About Hydrogen and Methane Breath Testing
Historically, two tests have been used to diagnose SIBO: endoscopy followed by bacterial culture and hydrogen breath testing. Although endoscopy followed by bacterial culture is a direct measure of SIBO, this procedure is invasive, expensive, requires unique technical expertise and training to perform, and is limited in that SIBO samples can only be collected from a limited number of sites which are usually restricted to the proximal small intestine. This is a significant limitation in that most SIBO is believed to be found in the distal small intestine. Moreover, culturing anaerobic bacteria using current methods is problematic and inaccurate. Although, hydrogen breath testing is an indirect measurement of SIBO, it is non-invasive, affordable, quantitative, and can approximate the location of the bacterial overgrowth in the small intestine.
Hydrogen and methane are produced in the digestive tract primarily by bacteria as a result of carbohydrate (sugars, starches, and vegetable fiber) fermentation. When either hydrogen or methane appears in the expired air, it is usually a signal that carbohydrates have been exposed to bacteria and undergone fermentation. The diffusion of these gases into the bloodstream occurs at the site of metabolism. The blood carrying the gases eventually enters the lungs, where the gases again diffuse out of the bloodstream and into the expired air.
Bacteria are not normally found in the small intestine where digestion and absorption of sugars take place. If, however, bacteria are present in the small intestine, as seen in SIBO, then an ingested sugar will produce hydrogen and/or methane as a result of bacterial fermentation. Because the transit time of ingested sugars through the small intestine is typically 90-120 minutes, detection of methane or hydrogen in the expired air in <120 minutes after sugar ingestion indicates the presence of SIBO.
Two sugar substrates are typically used in hydrogen breath testing to detect and diagnose SIBO: glucose and lactulose. Both sugars can serve as suitable substrates for bacterial fermentation, although, due to its rapid absorption within the small intestine, glucose may be unable to identify distal SIBO. Lactulose cannot be absorbed or metabolized in the small intestine unless bacteria are present, and so lactulose is generally a superior substrate to detect the presence of SIBO along the entirety of the small intestine.