Intestinal gas (flatus) produced in the human gut comes from two sources – swallowed air (exogenous) and the air produced by colonic bacteria (endogenous). It is composed of various amounts of oxygen, nitrogen, carbon dioxide, hydrogen, and methane. Oxygen, nitrogen, and carbon dioxide mostly come from exogenous air and hydrogen and methane are by-products of the breakdown of food residue by probiotic bacteria that naturally inhabit the intestines.
Hydrogen (H2) and methane (CH4) are produced in the large intestine as a result of normal anaerobic bacterial activity. CH4 is absorbed in portal blood and excreted through breathing. For that reason pulmonary CH4 excretion is used as an indicator of methane production in the colon. H2 production can fluctuate rapidly depending on the availability of fermentable substrates in the colon, whereas CH4 excretion remains relatively constant throughout the day and doesn’t depend on the meals consumed. High pH level and low dilution rates stimulate the growth of anaerobic methanogenic bacteria. Hydrogen is released in the colon when undigested carbohydrates undergo bacterial fermentation. Carbon dioxide (CO2) is a byproduct of a chemical reaction with acid in the stomach. Additional amounts of carbon dioxide can be produced in the intestines as the byproduct of bacterial fermentation of unabsorbed sugars and starches (1).
Flatus removal from the gastrointestinal tract occurs through eructation, absorption, bacterial consumption and anal evacuation.