Therapeutic potential of hydrogen-rich water (HRW) in oxidative stress-related diseases

Abstract

Oxidative stress is a central mechanism in metabolic, cardiovascular, and neurodegenerative diseases, contributing to inflammation, mitochondrial dysfunction, and apoptosis. Consequently, redox-modulating therapies are increasingly explored as potential therapeutic strategies. This review evaluates the mechanistic basis, experimental evidence, and clinical applicability of hydrogen-rich water (HRW) in oxidative stress-related diseases. A structured literature search identified mechanistic, preclinical, and clinical studies investigating HRW or molecular hydrogen (H₂) on oxidative stress, inflammation, mitochondrial regulation, and disease-related outcomes. Due to substantial methodological heterogeneity, findings were synthesized qualitatively. HRW has been proposed to selectively neutralize highly reactive species such as hydroxyl radicals (^•OH) and peroxynitrite (ONOO⁻) while preserving physiological reactive oxygen species signalling. Mechanistic studies demonstrate activation of nuclear factor erythroid 2–related factor 2/antioxidant response element (Nrf2/ARE) pathways, suppression of nuclear factor kappa B (NF-κB) and mitogen-activated protein kinase (MAPK) inflammatory cascades, modulation of JAK/STAT signalling, preservation of mitochondrial bioenergetics, and enhancement of autophagic regulation. Preclinical and clinical studies report improvements in glycaemic control, endothelial function, cardiometabolic health, neuroprotection, exercise performance, and treatment-related fatigue. Despite an excellent safety profile, standardization of HRW preparation and large multicentre randomized controlled trials (RCTs) remain necessary to establish clinical efficacy and translational potential.

Introduction

Oxidative stress, defined as an imbalance between the excessive production of reactive oxygen species (ROS) and the limited capacity of endogenous antioxidant defense systems, is widely recognized as a pivotal driver in the initiation and progression of chronic and degenerative diseases.^(1,2) Under physiological conditions, controlled ROS generation participates in essential cellular processes including signal transduction, host defense, and redox homeostasis. However, persistent or uncontrolled oxidative burden leads to cumulative damage of vital macromolecules such as lipids, proteins, and nucleic acids, disrupting membrane integrity, enzymatic activities, genomic stability, and mitochondrial function.⁽³⁾ This biochemical cascade not only accelerates cellular senescence and apoptosis but also perturbs metabolic pathways and inflammatory signaling networks, thereby exacerbating tissue dysfunction. As a result, oxidative stress is increasingly acknowledged as a unifying mechanism underpinning diverse pathological conditions, such as diabetes mellitus (DM), cardiovascular disorders, neurodegenerative diseases (e.g., Alzheimer’s and Parkinson’s disease), cancer, chronic kidney disease, and the biological process of aging.^(4,5) Given its broad relevance across organ systems and disease phenotypes, targeting oxidative stress through antioxidants, redox modulators, and lifestyle interventions represents a key therapeutic and preventive strategy under extensive scientific investigation.

Hydrogen-rich water (HRW), an emerging therapeutic strategy, serves as a novel delivery system for molecular hydrogen (H₂), a biologically inert and highly diffusible gas with demonstrated antioxidant, anti-inflammatory, and cytoprotective activities.^(6,7) Unlike conventional antioxidants that broadly scavenge ROS and may disrupt redox-dependent physiological signaling, H₂ selectively targets the most cytotoxic species, particularly hydroxyl radicals (^•OH) and peroxynitrite (ONOO⁻), while sparing beneficial ROS involved in cellular homeostasis and signal transduction.^(8,9) This unique selectivity provides an important mechanistic advantage, reducing the risk of impairing redox-regulated biological processes. In addition to its radical-scavenging properties, molecular H₂ exhibits pleiotropic biological effects, including modulation of mitochondrial function, attenuation of inflammatory cascades, and enhancement of endogenous antioxidant defense systems.⁽¹⁰⁾ Collectively, these features position HRW as a promising, safe, and targeted redox-modulating intervention with potential applications across oxidative stress-mediated diseases.

Since the landmark study by Fukuda et al. (2007) demonstrating that molecular H₂ inhalation significantly reduced brain infarct volume in a rodent model of cerebral ischemia–reperfusion injury,⁽¹¹⁾ extensive research has broadened its therapeutic scope. Accumulating evidence indicates that HRW exerts beneficial systemic effects across diverse pathologies, including metabolic syndrome, type 2 diabetes (T2DM), cardiovascular dysfunction, neurodegenerative diseases, and inflammatory disorders.^(12–15) These benefits extend beyond simple free-radical scavenging. Molecular H₂ modulates multiple cellular programs, acting not only as a selective antioxidant but also as an anti-inflammatory, anti-apoptotic, and cytoprotective signaling molecule. Mechanistically, H₂ influences key redox-sensitive and survival pathways, including nuclear factor erythroid 2–related factor 2/antioxidant response element (Nrf2/ARE)-mediated antioxidant defense, Nuclear Factor kappa-light-chain-enhancer of activated B cells (NF-κB)–driven inflammatory cascades, Mitogen-Activated Protein Kinase (MAPK) stress signaling, and Janus Kinase/Signal Transducer and Activator of Transcription (JAK/STAT) transcriptional regulation.⁽¹⁶⁾ Additionally, H₂ has been shown to promote mitochondrial homeostasis, support autophagic clearance of damaged organelles, regulate gene expression, and preserve cellular bioenergetics.⁽¹⁰⁾ Collectively, these pleiotropic biological actions underscore the therapeutic versatility of HRW and support its development as a promising adjunct strategy for oxidative stress- and inflammation-driven diseases.

However, most existing reviews evaluate molecular H₂ as a broad therapeutic category, combining findings from inhaled H₂ gas, H₂-rich saline, and H₂-induced biological generation without distinguishing the distinct pharmacokinetic, mechanistic, and translational characteristics of HRW. This represents an important gap in the literature. HRW is the most accessible and widely used modality in human studies, offering non-invasive oral administration, high patient compliance, low cost, and favourable safety. Emerging evidence suggests that HRW may exert biological effects beyond those attributable to dissolved H₂ concentration alone, including gastrointestinal absorption dynamics, mitochondrial targeting, modulation of metabolic redox states, and potential involvement of gut signaling mediators such as ghrelin. Yet, despite increasing clinical trials in metabolic syndrome, diabetes, vascular dysfunction, exercise physiology, cognitive impairment, and treatment-related fatigue, no comprehensive synthesis has specifically evaluated HRW as a distinct therapeutic platform. This review synthesizes current evidence on the therapeutic potential of HRW in oxidative stress-related diseases, integrating data from molecular, animal, and clinical studies. It also discusses safety considerations, methodological limitations, and future prospects for clinical translation.

