Biological Relevance of Intestinal Gases in Healthy Humans

Abstract

Intestinal gases are usually discarded as physiologically inert, useless sub-products of colonic fermentation that must be expelled to prevent discomfort and meteorism. Starting from the observation that many living beings use exogenous and/or endogenous gases to attain evolutionary benefits, we question whether intestinal gases in healthy humans could have underestimated physiological effects, either intestinal or extra-intestinal. We examine gaseous volume, composition, source and local distribution in proximal as well as distal gut, providing extensive data that may serve as reference for future studies. We analyze each one of the most abundant intestinal gases and describe their diffusive patterns, active trans-barrier transport dynamics, chemical properties, intra-/extra-intestinal metabolic effects mediated by intracellular, extracellular, paracrine and distant actions. Discussing the physical properties of the whole intestinal gaseous mixture, we illustrate how changes in volume/pressure can be generated by two different mechanisms, namely, physical muscular gut contraction and biological colonic fermentation, with quite different metabolic outcomes. The experimental gas laws suggest that the gaseous exchanges between lumen and bloodstream are impaired by muscular contraction and improved by muscular relaxation. In turn, the surface-area-to-volume ratio suggests that the gaseous exchanges are impaired by microorganismal overproduction and improved by microorganismal reduction. Further, theoretical stochastic approaches from probability theory indicate that the non-turbulent, random paths of gas molecules inside colonic haustra do not homogenously spread over the whole mucosal surface. This means that the intestinal area available for lumen/blood metabolic exchanges is much less than expected not just in disease states, but also in healthy individuals.