Lactose Intolerance: Lactose intolerance is a condition in which milk and milk products (lactose products) are not digestible. Lactose intolerance is caused by a deficiency of lactase in the intestinal wall. Lactose intolerance may appear gradually after childhood. However, the condition can be present at birth. Common symptoms of lactose intolerance include gas, bloating, abdominal pain, diarrhea, and nausea. The treatment for lactose intolerance are dietary changes to exclude products that contain lactase.