Oral Molecular Hydrogen and Intestinal Motility: Exposure, Timing, and Endogenous Methane Antagonism—A Testable Hypothesis Framework

Background The claim that molecular hydrogen promotes intestinal peristalsis recurs throughout Chinese-language hydrogen-medicine writing and in industry material, where it is usually invoked without citation, yet it rests on a single ex vivo animal experiment. No controlled human study exists. That gap between how often the claim is repeated and how little supports it is itself a methodological problem worth examining.Argument This paper does not argue that hydrogen treats constipation. It argues that the evidence permits no conclusion in either direction, and that this is not because the hypothesis has failed but because three identifiable and correctable methodological errors have been made: the oral route has been treated as low-dose systemic dosing rather than as regionally targeted delivery; minute-scale acute responses and day-scale transit changes have been collapsed into a single endpoint; and non-response has been read as absence of effect, leaving the effect heterogeneity introduced by an endogenous gas out of account.Evidence base In isolated guinea-pig colon, direct intraluminal insufflation of hydrogen shortened transit time by 47% proximally and 10% distally. In dogs, methane slowed small-intestinal transit by 59%. One single-arm open-label study of 30 patients (28 completing) reported a 29.8% rise in complete spontaneous bowel movements with electrolysed hydrogen-rich alkaline reduced water. A double-blind trial in healthy adults showed stool form shifting toward Bristol type 4. Quantitative colonic gas studies supply the anchors: net hydrogen excretion was (35 ± 6.1) mL/24 h on a starch- and fibre-free diet, and (227.0 ± 60.7) mL after a 15 g lactulose load.Inferences Three falsifiable inferences follow. H1: because the oral route delivers hydrogen to the luminal surface while keeping systemic levels low, the exposure that should matter is the local one at the gut wall, not the concentration in blood. H2: any genuine acute component should fall inside the postprandial colonic motor response window (seconds to 2.5 h) and must be separated from the volume effect of water itself. H3: baseline breath methane acts as an effect modifier, blunting response in high-methanogenic phenotypes.Conclusion Section 6 sets out the evidence unfavourable to the framework, in order of severity. The most serious item is quantitative rather than conceptual: the ex vivo cornerstone holds intraluminal hydrogen at about 1 atm for the duration of the experiment, whereas one oral serving carries 0.124 mmol and, once partitioned into the lumen gas phase, sustains a tension of about 0.03 atm, one to two orders lower, with no concentration–response data to say where the threshold lies. The stratification hypothesis cannot separate cause from consequence, and the methane-flux arithmetic leaves only one of its two arms standing. Replacing stratified group comparison with the within-subject correlation between effect size and breath methane cuts the sample needed to screen for H3 from roughly 1000 to roughly 70; a two-arm design that keeps randomisation sits between the two at about 250. That reduction is the paper's main methodological contribution.