Hydrogen breath testing can be used to test for lactose intolerance. Lactose is a sugar found in milk. Individuals with lactose intolerance are unable to properly digest this sugar. Undigested lactose reaches the small and large intestines where bacteria feed off of it and produces excesses gasses as a result of fermentation. The resulting symptoms of gas, bloating, pain and diarrhea typically occur 30 minutes to 2 hours after consuming dairy rich foods.
Lactose is normally broken down by an enzyme called lactase, produced by cells found in the lining of the small intestine. Lactose intolerance in adulthood is most often caused by a reduction in the production of lactase after infancy. Most people with lactase intolerance retain some lactase activity and are able to consume varying amounts of lactose in their diets without symptoms. Affected individuals may have difficulty digesting fresh milk but can eat certain dairy products such as cheeses or yogurts without discomfort. These foods are made using fermentation processes that break down much of the lactose in milk.