Lactose intolerance is the inability to break down a type of natural sugar called lactose. Lactose is commonly found in dairy products, such as milk and yogurt. A person becomes lactose intolerant when his or her small intestine stops making enough of the enzyme lactase to digest and break down the lactose. When this happens, the undigested lactose moves into the large intestine. The bacteria that are normally present in the large intestine interact with the undigested lactose and cause symptoms such as bloating, gas, and diarrhea. The condition may also be called lactase deficiency.
Lactose intolerance usually causes gastrointestinal symptoms, such as gas, bloating, and diarrhea, about 30 minutes to two hours after ingesting milk or other dairy products containing lactose. People who are lactose intolerant may need to avoid eating these products or take medicines containing the lactase enzyme before doing so.
The signs and symptoms of lactose intolerance usually begin 30 minutes to two hours after eating or drinking foods that contain lactose. Common signs and symptoms include: