Definition of Lactose intolerance:

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

Causes, incidence, and risk factors:

Lactose intolerance happens when the small intestine does not make enough of the enzyme lactase. Babies' bodies make this enzyme so they can digest milk, including breast milk. Before humans became dairy farmers, most people did not continue to drink milk, so their bodies did not make lactase after early childhood.

Lactose intolerance is more common in Asian, African, African-American, Native American, and Mediterranean populations than it is among northern and western Europeans.

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

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.

Lactose intolerance is sometimes seen in premature babies. Children who were born at full term generally do not show signs of lactose intolerance until they are at least 3 years old.

Not having enough lactase (lactase deficiency) may also occur as a result of intestinal diseases such as celiac sprue and gastroenteritis, or after bowel surgery. Temporary lactase deficiency can result from viral and bacterial infections, especially in children, when the cells lining the intestine are injured.