Sixty-minute inhalation of molecular hydrogen decreases blood oxygen saturation but does not alter autonomic cardiac regulation at rest in healthy females: a randomized, double-blind, placebo-controlled crossover study

Molecular hydrogen has gained attention for its potential antioxidant, anti-inflammatory, and ergogenic effects. However, its impact on blood oxygen saturation and autonomic cardiac regulation under resting conditions remains unclear. This randomized, double-blind, placebo-controlled crossover study investigated the effects of a 60-minute molecular hydrogen inhalation on oxygen saturation and heart rate variability in healthy females. Twenty physically active participants (22.1 ± 1.6 years) completed two inhalation sessions with either molecular hydrogen or placebo (ambient air), separated by a seven-day washout period. Oxygen saturation and heart rate variability were continuously monitored during the 60 minutes of inhalation. The results using 60-minute averages showed a statistically significant (p ≤ 0.007) decrease in oxygen saturation during molecular hydrogen inhalation (95.9 ± 1.0%) compared to placebo (96.7 ± 0.7%), but this decrease should not be considered clinically significant. No significant changes (all p ≥ 0.32) were observed in time-domain or frequency-domain heart rate variability indices. These findings suggest that, under resting conditions, molecular hydrogen inhalation does not induce functional changes in autonomic cardiac regulation, and the observed reduction in blood oxygen saturation does not compromise homeostatic stability.