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]. By mid-2026, over 1,500 basic research papers and 60 clinical trials had been published on hydrogen medicine[^c29].
The mechanistic understanding of molecular hydrogen underwent a paradigm shift in 2026. A major review established that H₂'s biological effects cannot be fully explained by direct radical scavenging alone — the "stoichiometric mismatch" between H₂'s micromolar in vivo concentration and the magnitude of its biological effects prompted a conceptual move from a framework of "passive free radical scavenging" to "active regulation of the redox–mitochondria–metabolism network"[^c32]. Key advances include the identification of the Nrf2/PINK1/Parkin-mediated mitophagy pathway as a central mechanism of H₂ neuroprotection[^c33], the discovery that H₂ inhibits ferroptosis via the Nrf2/GPX4/HO-1 pathway in cerebral ischemia-reperfusion injury[^c38], the recognition that H₂ coordinates mitochondrial biogenesis, dynamics, and mitophagy through the AMPK/Sirtuins/PGC-1α signaling axis[^c32], and the demonstration that H₂ inhalation reduces hippocampal ROS, Aβ42 accumulation, and neuroinflammation in the 5xFAD Alzheimer's mouse model[^c34]. The Rieske iron–sulfur protein (RISP) in mitochondrial complex III was identified as a primary molecular target, triggering a mitohormetic adaptive response through the mitochondrial unfolded protein response[^c13]. In 2024, oxidized heme (Heme-Fe(III)-OH) was identified as an additional core molecular target, providing a unified explanation for H₂'s selective antioxidant mechanism[^c22]. In sepsis-associated encephalopathy, a 2026 study identified SIRT1-mediated regulation of PINK1-dependent mitophagy as the core mechanism through which H₂ attenuates neuroinflammation and neuronal apoptosis[^c51].
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 in 2026 continued to expand across multiple therapeutic areas. In cardiovascular medicine, a pioneering injectable hydrogel encapsulating living photosynthetic bacteria that produces H₂ on demand when exposed to light demonstrated reduced infarct size and improved cardiac function after myocardial ischemia-reperfusion injury in rodent and porcine models[^c28]. The first systematic review on H₂ in heart failure models, published November 2025, concluded that H₂ reduces cardiac oxidative stress, inflammation, and cardiomyocyte death while improving mitochondrial function and attenuating cardiac remodeling across multiple HF subtypes[^c49]. A 2026 study demonstrated that hydrogen-rich water attenuates atherosclerosis in ApoE⁻/⁻ mice by modulating the gut microbiota-propionate-macrophage axis[^c50]. A head-to-head preclinical comparison of H₂-rich water versus 4% H₂ inhalation in radiation-induced heart disease showed both routes effectively decreased oxidative stress and normalized the Nrf2/Keap1 pathway, with a trend favoring inhalation[^c36]. In sports medicine, a randomized controlled trial of alkaline hydrogen-rich water in 40 physically active adults demonstrated a significant reduction in post-exercise IL-6 levels, with a large effect size, and stable malondialdehyde levels compared to controls[^c35]. Low-concentration hydrogen inhalation improved cognitive function in elderly women with mild cognitive impairment, shifting MMSE scores from the suspected dementia range to the normal range[^c18]. A closed-loop glucose-responsive wound dressing combining nitric oxide and molecular hydrogen demonstrated complete wound closure in diabetic mouse models[^c20]. 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].
A landmark 2025 study in Nature Microbiology identified the group B [FeFe]-hydrogenase as the primary driver of fermentative H₂ production in the healthy human gut, with Bacteroides species as major producers. This enzyme was shown to be significantly depleted in Crohn's disease patients, who exhibited a restructured gut hydrogen economy[^c37]. In dermatology, a 2026 study demonstrated that hydrogen-rich water inhibits the cGAS-STING signaling pathway in psoriasis models, identifying a novel molecular mechanism for H₂'s anti-psoriatic effects[^c41]. A 2026 study in lung cancer models under chronic intermittent hypoxia showed that H₂ suppresses tumor growth by modulating macrophage polarization toward the M1 phenotype and suppressing the CCL2-CCR2 axis[^c45]. The first human pharmacokinetic study for inhaled H₂ established a minimum effective blood concentration range of 0.42 to 1.05 µg/L[^c43], and the first ratiometric fluorescent bioprobe provided direct visual confirmation that H₂ crosses the blood-brain barrier within 5 minutes of inhalation[^c42]. A randomized controlled trial of hydrogen-oxygen inhalation in 66 participants with sleep disorders demonstrated significant improvements in total sleep time, sleep efficiency, and mood[^c40]. The HYDRAPPET trial found that daily hydrogen-rich water consumption for eight weeks increased GLP-1 levels, reduced food cravings, improved sleep quality, and lowered cholesterol in obese adults[^c39]. A 2025 perspective article further proposed a mechanistic framework for H₂ in obesity management involving modulation of PGC-1α, irisin, and GLP-1 signaling pathways[^c53]. A 2025 comprehensive review of molecular hydrogen in liver diseases detailed H₂'s multi-target mechanisms regulating redox signaling, inflammatory cascades, glucolipid metabolism, and gut microbiota remodeling[^c44].
