The hydrogen breath test is a test that uses the measurement of hydrogen in the breath to diagnose several conditions that cause gastrointestinal symptoms. In man, only bacteria-specifically, anaerobic bacteria in the colon–are capable of producing hydrogen. The bacteria produce hydrogen when they are exposed to unabsorbed food, particularly sugars and carbohydrates.
Although limited hydrogen is produced from the small amounts of unabsorbed food that normally reach the colon, large amounts of hydrogen may be produced when there is a problem with the digestion or absorption of food in the small intestine that allows more unabsorbed food to reach the colon. Large amounts of hydrogen also may be produced when the colonic bacteria move back into the small intestine, a condition called bacterial overgrowth of the small bowel.
In this latter instance, the bacteria are exposed to unabsorbed food that has not had a chance to completely traverse the small intestine to be fully digested and absorbed. Some of the hydrogen produced by the bacteria is absorbed into the blood flowing through the wall of the small intestine and colon . The hydrogen-containing blood travels to the lungs where the hydrogen is released and exhaled in the breath where it can be measured.