Most of this seems to be eructated and, apparently, very little passes into the duodenum. Intraluminal generation of gases results from two major processes. The amount of gas generated by this pathway is not great, because the lumenal contents do not contain carbonic anhydrase and the dissociation of H 2 CO 3 is thus quite slow. Additionally, most of the CO 2 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, 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.