Often confused with a milk allergy, which is entirely different, lactose intolerance refers to the inability to digest milk due to the lack of an enzyme called lactase. Lactase is needed to digest lactose, the kind of sugar found in milk. Over and over, research has shown that the inability to digest milk and milk products (known as lactose intolerance) is actually the rule, not the exception. 60% of adults can't digest milk, and when these individuals consume dairy products, they experience a host of unpleasant side effects.