Intestinal Gas Production

A considerable amount of gas is present in the gastrointestinal contents of all animals, and much of this is eliminated through the anus as flatus. Complaints of excessive gastrointestinal gas production in people and their pets are common. What we know about intestinal gas production and disposition has largely be gathered from studies with humans.

Five gases constitute greater than 99% of the gases passed as flatus: N₂, O₂, CO₂, H₂ and methane. None of these gases has an odor, and the characteristic odor of feces is due to very small quantities of a few other gases, including hydrogen sulfide, scatols and indoles. There is considerable individual variation in the contribution of each of these gases to total gas, but nitrogen typically predominates. Volume of gas elaborated also varies widely. In normal adult humans, the rate of excretion of gas per rectum ranges between 200 and 2000 ml per day. Ingestion of certain foods, beans being the classical example, is widely recognized to increase the rate of gas production.

There are three principal sources of the five major intestinal gases:

1. Air swallowing is the major source of gas in the stomach. Several milliliters of air are swallowed with every bolus of food or saliva. Most of this seems to be eructated and, apparently, very little passes into the duodenum.

2. Intraluminal generation of gases results from two major processes;

First, in the proximal intestine, the interaction of hydrogen and bicarbonate ions (principally from gastric and pancreatic secretions) leads to generation of CO₂. The amount of gas generated by this pathway is not great, because the lumenal contents do not contain carbonic anhydrase and the dissociation of H₂CO₃ is thus quite slow. Additionally, most of the CO₂ produced in this way is absorbed into blood.

The second and much more productive source of gas is fermentation by colonic bacteria. Microbes appear to be the sole source of all of the hydrogen and methane produced in the intestine. Fermentable substrates that escape digestion or absorption in the small intestine are often prime substrates for bacteria in the large intestine. A variety of fruits and vegetables contain polysaccharides that are not digested in the small intestine and lead to voluminous gas production by microbes. Indeed, the primary medical treatment for excessive gas production is dietary manipulation to eliminate foodstuffs that the individual cannot digest and absorb.

3. Gases readily diffuse across the mucosa. The direction of diffusion is dictated by the partial pressure of that gas in blood versus luminal contents. For methane and hydrogen, diffusion is always out of the lumen into blood. Nitrogen and CO₂ diffuse in either direction, depending on specific conditions within the individual.

Intestinal gases are a frequent cause of minor, occasionally major, social embarrassment, but can they ever be of truly dangerous? Both H₂ and CH₄ are combusible and potentially explosive. In human hospitals, there have been many explosions in the colon triggered by use of electrocautery performed through a proctosigmoidoscope. Many of these cases occurred when mannitol, a fermentable carbohydrate, was used as a purgitive to cleanse the colon. Use of non-fermentable cleansing agents has virtually eliminated this kind of accident.