In liver disease, a 2026 study from Ohsawa's group demonstrated that hydrogen-water protects against acetaminophen-induced hepatotoxicity in diabetic mice and synergizes with N-acetylcysteine, with H₂ being more effective than NAC at reducing mitochondrial oxidative stress[^c52]. In autoimmune disease, a 2026 case report documented marked clinical stabilization in a 72-year-old patient with refractory primary Sjögren's syndrome-associated interstitial lung disease after oral molecular hydrogen capsule therapy, including improved pulmonary function, decreased anti-Ro antibodies, and normalization of immunophenotypic markers[^c46]. A randomized double-blind controlled trial published in BMC Women's Health found that daily hydrogen-rich water consumption significantly reduced premenstrual syndrome symptoms, including fatigue, mood swings, pain, and sleep disruption, over three menstrual cycles[^c47]. A randomized controlled pilot trial during COVID-19 lockdowns found that hydrogen-rich water consumption in adults over 70 years was associated with a 4% increase in telomere length over six months, compared to an 11% decline in controls, along with reduced C-reactive protein and improved sleep and mobility[^c48]. In oral medicine, a comprehensive review described H₂ applications in periodontitis, peri-implantitis, oral cancer, radiotherapy-related damage, and maxillofacial wound healing. A randomized trial completed in May 2026 evaluated hydrogen-rich water in 60 overweight and obese adolescents during a weight loss retreat, with results pending publication.
A 2026 study of overnight breath hydrogen dynamics in 166 healthy adults aged 20–85 years found that older adults had significantly lower morning breath hydrogen levels and larger overnight declines compared to younger adults, identifying exhaled hydrogen as a potential noninvasive biomarker for aging-related physiological changes[^c54]. Bone regeneration research advanced with a calcium silicide nanomaterial (CaSi₂) achieving a hydrogen yield of 911 mL/g and enabling sustained in situ H₂ release within scaffolds, producing a local H₂ concentration 46,000-fold higher than hydrogen-rich saline injection and enabling effective bone defect repair in aged mice[^c55].
A 2026 systematic review of 45 studies evaluated molecular hydrogen therapy across musculoskeletal conditions, reporting symptomatic benefits but rating the GRADE evidence as low or very low across all trials[^c23]. In critical care, inhaled hydrogen improved survival and preserved cognitive function in sepsis models through Nrf2/HO-1 signaling and mitochondrial quality control[^c24]. A 2026 study identified SIRT1-mediated PINK1/Parkin-dependent mitophagy as the core neuroprotective mechanism of H₂ in sepsis-associated encephalopathy, with 2% H₂ inhalation increasing survival and preserving cognitive function in septic mice[^c51]. Cardiovascular studies expanded with a Keio University trial reporting that hydrogen-rich water enhanced heart rate variability at rest in healthy adults[^c25], and a prospective observational study associated routine HRW consumption with improved lower extremity function in older adults[^c26]. In dermatology, a double-blind randomized trial found that hydrogen-rich water reduced pain and itching in keloid patients[^c27], and a mouse study showed that continuous H₂ administration delayed UVB-induced skin carcinogenesis. In oncology, a systematic review of 27 studies reported significantly improved progression-free survival in advanced NSCLC patients receiving adjunctive H₂ inhalation[^c17].
Administration routes include inhalation of hydrogen gas, drinking hydrogen-rich water, hydrogen baths, and oral solid supplements, each with distinct pharmacokinetic characteristics[^c7]. Conventional delivery routes face critical limitations in stability, bioavailability, and targeted delivery, spurring development of advanced delivery systems including H₂-containing carriers, in situ H₂-generating materials, and externally stimulated platforms[^c31]. 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]. However, multiple severe explosion accidents have been documented from high-concentration hydrogen inhalers (67–100 vol%) that far exceed the verified safe threshold of 10 vol%[^c19].
Despite encouraging findings, the field faces significant challenges. Many studies rely on small sample sizes and exhibit methodological variability; the GRADE evidence for hydrogen in musculoskeletal conditions was rated as "low" or "very low" across all included studies[^c15]. 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].