Close to 50 million American adults are lactose intolerant. Certain ethnic and racial populations are more widely affected than others. The condition is least common among persons of northern European descent. Lactose intolerance results from a shortage of the enzyme lactase, which is normally produced by the cells that line the small intestine.
Lactase breaks down milk sugar into simpler forms that can then be absorbed into the bloodstream.
When there is not enough lactase to digest the amount of lactose consumed, the results, although not usually dangerous, may be very distressing. Common symptoms include nausea, cramps, bloating, gas, and diarrhea, which begin about 30 minutes to 2 hours after eating or drinking foods containing lactose. The severity of symptoms varies depending on the amount of lactose each individual can tolerate. Some causes of lactose intolerance are well known. For instance, certain digestive diseases and injuries to the small intestine can reduce the amount of enzymes produced.