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Lactose intolerance: Inability to digest lactose, a 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 called lactase in the small intestine. Lactase is essential to digest lactose. Without enough lactase, there is lactose intolerance.

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

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

The most common symptoms of lactase deficiency 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.

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