Lactose intolerance is a common problem. It happens when your body does not have enough lactase, which is an enzyme produced in the small intestine. Lactase is necessary to digest lactose—the natural sugar found in milk and other dairy products. In the intestines, undigested lactose leads to the buildup of gas. Within 30 minutes to 2 hours after eating dairy products containing lactose, people with lactose intolerance start to develop stomach cramps and diarrhea. These two symptoms must be present for a person to be diagnosed with lactose intolerance.
Lactose intolerance is a common condition that is more likely to occur in adulthood, with a higher incidence in older adults. Some ethnic and racial populations are more affected than others, including African Americans, Hispanic Americans, American Indians, and Asian Americans. The condition is least common among Americans of northern European descent.