Methods

This narrative review synthesizes current evidence on HRW as a redox-modulating intervention in oxidative stress related diseases. Although structured search elements were applied to enhance transparency, the intent was interpretive rather than to conduct a systematic review or meta-analysis. A literature search was performed in PubMed, Scopus, Web of Science, and Google Scholar from database inception to October 2025 using combinations of the terms “hydrogen-rich water”, “molecular hydrogen”, and “H₂ water”, together with “oxidative stress”, “inflammation”, “mitochondria”, “neuroprotection”, “metabolic disorders”, and “cardiovascular disease”.

Reference lists of relevant publications were additionally screened for missed sources. Studies were included if they examined HRW or orally delivered dissolved H₂ and reported mechanistic, preclinical, or clinical outcomes related to oxidative stress biomarkers, mitochondrial regulation, inflammatory mediators, metabolic control, cardiovascular function, or neurological performance. Non-English articles, abstracts without full data, commentaries, and studies assessing H₂ gas or H₂-rich saline without an HRW arm were excluded. Eligible studies were grouped into in vitro mechanistic models, in vivo animal models, and human clinical investigations. Data extracted where available included model characteristics, HRW preparation method, dissolved H₂ concentration, duration of exposure, comparator conditions, and primary mechanistic or clinical outcomes with emphasis on redox-related signalling pathways. In total, our search identified 72 relevant HRW studies, comprising 18 in vitro studies, 28 in vivo animal models, and 26 human clinical investigations. The majority of preclinical studies focused on metabolic and cardiometabolic disorders, followed by cardiovascular and neurodegenerative models, while most clinical trials involved metabolic syndrome, T2DM, hyperlipidaemia, rheumatoid arthritis, and exercise-related fatigue. Findings were synthesized qualitatively due to heterogeneity in dosing strategies, biomarker selection, and outcome reporting. This methodological approach maintains the narrative nature of the review while providing increased transparency and clearer mapping of evidence distribution to guide future mechanistic and clinical research priorities. Figure 1 summarizes the PRISMA-guided selection process, from 1,288 records identified to 72 studies included in the final qualitative synthesis. The literature search was updated prior to revision to include recently published clinical studies on oral HRW administration.

Fig. 1.

PRISMA flow diagram illustrating the identification, screening, eligibility assessment, and inclusion of studies for the review. Records were retrieved from major scientific databases, duplicates removed, titles and abstracts screened, full texts evaluated, and studies meeting the inclusion criteria incorporated into the qualitative synthesis.

Oxidative Stress and Its Role in Disease

Mechanisms of oxidative stress

ROS such as superoxide anion (O₂^•−), hydrogen peroxide (H₂O₂), and hydroxyl radical (^•OH) are inevitable byproducts of mitochondrial oxidative phosphorylation and various enzymatic oxidases, including NADPH oxidase and xanthine oxidase.⁽¹⁶⁾ Under physiological conditions, ROS generation is tightly regulated and balanced by a complex antioxidant defense network. This includes enzymatic antioxidants such as superoxide dismutase (SOD), catalase (CAT), and glutathione peroxidase (GPx), which catalytically convert reactive intermediates into less harmful molecules, as well as non-enzymatic antioxidants such as vitamins C and E, reduced glutathione (GSH), and uric acid that scavenge free radicals and maintain redox equilibrium.⁽¹⁷⁾

However, in pathological states such as persistent hyperglycemia, ischemia–reperfusion injury, or chronic inflammation, this redox balance is disrupted, leading to excessive accumulation of ROS and the onset of oxidative stress.⁽³⁾ Mitochondrial overproduction of ROS damages essential cellular macromolecules, including membrane lipids, proteins, and nucleic acids, thereby impairing electron transport chain efficiency and reducing ATP synthesis. Oxidative modification of mitochondrial DNA and cardiolipin destabilizes the mitochondrial membrane, promoting the release of cytochrome c into the cytosol and activating caspase-dependent apoptotic pathways.⁽¹⁸⁾ Concurrently, elevated ROS levels act as secondary messengers that stimulate pro-inflammatory signaling cascades particularly the NF-κB, MAPK, and JAK/STAT pathways that further amplifying cellular injury, inflammation, and programmed cell death. Collectively, this vicious cycle of oxidative damage and inflammatory activation contributes to the pathogenesis of numerous chronic and degenerative diseases.

Oxidative stress in major diseases

Diabetes mellitus

Chronic hyperglycemia in DM triggers a cascade of oxidative and metabolic disturbances that underlie both β-cell dysfunction and peripheral insulin resistance. Elevated glucose levels accelerate glucose autoxidation, nonenzymatic protein glycation, and polyol pathway flux, each contributing to excessive generation of ROS such as O₂^•−, H₂O₂, and ^•OH.^(19,20) Mitochondrial overload resulting from enhanced substrate oxidation further amplifies electron leakage from complexes I and III of the respiratory chain, producing a continuous oxidative insult.⁽¹⁸⁾ In pancreatic β-cells, which possess relatively low antioxidant enzyme capacity (notably reduced CAT and GPx activities), ROS accumulation causes mitochondrial DNA damage, lipid peroxidation, and inhibition of insulin gene transcription and secretion.⁽²¹⁾ Simultaneously, in insulin-sensitive tissues such as skeletal muscle, adipose tissue, and liver, oxidative stress disrupts insulin signaling via serine phosphorylation of insulin receptor substrate (IRS) proteins and activation of stress kinases including JNK and p38 MAPK.⁽²²⁾ These events culminate in impaired glucose uptake and systemic insulin resistance.

Moreover, the sustained oxidative milieu accelerates the onset and progression of chronic diabetic complications including nephropathy, neuropathy, retinopathy, and cardiomyopathy through endothelial dysfunction, advanced glycation end-product (AGE) accumulation, and activation of redox-sensitive transcription factors such as NF-κB and AP-1.^(23–26) Thus, oxidative stress represents a unifying molecular mechanism linking hyperglycemia to both pancreatic β-cell failure and multisystem diabetic complications. Figure 2 illustrates how oxidative stress serves as a central mechanistic link between chronic hyperglycemia, β-cell dysfunction, and insulin resistance, highlighting the role of excessive ROS generation, impaired antioxidant defense, and disrupted insulin signaling in driving the progression of diabetes.

