Intolerance, lactose: The inability to digest lactose, a sugar component of milk and most other dairy products. Lactose is sometimes also used as an ingredient in other foods, so those with a lactase deficiency should check labels carefully. The basis for this condition is lack of an enzyme in the small intestine called lactase. Lactase is essential to digest lactose. Without enough lactase, there is lactose intolerance.

Underproduction of lactase can cause difficulty digesting lactose- containing products, which include dairy products themselves and foods containing dairy products as ingredients.

The most common symptoms of lactose intolerance are diarrhea, bloating, and gas. The diagnosis may be made by a trial of a lactose- free diet or by special testing. In some cases, other diseases of the intestine may need to be excluded by further medical evaluation.

Most people are born with the ability to make adequate amounts of lactase but lactase production normally declines with age, more so in some persons than others, and there are also significant differences relative to lactase production among ethnic groups.