Lactose intolerance is the inability to digest lactose. Lactose is a type of sugar found in milk and other dairy products.

Lactase deficiency; Milk intolerance; Disaccharidase deficiency; Dairy product intolerance

Lactose intolerance happens when the small intestine does not make enough of the enzyme lactase. Enzymes help the body absorb foods. Not having enough lactase is called lactase deficiency.

Babies' bodies make this enzyme so they can digest milk, including breast milk.

Premature babies sometimes have lactose intolerance. Children who were born at full term usually do not show signs of lactose intolerance until they are at least 3 years old.

Lactose intolerance can begin at different times in life. In Caucasians, it usually affects children older than age 5. In African Americans, lactose intolerance often occurs as early as age 2.

Lactose intolerance is more common in people with Asian, African, Native American, or Mediterranean ancestry than it is among northern and western Europeans.

Lactose intolerance is very common in adults and is not dangerous. Approximately 30 million American adults have some amount of lactose intolerance by age 20.

Causes of lactose intolerance include:

• Bowel surgery
• Infections in the small intestine from viruses or bacteria, which may damage the cells lining the intestine (most often in children)
• Intestinal diseases such as celiac sprue

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Digestive system organs

Review Date: 07/07/2010
Reviewed By: David C. Dugdale, III, MD. Professor of Medicine, Division of General Medicine, Department of Medicine, University of Washington School of Medicine; and George F. Longstreth, MD, Department of Gastroenterology, Kaiser Permanente Medical Care Program, San Diego, California. Also reviewed by David Zieve, MD, MHA, Medical Director, A.D.A.M., Inc.

A.D.A.M., Inc. is accredited by URAC, also known as the American Accreditation HealthCare Commission (www.urac.org)