Fig. 2.

Oxidative stress as a central mechanism linking hyperglycemia to β-cell dysfunction and insulin resistance in diabetes mellitus. Illustrates how oxidative stress acts as a central mechanistic link between chronic hyperglycemia, β-cell dysfunction, and insulin resistance in diabetes mellitus. Persistent elevation of glucose enhances mitochondrial reactive oxygen species (ROS) production, activates NADPH oxidase, and impairs antioxidant defense systems. Excess ROS leads to oxidative damage of pancreatic β-cells, reducing insulin synthesis and secretion. Simultaneously, oxidative stress disrupts insulin receptor signaling pathways in peripheral tissues, promoting insulin resistance. The combined effects accelerate metabolic dysregulation, forming a self-perpetuating cycle that contributes to the onset and progression of diabetes. Reprinted from Dludla et al., 2023.⁽⁷⁴⁾

Cardiovascular diseases

In the cardiovascular system, oxidative stress represents a central pathological driver linking metabolic imbalance to endothelial dysfunction and atherogenesis. Excess production of ROS impairs endothelial nitric oxide synthase (eNOS) activity and reduces nitric oxide (NO) bioavailability, largely through increased superoxide formation and subsequent peroxynitrite generation, thereby disrupting vascular tone and homeostasis.^(27–29) Oxidative modification of low-density lipoprotein (LDL) promotes its conversion into oxidized LDL (oxLDL), which accelerates foam-cell formation and initiates early atherosclerotic changes within the arterial intima.⁽³⁰⁾ Mitochondrial dysfunction and overactivation of NADPH oxidase isoforms (particularly NOX1, NOX2, and NOX4) further amplify oxidative burden, contributing to vascular smooth muscle cell proliferation, extracellular matrix deposition, and arterial stiffening.⁽³¹⁾ In parallel, elevated ROS activates key inflammatory pathways NF-κB and the NLRP3 inflammasome leading to increased expression of TNF-α, IL-1β, and IL-18, which perpetuate vascular inflammation and lesion progression.^(32,33) Oxidative stress also induces matrix metalloproteinases (MMP-2 and MMP-9), which degrade extracellular matrix components, destabilize the fibrous cap, and enhance the likelihood of plaque rupture and thrombosis, the major cause of acute cardiovascular events.^(34,35) As illustrated in Fig. 3, oxidative stress acts as a key driver of endothelial dysfunction and atherosclerosis by promoting excessive ROS generation, eNOS uncoupling, inflammatory pathway activation, and vascular smooth muscle cell remodeling, ultimately accelerating plaque formation.

Fig. 3.

Oxidative stress driven molecular pathways leading to endothelial dysfunction and atherosclerosis. This schematic illustrates the major enzymatic and mitochondrial sources of reactive oxygen species (ROS) involved in vascular oxidative stress, including NADPH oxidase isoforms (NOX1/NOX2/NOX4) and mitochondrial ROS (mtROS). Excessive ROS generation promotes peroxynitrite (ONOO⁻) formation and causes endothelial nitric oxide synthase (eNOS) uncoupling, resulting in diminished nitric oxide bioavailability and impaired endothelial relaxation. Oxidized low-density lipoprotein (oxLDL) further amplifies oxidative stress and triggers downstream inflammatory cascades. Activation of JAK/STAT, MAPK, and NF-κB signaling pathways induces the release of pro-inflammatory cytokines (IL-1β, IL-18, and TNF-α), contributing to chronic vascular inflammation. ROS and oxLDL also stimulate vascular smooth muscle cell (VSMC) proliferation and migration and activate matrix metalloproteinases (MMP-2 and MMP-9), leading to extracellular matrix degradation. Collectively, these processes drive endothelial dysfunction and promote progressive atherosclerotic plaque development. Reprinted from Yan et al., 2023.⁽⁷⁵⁾

Neurodegenerative disorders

The brain’s exceptionally high oxygen consumption, abundant polyunsaturated fatty acids, and comparatively weak antioxidant defense make it highly susceptible to oxidative damage.⁽³⁶⁾ Excess ROS and mitochondrial dysfunction are now recognized as central drivers of neuronal loss across major neurodegenerative disorders including Alzheimer’s disease (AD), Parkinson’s disease (PD), and ischemic stroke.⁽³⁷⁾ In AD, oxidative modification of amyloid precursor protein (APP) upregulates β-secretase (BACE1) activity, enhances amyloid-β (Aβ) aggregation, and contributes to tau hyperphosphorylation and synaptic dysfunction. Lipid peroxidation products such as 4-hydroxynonenal (4-HNE) and malondialdehyde (MDA) further destabilize neuronal membranes and exacerbate mitochondrial impairment.⁽³⁸⁾

In PD, neurotoxic ROS originate from mitochondrial complex I deficiency, dopamine auto-oxidation, monoamine oxidase-B (MAO-B) metabolism, and microglial NADPH oxidase (NOX2) activation. Oxidative modification of α-synuclein promotes its misfolding and fibrillization, forming Lewy bodies and impairing proteasomal/autophagic clearance. This cascade leads to dopaminergic neuronal death within the substantia nigra, a hallmark of PD pathology.⁽³⁹⁾

In ischemic stroke, reperfusion triggers an explosive burst of ROS, amplifying glutamate excitotoxicity, NMDA receptor overactivation, Ca²⁺ overload, and mitochondrial permeability transition pore (mPTP) opening, culminating in apoptotic and necroptotic injury. ROS also activate neuroinflammatory pathways such as NF-κB and NLRP3 inflammasomes, perpetuating cytokine release, microglial activation, and secondary neuronal injury.⁽³⁷⁾

Collectively, these processes position oxidative stress as a unifying pathogenic mechanism across AD, PD, and stroke, supporting the rationale for developing antioxidant or redox-modulatory therapies such as HRW, Nrf2 activators, MAO-B inhibitors, and NOX inhibitors as disease-modifying interventions.^(40,41) Figure 4 highlights oxidative stress as a central pathogenic mechanism across major neurodegenerative disorders, demonstrating how excessive ROS production, mitochondrial dysfunction, and chronic neuroinflammation collectively drive neuronal injury, aberrant protein aggregation, and progressive neurodegeneration.

Fig. 4.

