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, especially beans, 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.
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 CO 2 diffuse in either direction, depending on specific conditions within the individual. Both H 2 and CH 4 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.