Lactose intolerance is the inability or insufficient ability to digest lactose, a sugar found in milk or milk products. It is caused by a deficiency of the enzyme lactase, which is produced by the cells lining the small intestine. Lactase breaks down lactose into two simpler forms of sugar, glucose and galactose, which are then absorbed into the bloodstream.
The majority of humans have some degree of decreased lactase production. The frequency of decreased lactase production varies by region. For example, about 5% of people in northern Europe are lactase deficient, while some regions in Africa and Asia have a 90% prevalence. Not all people with lactase deficiency have digestive symptoms, but those who do may have lactose intolerance. Most people with lactose intolerance can tolerate some amount of lactose in their diet.
People sometimes confuse lactose intolerance with cow’s-milk allergy. Milk allergy is a reaction by the immune system to one or more milk proteins and can be life threatening when just a small amount of milk or milk product is consumed.
Milk allergy most commonly appears in the first year of life, while lactose intolerance usually develops in adulthood.
— Source: National Digestive Diseases Information Clearinghouse