Oxidative stress as a central pathogenic mechanism in neurodegenerative disorders. Illustrates how oxidative stress serves as a key pathogenic driver in major neurodegenerative disorders, including Alzheimer’s disease, Parkinson’s disease, amyotrophic lateral sclerosis, and stroke. Excessive production of reactive oxygen species (ROS), combined with impaired antioxidant defense systems, leads to mitochondrial dysfunction, lipid peroxidation, protein oxidation, and DNA damage. These redox-mediated insults promote neuronal apoptosis, synaptic degeneration, neuroinflammation, and abnormal protein aggregation, such as β-amyloid plaques, α-synuclein fibrils, and tau pathology. The figure highlights the interplay between mitochondrial impairment, microglial activation, excitotoxicity, and chronic inflammatory signaling, demonstrating how oxidative stress accelerates neuronal loss and functional decline across diverse neurological conditions. Reprinted with permission from Singh et al., 2025.⁽⁷⁶⁾

HRW: Production and Properties

HRW can be generated through multiple physical and chemical methods, each differing in H₂ yield, stability, and suitability for experimental or clinical application. The three principal approaches include electrolysis of water, magnesium–water chemical reaction, and direct gas infusion under pressure.^(7,9,42)

Electrolysis-based generation

Electrolysis is the most widely adopted technique for producing HRW in laboratory and clinical research due to its reproducibility, safety, and controllable H₂ concentration. In this process, purified water is split into H₂ and oxygen using inert electrodes, typically platinum-coated titanium or palladium, under low-voltage current (2–5 ‍V).⁽⁸⁾ Under atmospheric pressure, electrolytic generation yields dissolved H₂ concentrations in the range of 0.6–1.6 ‍ppm, corresponding to approximately 0.4–0.8 ‍mM H₂,⁽⁴³⁾ which closely reflects the solubility limit of hydrogen in water at room temperature.

Dissolved H₂ concentrations exceeding atmospheric solubility are not achievable through electrolysis alone and require additional constraints such as pressurization, reduced temperature, sealed or pressure-resistant containers, and immediate measurement or consumption to minimize degassing. Under such controlled conditions, higher concentrations (typically up to ~3 ‍ppm) have been reported, consistent with pressurization-based solubility limits.⁽⁴⁴⁾ The electrolytic method nevertheless offers the advantage of real time generation, which helps reduce H₂ loss due to rapid diffusion when water is exposed to air or agitation.

Magnesium–water reaction

Another practical approach involves the reaction between metallic magnesium (Mg) and water, typically implemented using magnesium sticks or tablets composed of high-purity Mg granules. The reaction (Mg + 2H₂O → Mg (OH)₂ + H₂) liberates molecular H₂ while producing magnesium hydroxide as a benign by-product.⁽⁴⁵⁾ This method is inexpensive and portable, making it ideal for field or outpatient applications. However, the concentration of generated H₂ varies with water temperature, pH, and Mg particle size. Excessive magnesium hydroxide accumulation can increase alkalinity, necessitating pH control for safety and consistency.⁽¹⁴⁾

Direct gas infusion and pressurization

Direct H₂ gas infusion under pressurized conditions has been used primarily to define the physicochemical limits of H₂ solubility in aqueous solutions. At pressures of approximately 0.4 ‍MPa, dissolved H₂ concentrations of up to ~1.6 ‍mM (≈3.2 ‍ppm) has been reported, consistent with Henry’s law-based solubility constraints.⁽¹²⁾ Achieving such concentrations requires pressure-resistant containers, minimal headspace, low-permeability materials, and immediate measurement or use, as H₂ rapidly dissipates upon depressurization.

This approach is referenced here solely for contextual comparison, as it illustrates the conditions required to exceed atmospheric solubility limits and highlights why such concentrations are not representative of routine oral HRW preparation or consumption. H₂ saturated saline produced via pressurized infusion has been widely used in experimental models, including cardiac ischemia reperfusion injury and neuroprotection studies,^(9,11) but these studies are not considered evidence of HRW efficacy within the scope of this review.

Physicochemical and biological characteristics

Molecular H₂ is the smallest diatomic molecule (molecular weight 2.016 Da) and exhibits high diffusibility across biological barriers. Due to its nonpolar nature and minimal molecular size, H₂ readily penetrates cell membranes, mitochondria, and even the blood–brain barrier (BBB) within minutes of administration.⁽⁴⁶⁾ Pharmacokinetic studies have shown that following oral ingestion of HRW, plasma H₂ levels peak within 5–10 ‍min, distributing rapidly to multiple organs including the liver, kidney, brain, and skeletal muscle.⁽⁴⁷⁾ Excess H₂ is exhaled through the lungs without undergoing metabolic conversion, ensuring physiological safety and minimal interference with cellular signaling.⁽⁴⁸⁾ Additionally, H₂’s nonreactivity toward essential signaling ROS (e.g., O₂^•− and H₂O₂) distinguishes it from conventional antioxidants, which may cause reductive stress or disrupt redox homeostasis.⁽⁴⁹⁾ This selective reactivity allows HRW to mitigate oxidative injury while preserving physiological ROS-mediated functions such as immune defense and mitochondrial signaling.

Pharmacokinetic considerations and temporal biological effects of HRW

Molecular H₂ exhibits rapid pharmacokinetics characterized by fast gastrointestinal absorption, wide tissue distribution, and swift elimination primarily via pulmonary exhalation.^(9,44,46,47) Following oral ingestion of HRW, peak H₂ concentrations in blood and tissues typically occur within minutes and decline rapidly thereafter, reflecting a short biological half-life.^(9,41,44,47) This transient exposure profile raises important questions regarding how HRW can elicit sustained biological and clinical effects.^(8,10,41,50)

Accumulating experimental evidence suggests that the biological actions of H₂ are not solely dependent on continuous radical scavenging but rather on its ability to act as a brief redox-modulating and signaling stimulus.^(8,9,41) Short-lived H₂ exposure has been shown to influence redox-sensitive molecular pathways, including activation of Nrf2-dependent antioxidant responses, suppression of NF-κB mediated inflammation, modulation of mitochondrial function, and regulation of gene expression.^(10,15,50) Once initiated, these downstream responses may persist well beyond the presence of H₂ itself, thereby providing a mechanistic basis for longer-lasting physiological effects following intermittent HRW consumption.^(9,10,41,50)

In this context, oral HRW administration may be best understood as delivering repeated transient H₂ pulses, which cumulatively modulate cellular redox homeostasis, mitochondrial resilience, and inflammatory tone over time.^(9,41,50) Such a trigger-based mechanism is consistent with observations that regular HRW intake is associated with improvements in metabolic, vascular, and functional outcomes despite the short systemic residence time of molecular H₂.^(13,42) Nonetheless, the relationship between H₂ pharmacokinetics, tissue-specific exposure, and clinical efficacy remains incompletely defined, highlighting the need for future studies integrating H₂ concentration–time profiling with mechanistic and clinical endpoints.^(41,44,47)

