The goal of this clinical trial is to evaluate if molecular hydrogen can reduce the metabolic response during graded exercise in healthy women aged 18-30 years. The main questions it aims to answer are:

• Does molecular hydrogen reduce the respiratory exchange ratio?
• Does molecular hydrogen reduce oxygen uptake?

Researchers will compare molecular hydrogen to a placebo (ambient air containing no molecular hydrogen) to see if molecular hydrogen reduces the metabolic response.

Participants will:

• Inhale molecular hydrogen for 60 minutes each day for 7 days.
• Undergo an exercise testing twice after a two-week washout period.

This crossover trial examines the effect of repeated inhalation of molecular hydrogen on the metabolic response during graded exercise in healthy female participants. Each participant undergoes two experimental interventions (molecular hydrogen and placebo) in random order, separated by a two-week washout period. The order of the interventions is randomized using a computer random number generator. Each intervention consists of 7 days, during which participants inhale either molecular hydrogen or placebo gas for 60 minutes daily under resting conditions. Molecular hydrogen is produced by electrolysis of water and mixed with ambient air in the chamber to achieve a hydrogen concentration of 3.75 % which is below the lower explosive limit. This gas is administered via a face mask connected to an Ambu bag system with a one-way valve to ensure standardized delivery. Placebo is produced by pumping ambient air without molecular hydrogen. Molecular hydrogen is colorless, odorless, and tasteless, making it indistinguishable from placebo by human senses. On the 7th day of each intervention, participants undergo a graded exercise test on an bicycle ergometer immediately following the final inhalation session. The protocol consists of 8 incremental steps with the workload increasing from 0.5 to 2.25 W/kg. Outcome measures collected at rest before exercise and during exercise include cardiorespiratory variables, capillary blood biomarkers, and perceived exertion. Statistical analysis is based on a two-way analysis of variance for repeated measures, and p < 0.05 is considered statistically significant.

Inclusion Criteria:

• Healthy female participants.
• Regular menstrual cycle with exercise testing scheduled outside the menstrual phase.
• Signed informed consent.

Exclusion Criteria:

• Pregnancy or breastfeeding.
• History or presence of cardiovascular disease.
• History or presence of respiratory disease.
• Regular use of medications affecting metabolism.
• Use of dietary supplements affecting metabolism within 14 days prior to intervention.
• Strenuous physical activity within 24 hours prior to exercise testing.
• Any contraindication to exercise testing or study participation determined by a physician.

Cisgender women.

All IPD that underlie results in a publication.

Beginning 1 month after the publication of results with no end date.

Anonymized IPD will be uploaded to an open-access data repository.