Acute Effects of Molecular Hydrogen Inhalation on Exercise-Induced Oxidative Stress, Vascular Responses, and Performance in National-Level Female Rowers: An Exploratory Pilot Case Series

Background: Evidence for the acute effects of molecular hydrogen (H2) inhalation on exerciserelated responses remains limited, particularly in elite female athletes exposed to high oxidative stress during intense exercise. Objective: To generate hypotheses and assess feasibility of near-lower explosive limit (LEL) hydrogen inhalation (approximately 4% H2 in air) on post-exercise oxidative stress markers, peripheral venous diameter, and exercise performance in nationally competitive female rowers. Methods: This participant-blinded, fixed-order (placebo then H2) exploratory pilot case series enrolled four nationally competitive female rowers. On the same day, each participant completed 30 min of placebo inhalation followed by an incremental exercise test (Wattbike), and, after a pragmatic 60-min recovery interval, completed 30 min of approximately 4% H2 inhalation followed by the same test. Blood/urine sampling and ultrasound measures were obtained at baseline, immediately post-exercise, and 60 min post-exercise. Within-participant differences in change scores (H2 minus placebo) were summarized using Hodges-Lehmann (HL) median differences with percentile bootstrap 95% confidence intervals. Exact Wilcoxon signed-rank pvalues were treated as descriptive; with n=4 the smallest two-sided p-value is 0.125. Results: Post-exercise change differences were directionally consistent across participants for HDL-C (HL +2.5 mg/dL), LDL-C (HL +6.5 mg/dL), cholinesterase (HL +10.5 U/L), and venous diameter (HL +2.98 mm) (all exact p=0.125, reflecting the minimum possible two-sided value with n=4). During recovery, urinary 8-OH-dG tended to be lower under H2 (HL-9.9; exact p=0.25), suggesting a potential attenuation of oxidative DNA damage, and venous diameter also showed consistent differences (HL-2.88 mm; exact p=0.125). Mean power output (mean WATT) was higher under H2 in three of four participants; one participant had lower mean WATT due to a transient orthostatic hypotension-like event requiring brief interruption. Conclusions: In this feasibility-constrained, fixed-order pilot case series (n=4), hydrogen inhalation showed directionally favorable signals in selected biochemical, vascular, and performance indices, but efficacy cannot be inferred because learning, fatigue, and carry-over effects cannot be separated. Larger double-blind, counterbalanced randomized crossover trials with adequate washout, plasma volume correction, and pre-specified safety endpoints are warranted.