Relevance of HRW production methods to biological and clinical outcomes

Differences in HRW production methods represent an important, yet often underappreciated, source of variability in reported biological and clinical outcomes.^(7,9,41) Electrolysis-based generation, magnesium water reactions, and pressurized gas infusion differ substantially in their ability to achieve and maintain dissolved H₂ concentrations, as well as in H₂ stability during storage and consumption.^(7,42,44,45) These factors directly influence the magnitude, duration, and reproducibility of systemic H₂ exposure following oral intake.^(9,41,47)

For example, HRW produced under atmospheric-pressure electrolysis typically yields transient H₂ concentrations near the solubility limit, whereas pressurized or sealed systems can achieve higher initial concentrations but require strict handling to prevent rapid degassing.^(9,43,44) Variability in container material, headspace volume, temperature, and time between preparation and consumption further contributes to inconsistent H₂ delivery across studies.^(41,44,47) As a result, differences in HRW production and handling may partially account for heterogeneity in reported biological effects and clinical efficacy.^(9,41,47)

Recognizing HRW preparation as a determinant of effective H₂ exposure, rather than a purely technical detail, is therefore essential for interpreting existing evidence and for designing future studies.^(9,41,47) Standardized reporting of production methods, dissolved H₂ concentration, and stability parameters will be critical to improving reproducibility and strengthening the translational relevance of HRW research.^(41,44,47)

Experimental Evidence

In vitro studies

In vitro investigations have provided robust mechanistic insight into the cytoprotective effects of HRW and H₂. Numerous studies have demonstrated that exposure to H₂-enriched media markedly attenuates oxidative damage and apoptosis in neuronal, endothelial, and hepatocytic cell models. In neuronal systems, studies using SH-SY5Y neuroblastoma and PC12 cultured cells demonstrate that H₂-enriched media activates the Nrf2/HO-1 antioxidant cascade, upregulates endogenous antioxidant enzymes (SOD, CAT, and GPx), and suppresses Fyn-mediated oxidative signaling, restoring intracellular redox balance and reducing apoptotic death. Similar cytoprotective effects have been observed in primary cortical and hippocampal neurons exposed to excitotoxic or ischemic conditions.⁽⁴⁹⁾

In immune and inflammatory cell systems, H₂ exposure attenuated lipopolysaccharide (LPS) and interferon-γ (IFN-γ)-induced overproduction of nitric oxide and TNF-α through suppression of the NF-κB, JNK, and p38 MAPK pathways, highlighting its potent anti-inflammatory and immunomodulatory potential.^(15,50,51)

Likewise, in human umbilical vein endothelial cells (HUVECs), treatment with H₂ rich medium significantly reduced H₂O₂-induced mitochondrial depolarization, caspase-3 activation, and DNA fragmentation, indicating that H₂ exerts direct mitochondrial protection against oxidative insult.⁽⁵¹⁾ These in vitro mechanistic trends collectively reinforce that HRW influences not only radical scavenging but also transcriptional regulation, mitochondrial homeostasis, and inflammatory signaling control.^(15,49–51)

In vitro mechanistic studies and relevance to HRW

In vitro studies provide important mechanistic insight into the antioxidant, anti-inflammatory, and cytoprotective actions of molecular H₂ by allowing controlled interrogation of redox-sensitive signaling pathways and mitochondrial function.^(15,49,51) In the context of HRW, however, their value lies primarily in mechanistic elucidation rather than direct translational equivalence.^(9,41,47)

Most in vitro experiments involve prolonged exposure of cells to H₂ containing gas or H₂ enriched media, which differs substantially from HRW ingestion in vivo, where dissolved H₂ produces brief systemic peaks and is rapidly eliminated via exhalation.^(9,44,47) As such, in vitro exposure paradigms more closely resemble continuous H₂ delivery (e.g., inhalation or hydrogen-rich saline) than oral HRW intake.^(11,41)

Despite this limitation, in vitro findings consistently identify H₂ responsive pathways that may be transiently engaged following HRW consumption.^(9,41,50) These include activation of the Nrf2/ARE antioxidant defense system, suppression of NF-κB and MAPK inflammatory signaling, preservation of mitochondrial membrane potential, and attenuation of apoptosis across neuronal, endothelial, and immune cell models.^{(10,15,49,51,52)} These effects highlight mitochondria and redox-sensitive transcriptional programs as key intracellular targets of H₂.^(10,41,50)

Accordingly, in vitro studies should be interpreted as defining biological plausibility and pathway sensitivity to H₂, rather than as direct surrogates of HRW pharmacokinetics or efficacy.^(9,41,50) When integrated with in vivo and clinical evidence, they contribute to a coherent mechanistic framework supporting the potential therapeutic actions of HRW.^(10,41)

In vivo animal studies

Building on in vitro mechanistic insights, in vivo animal studies evaluate how transient systemic exposure to molecular H₂ following HRW intake translates into integrated physiological outcomes. Animal models corroborate the cell-based findings and highlight the multi-systemic efficacy of HRW and H₂ across metabolic, cardiovascular, and neurological systems. In chronically stress mice, HRW provides hepatoprotection by improving mitochondrial membrane potential and inhibits endoplasmic-reticulum-stress–induced apoptosis.⁽⁵³⁾ In T2DM rodent models, chronic HRW supplementation significantly lowered fasting blood glucose, improved glucose tolerance, and decreased lipid peroxidation markers such as MDA, while enhancing antioxidant enzyme activities, SOD, CAT, and GPx, indicating restoration of redox balance and amelioration of oxidative stress–induced β-cell dysfunction.⁽⁵⁴⁾ HRW treatment also modulated insulin signaling through upregulation of phosphorylated Akt and suppression of inflammatory mediators TNF-α and IL-6, contributing to improved glycemic control and insulin sensitivity.⁽⁵⁵⁾

In cardiac ischemia–reperfusion injury models, administration of HRW markedly reduced infarct size, preserved myocardial ATP levels, and improved left ventricular ejection fraction.⁽⁵⁶⁾ These cardioprotective effects were mediated by the suppression of mPTP opening, inhibition of NF-κB and p38 MAPK activation, and attenuation of apoptosis via modulation of Bcl-2/Bax ratio and caspase-3 activity. Furthermore, HRW administration mitigated oxidative DNA damage and maintained mitochondrial integrity, thereby preventing cardiac remodeling in post-infarction models.⁽⁵⁶⁾

