Lactose is a major type of sugar found in milk and milk products, including human milk. Lactose makes up less than eight percent of the solids in milk. Lactose is not found naturally in any other food aside from dairy products.
Lactose intolerance is caused by the inability of the body to break down milk sugars. In a normal person, lactase, an enzyme produced by the small intestine, breaks down lactose so it can be absorbed into the bloodstream. People who are lactose intolerant cannot produce enough lactase and are unable to process lactose.