Intestinal gases as markers to study the diet–gut microbiota–host metabolism axis in humans and their relationship to metabolic health

The gut microbiota‒host metabolism axis is a critical determinant of metabolic health, yet its functional activity remains difficult to monitor in vivo. The gut microbiota ferments undigested food components such as dietary fibers and proteins, yielding various important metabolites and gases that impact human metabolism. This review synthesizes current evidence on intestinal gases, primarily hydrogen (H2), methane (CH4) and hydrogen sulfide (H2S), as non-invasive markers of the trade-off between saccharolytic and proteolytic fermentation. Within the current review, we evaluate the concentrations of important gut metabolites and gases and techniques to measure intestinal gases, including stable isotope breath tests, volatile organic compound (VOC) profiling, and respiration chambers. Crucially, we highlight that these gases may function not only as metabolic byproducts and biomarkers of microbial activity but as active signaling molecules influencing gastrointestinal transit, satiety, and systemic inflammation. This review concludes that real-time gas monitoring provides a unique opportunity to study real-time gut microbial fermentation. We propose a framework for phenotyping individual or subgroup-based fermentation patterns to guide personalized nutritional interventions for obesity and type 2 diabetes.