In the central nervous system (CNS), numerous studies have confirmed H₂’s neuroprotective effects across models of neurodegeneration, ischemia, and neuroinflammation. HRW significantly attenuated dopaminergic neuronal loss and microglial activation in MPTP-induced PD mice, improving motor performance and reducing nigrostriatal oxidative burden.⁽⁵⁷⁾

Similarly, in experimental models of neurodegeneration and metabolic-associated cognitive impairment, oral administration of HRW has been shown to preserve neuronal integrity, reduce lipid peroxidation, and attenuate oxidative damage. Behavioural analyses, including the Morris Water Maze (MWM) and novel object recognition tasks (NORT), demonstrate that HRW improves spatial learning and memory, consistent with reduced neuronal apoptosis and neuroinflammation. Mechanistically, these neuroprotective effects are largely attributed to activation of the Nrf2/ARE antioxidant pathway, suppression of inflammatory signaling, and modulation of apoptotic regulators, including downregulation of Bax and caspase-3 alongside upregulation of Bcl-2.^(54,55,57) Collectively, these findings indicate that orally administered HRW exerts system-wide protective effects by reprogramming oxidative stress responses, inflammatory pathways, and apoptotic signaling across multiple organ systems. More recent in vivo studies further support these findings, reporting improvements in oxidative stress markers, lipid profiles, glycaemic control, and inflammatory indices following sustained oral HRW intake across metabolically at-risk populations.^(58–60) Additional HRW studies have reported favourable effects on endothelial function, vascular stiffness, and cardiometabolic risk markers following oral administration, reinforcing the vascular relevance of redox modulation by HRW.^(61–63) Table 1 provides an integrated summary of in vivo, and clinical evidence demonstrating the cytoprotective effects of HRW, detailing the experimental models, disease conditions, key outcomes, and underlying mechanisms reported across studies.

Table 1.

Summary of in vivo and human studies demonstrating cytoprotective effects of hydrogen-rich water

Study type	Model/Participants	Disease/Condition	Main findings	Proposed mechanisms	References
In vivo (animal)	Mice (CDAA/CSAA diet); primary hepatocytes; RAW264 macrophages	Non-alcoholic steatohepatitis (NASH) & liver fibrosis	HRW reduced ALT/AST, inflammation, steatosis, and fibrosis, improved lipid metabolism.	↑ HO-1, ↑ IL-10, ↑ Sirt1; activation of HO-1/AMPK/PPAR-α/PPAR-γ axis; inhibition of LPS-induced cytokines; ↓ oxidative stress.	Li et al., 2021(43)
	Chronic restraint stress rats	Cognitive dysfunction	HRW improved spatial memory; ↓ hippocampal apoptosis	Restoration of hippocampal Nrf2 and BDNF signaling	Nagata et al., 2009(46)
	Sprague–Dawley rats	Traumatic brain injury (TBI)	HRW improved survival, reduced neurological deficits, ↓ MDA, ↑ GPx/CAT.	Activation of Nrf2, nuclear translocation; ↑ HO-1 and ↑ NQO1 expression; suppression of oxidative stress.	Yuan et al., 2018(48)
	Kunming mice (CUMS model)	Chronic stress-induced liver injury	Long-term HRW improved liver histology, normalized ALT/AST, ↑ antioxidants, ↓ lipid peroxidation & ferroptosis, improved mitochondria.	Activation of Nrf2/HO-1, ↑ GPX4/SLC7A11, ↑ PGC-1α/Nrf1/Tfam; improved mitochondrial biogenesis and quality control; ↓ Bax/caspase-3, ↑ Bcl-2.	He et al., 2025(53)
	db/db mice	Obesity and diabetes	HRW administration ↓ Blood glucose, ↓ hepatic lipids, ↑ FGF21	Activation of FGF21–AMPK metabolic pathway	Kamimura et al., 2011(54)
	STZ-induced diabetic rats	T2DM	HRW administration ↓ FBG, ↓ MDA, ↑ SOD, ↑ CAT; improved insulin sensitivity	Reduction of oxidative stress; modulation of Nrf2 and NF-κB	Zheng et al., 2021(55)
	Rat model of myocardial ischemia–reperfusion	Cardiac ischemia	HRW administration ↓ Infarct size, ↓ MDA, improved cardiac output	Antioxidant protection; suppression of mitochondrial permeability	Hayashida et al., 2014(56)
	MPTP-induced mice	Parkinson’s disease	HRW administration ↓ Dopaminergic neuron loss, ↓ microglial activation	Downregulation of TNF-α, IL-1β, and NF-κB	Fu et al., 2009(57)
	LDL receptor-deficient mice (Ldlr−/−)	Atherosclerosis/vascular aging (aortic endothelial senescence)	HRW reduced endothelial senescence markers (p16INK4a, p21), ↓ macrophage infiltration (MOMA-2) in aortic intima, ↓ TNF-α and CD36 expression (foam cell formation marker)	H2 reduces ROS → ↓ p16/p21 expression, ↓ TNF-α and macrophage activation, Nrf2 activation, Inhibition of CD36, modulation of TLR4 signaling	Iketani et al., 2018(58)
	Adult male Sprague–Dawley rats	Exercise-induced fatigue/reduced endurance performance	HRW significantly increased endurance time (~55% improvement), ↑ SOD, ↑ mtDNA, ↑ PGC-1α, PPARγ, TFAM expression, ↓ DAO, D-lactate, ↑ glycolysis genes (Pgk, Pkm), ↓ pyruvate, acetyl-CoA), ↓ CK	Mitochondrial biogenesis activation, antioxidant effect, gut–muscle axis modulation, metabolic reprogramming, and improved mitochondrial function	Luo et al., 2022(59)
	rd6 mice (genetic model of retinal degeneration; MFRP mutation) intervention: hydrogen-rich water (1.2–1.6 ‍ppm)	Retinitis pigmentosa (RP)/retinal degeneration	Increased outer retinal thickness (OCT), ↑ outer nuclear layer thickness, ↑ photoreceptor segments, ↑ ERG b-wave; preserved rod response, ↑ opsin red/green-positive cells, regulation of phototransduction-related genes, No significant change in global inflammatory markers (GFAP, Iba-1, MOMA-2)	Reduced oxidative damage to photoreceptors, inhibition of photoreceptor apoptosis (e.g., ↑ Bcl-2 expression), upregulation of phototransduction pathways (rhodopsin, transducin, PDE, ion channels) and reduced ROS-induced mitochondrial dysfunction.	Igarashi et al., 2022(60)
	Mouse model of sepsis	Endotoxin shock in mice	HRW reduced the increase in both apoptosis and oxidative stress, enhanced expression of heme oxyganase-1 and reduced endothelin-1 expression.	HRW preventing acute injury of the liver with attenuation of an increase in oxidative stress.	Iketani et al., 2017(61)
	DAL101 transgenic mice	Oxidative stress-induced neurodegeneration	In mice: improved memory, reduced oxidative stress, extended lifespan	↓ MDA, ↓ 8-OHdG; neuroprotection via reduced oxidative injury.	Nishimaki et al., 2018(62)
	Type 2 diabetic mice (db/db)	Diabetic retinopathy/retinal microvascular dysfunction	HRW significantly improved retinal blood flow (RBF), Reduced blood glucose (modest effect), ↓ GFAP expression, ↓ 3-nitrotyrosine, restored neurovascular coupling toward normal levels	Antioxidant effect: ↓ nitrotyrosine, Suppression of Müller cell gliosis: ↓ GFAP, Improvement of neurovascular coupling (retina).	Sugiyama et al., 2024(63)
Clinical (human)	20 subjects with metabolic syndrome	Metabolic disorder	↑ Serum antioxidant capacity; ↓ LDL oxidation	Enhanced systemic redox balance	Nakao et al., 2010(13)
	30 patients with T2DM	Diabetes mellitus	↓ FPG, ↓ triglycerides, ↑ HDL-C	Improved lipid metabolism, insulin sensitivity	Kajiyama et al., 2008(42)
	60 adults (men & women) with metabolic syndrome	Metabolic syndrome	↓ Total cholesterol, ↓ triglycerides, ↓ fasting glucose, ↓ HbA1c, ↓ BMI/WHR; ↑ vitamins C & E; ↓ TNF-α, IL-6, CRP	Antioxidant effects (↓ MDA, ↓ diene conjugates), improved redox balance, anti-inflammatory signaling modulation, improved glucose–lipid metabolism	LeBaron et al., 2020(64)
	10 middle-aged overweight women	Obesity/overweight	↓ Body fat%, ↓ arm fat index; ↓ insulin; ↓ triglycerides	Improved insulin sensitivity, modulation of lipid metabolism, mitochondrial-targeted antioxidative signaling, possible ghrelin upregulation	Korovljev et al., 2018(66)
	30 adults with NAFLD (Randomized, placebo-controlled trial)	Non-alcoholic fatty liver disease	HRW showed trends of ↓ weight, ↓ BMI (significant), ↓ LDH; mild ↑ lipid profile; ↑ MDA/8-OHdG (possible hormesis). No adverse effects.	Mild oxidative hormesis; ↓ NF-κB, ↓ MMP-9; modulation of inflammatory markers; metabolic improvement signals.	Kura et al., 2022(67)
	22 athletes	Exercise-induced fatigue	↓ Blood lactate, ↓ oxidative markers	Improved mitochondrial respiration	Mikami et al., 2019(68)
	Human MCI patients (n = 73)	Mild cognitive impairment	In humans: APOE4 carriers showed improved ADAS-cog scores with H2 water.	Enhanced memory circuits; selective benefit in APOE4 genotype.	Nishimaki et al., 2018(62)
	25 adult women diagnosed with panic disorder (PD)	Panic disorder (with comorbid anxiety, depression, and chronic inflammatory conditions)	Reduction in pro-inflammatory cytokines (IL-1β, IL-6, IL-8, IL-12, TNF-α, IFN-γ).	Antioxidant and anti-inflammatory effects (↓ ROS, ↓ pro-inflammatory cytokines), Modulation of microbiome–gut–brain axis (MGBA), normalization of HPA axis (↓ cortisol), ↓ cytokine production via stress reduction	Fernández-Serrano et al., 2022(69)

