hives, rashes, swelling of the mouth and esophagus, etc.