Lactose is the dietary sugar that is found in many foods in the typical Western diet, most notably dairy products such as milk, cheese, yogurt, and ice cream. The body makes an enzyme (called lactase) that breaks down the lactose sugar molecule so that it is easily digested. Some people do not make enough lactase to effectively digest lactose.
A buildup of this undigested lactose can cause painful bloating and gas. Lactose intolerance is generally acquired over time or after an illness (rarely present in infancy). It sometimes goes away on its own, but some people with have lactose intolerance for life. While not necessarily dangerous, it can be quite uncomfortable and cause distressing GI symptoms.
Lactose intolerance can be formally diagnosed with either a breath test (ingesting lactose and measuring the amount expelled in the breath over time) or through upper endoscopy.
Once diagnosed, the treatment typically involves dietary education to avoid lactose-containing foods. There are also over-the-counter lactase supplements that can be taken with lactose-containing meals to help with symptom management.