↑, increase; ↓, decrease; FBG, fasting blood glucose; MDA, malondialdehyde; SOD, superoxide dismutase; CAT, catalase; BDNF, brain-derived neurotrophic factor; FGF21, fibroblast growth factor 21; QoL, quality of life; Nrf2, nuclear factor erythroid 2–related factor 2; SOD, superoxide dismutase; CAT, catalase; GPx, glutathione peroxidase; NF-κB, nuclear factor kappa-light-chain-enhancer of activated B cells; JNK, c-Jun N-terminal kinase; TNF-α, tumour necrosis factor-alpha; ROS, reactive oxygen species; MDA, malondialdehyde; 4-HNE, 4-hydroxynonenal; ARE, antioxidant response element; IL-6, interleukin-6; IL-1β, interleukin-1 beta; NO, nitric oxide; NOX, NADPH oxidase; MAPK, mitogen-activated protein kinase; Bax, B-cell lymphoma-2 associated X protein; Bcl-2, B-cell lymphoma-2; mPTP, mitochondrial permeability transition pore; Casp-3, caspase-3; MCI, mild cognitive impairment; LDL, low-density lipoprotein; HDL, high-density lipoprotein; oxLDL, oxidized low-density lipoprotein.

Integrated evidence synthesis from preclinical studies

Evidence from in vivo investigations demonstrates a convergent mechanistic profile through which HRW exerts biological effects across oxidative stress-related disease pathways. Studies employing oral HRW administration have shown that this intervention selectively modulates oxidative stress without disrupting physiological redox signaling, thereby avoiding the redox-interference limitations associated with conventional antioxidants.^(9,41) Subsequent mechanistic investigations indicate that HRW activates endogenous antioxidant defence pathways through upregulation of Nrf2/ARE signaling, induction of HO-1 expression, and enhanced activities of SOD, CAT, and GPx, leading to restoration of intracellular redox balance under oxidative load.^{(43,48,54,55)} Parallel suppression of inflammatory cascades, including NF-κB and MAPK signaling, has been observed in HRW-treated models, accompanied by reductions in TNF-α, IL-1β, and other pro-inflammatory mediators.^(55,60)

Mitochondrial protection represents a recurring mechanistic hallmark across experimental models of HRW administration. Oral HRW has been shown to preserve mitochondrial membrane potential, maintain ATP production, and support mitochondrial biogenesis, thereby sustaining bioenergetic stability during oxidative stress.^(53,54,59) These mitochondrial effects are closely linked with apoptosis-regulating mechanisms, including reduced expression of pro-apoptotic proteins such as Bax and caspase-3 and increased expression of the anti-apoptotic protein Bcl-2, collectively promoting cellular survival.^(54,55,60) Functional outcomes derived from these mechanisms include improved glycaemic regulation and insulin sensitivity in diabetic models, reduced lipid peroxidation and improved lipid profiles in metabolic disorders,^(54,55) as well as vascular protection and enhanced endothelial function.⁽⁵⁸⁾ In addition, HRW has been associated with improved physical performance and mitochondrial efficiency in exercise models, reflecting enhanced systemic energy metabolism.⁽⁵⁹⁾ Collectively, these findings demonstrate that orally administered HRW exerts multi-system protective effects through coordinated regulation of mitochondrial function, oxidative stress, and apoptosis.

