Hydrogen, and sometimes methane, are generated by bacterial action in the gut. The normal fasting level is low but increases are seen after the ingestion of fermentable carbohydrate. If bacterial action occurs in the stomach the hydrogen increase is detected in a matter of minutes. The normal rise after a test dose of a fermentable but non -absorbed sugar occurs two to three hours later when the carbohydrate reaches the colon. Lactulose is used as it is not absorbed but it is fermentable. An additional use of the test is in the investigation of lactose intolerance. Lactose is usually broken down and absorbed so that little or no increase in breath hydrogen occurs after ingestion, but when it’s not absorbed an increase in breath hydrogen is observed. Low doses of the test substances are used in order to avoid clinical symptoms as a result of the ingestion of a fermentable sugar.