Molecular Hydrogen in Health and Medicine
Molecular hydrogen (H₂) is a diatomic gas that, in biological systems, functions as a selective antioxidant preferentially neutralizing the most cytotoxic reactive oxygen species while preserving physiologically important radicals involved in normal cell signaling[^c1][^c2]. This selectivity distinguishes it from conventional antioxidants and has made it the subject of extensive biomedical research since the landmark 2007 study by Ohsawa et al. in Nature Medicine[^c3]. Beyond direct radical scavenging, H₂ activates the Nrf2 transcription factor to upregulate endogenous antioxidant enzymes, inhibits the NF-κB pathway to reduce pro-inflammatory cytokines, and protects mitochondrial function[^c4][^c5]. Recent work has identified the Rieske iron–sulfur protein (RISP) in mitochondrial complex III as a primary molecular target of H₂, triggering a mitohormetic adaptive response—a significant advance in understanding how trace amounts of H₂ produce marked biological effects[^c13].
The biological effects of H₂ have been investigated across more than 38 disease conditions, including ischemia-reperfusion injury, neurodegenerative disorders, cardiovascular disease, metabolic syndrome, and inflammatory conditions[^c6]. Clinical research published in 2026 has extended these investigations to myalgic encephalomyelitis/chronic fatigue syndrome (ME/CFS), autoimmune diseases such as Behçet's disease, Sjögren's syndrome, and SLE-associated pulmonary arterial hypertension, and the first Phase III multicenter randomized controlled trial of supersaturated hydrogen-rich water for weight management (the HOPE trial) began recruitment in April 2026[^c14]. Administration routes include inhalation of hydrogen gas, drinking hydrogen-rich water, hydrogen baths, and oral solid supplements, each with distinct pharmacokinetic characteristics[^c7]. H₂ is a physiologically normal molecule produced by intestinal bacteria and has demonstrated no toxic side effects across exposures far exceeding therapeutic levels[^c8], though a 2026 safety study found that pure hydrogen inhalation causes a mild decrease in blood oxygen saturation from dilution of inspired oxygen[^c16]. In Japan, electrolyzed hydrogen water generators are certified as Class II medical devices for gastrointestinal symptom improvement, and hydrogen inhalation for post-cardiac arrest syndrome has been designated as an advanced medical therapy[^c9][^c10].
Despite encouraging findings, the field faces significant challenges. Many studies rely on small sample sizes and exhibit methodological variability; a 2026 evidence-based review rated the GRADE evidence for hydrogen in musculoskeletal conditions as "low" or "very low" across all included studies[^c15]. The mechanisms underlying H₂'s biological activity at low concentrations remain incompletely understood, though the discovery of the RISP-mediated mitohormetic pathway has begun to close this gap[^c13]. Commercial promotion of hydrogen products has often outpaced the scientific evidence, and no national mandatory standards exist for hydrogen water concentration or quality in most countries[^c12].