Although preclinical findings remain less consistent due to small sample sizes, short treatment durations, and heterogeneity in HRW concentration reporting, the mechanistic alignment across cellular and animal reinforces the biological plausibility of HRW as a therapeutic modality in oxidative stress-mediated disease processes. This cross-tier convergence highlights HRW as a candidate adjunctive intervention warranting larger, well-controlled translational trials with standardized H₂ dosing, validated biomarker frameworks, and longer follow-up intervals.

Clinical Studies

Human studies investigating HRW provide emerging support for its antioxidant, metabolic, vascular, performance-related, and neurocognitive benefits, although the strength of evidence varies across clinical domains. Most trials to date have been small, short in duration, and heterogeneous in HRW concentration, preparation methods, and outcome measures. Nevertheless, collectively they demonstrate favourable shifts in oxidative stress biomarkers and disease-relevant physiological indices following oral HRW consumption.

Metabolic and diabetic disorders

Human trials have confirmed HRW’s antioxidant and metabolic effects. In subjects with potential metabolic syndrome, daily ingestion of HRW for 8–10 weeks (≈0.9–2.0 ‍L/day) significantly increased serum antioxidant capacity, elevated SOD activity, reduced serum MDA levels, and improved composite oxidative stress scores, indicating a shift toward a more favourable redox balance. In the same clinical context, HRW lowered total and LDL cholesterol, decreased LDL oxidizability, and enhanced HDL functionality, suggesting that HRW improves both quantitative and qualitative aspects of lipid metabolism.^(13,64–67)

Kajiyama et al.⁽⁴²⁾ demonstrated that HRW supplementation in patients with T2DM or impaired glucose tolerance improved fasting plasma glucose, triglycerides, and HDL cholesterol, with parallel reductions in biomarkers of oxidative stress and inflammation, including oxidized LDL and pro-inflammatory cytokines such as TNF-α and IL-6. Subsequent trials and meta-analyses have broadly supported these findings, reporting that medium to high concentration HRW administered over 8–24 weeks can reduce total cholesterol, LDL cholesterol, and HbA1c while improving markers of systemic redox homeostasis and low-grade inflammation in populations with metabolic syndrome or related cardiometabolic risk.^(64–68) Collectively, these data support the concept that HRW ameliorates metabolic dysregulation primarily through modulation of mitochondrial redox status, attenuation of NF-κB–driven inflammatory signalling, and enhancement of insulin signalling efficiency, thereby improving both lipid and glucose handling in at-risk individuals.^(61,65,67)

Cardiovascular and exercise physiology

In individuals with impaired glucose tolerance, regular consumption of HRW was associated not only with improved lipid and glycaemic profiles but also with favourable vascular effects, including enhanced flow mediated dilation and reductions in arterial stiffness. These findings reflect improved endothelial NO bioavailability and attenuation of oxidative stress induced vascular dysfunction.^(44,64,67) Such vascular benefits align with observations that HRW reduces oxidized LDL levels, lowers inflammatory mediators, and improves surrogate markers of microvascular health in metabolically at-risk populations.^(13,64,65)

Beyond cardiometabolic settings, performance-related benefits have also been described. Athletes and physically active individuals consuming HRW demonstrated reduced post-exercise blood lactate accumulation, faster lactate clearance, and attenuation of perceived exertion and fatigue. These effects are consistent with improved mitochondrial efficiency, reduced exercise-induced ROS generation, and enhanced acid-base homeostasis during strenuous activity.⁽⁶⁸⁾ Controlled trials further report that HRW enhances endurance capacity and shortens recovery time following high-intensity exercise, supporting the hypothesis that HRW acts as a selective antioxidant that preserves mitochondrial function under metabolic stress.^(66,68)

Neurocognitive and other clinical contexts

Emerging clinical pilot studies suggest that oral HRW intake may confer benefits in neurocognitive and fatigue-related conditions. In individuals with mild cognitive impairment, daily consumption of HRW was associated with improvements in attention, processing efficiency, and mood related scores, accompanied by reductions in systemic oxidative stress markers. These findings imply that HRW may mitigate early neurodegenerative metabolic stress and support cognitive performance.⁽⁶³⁾

Complementary evidence has also been reported in oncology settings, where patients undergoing radiotherapy for liver tumours experienced reduced treatment related fatigue, lower lipid peroxidation, attenuated inflammatory burden, and improved quality-of-life indices following regular HRW intake.⁽¹⁴⁾ Together, these observations support the feasibility, safety, and potential supportive role of HRW as an adjunctive strategy in conditions characterised by elevated oxidative and inflammatory stress.

Emerging evidence also supports potential neurocognitive and functional benefits of oral HRW intake, including improved fatigue-related outcomes and reduction of oxidative stress in clinical populations, with growing recognition of the role of inflammatory and neuroimmune pathways in these effects.^{(21,23,27,69)}

Clinical evidence limitations

Across all categories, clinical research on HRW remains limited by recurring methodological constraints. Most trials employ small sample sizes, reducing statistical power and precision of effect estimates, and intervention durations are typically short, ranging from 4–12 weeks, which restricts evaluation of sustained therapeutic benefit.^{(13,65,66,68)} Considerable variability exists in HRW concentration, stability, and measurement techniques, with inconsistent reporting of preparation methods, dissolved levels (ppm), container type, and timing of consumption relative to assessment.^(65,67) Outcome measures are likewise heterogeneous, spanning oxidative stress biomarkers, metabolic indices, symptom scales, and functional performance, making cross-study comparison difficult.^(64–68) Collectively, these limitations impede data harmonization and weaken clinical interpretability. To advance the field, future studies require standardized HRW preparation protocols, validated H₂ quantification methods, unified core outcome sets, and adequately powered multicentre randomized controlled trials (RCTs) to clarify dose–response relationships, long-term safety, and the therapeutic role of HRW alongside established pharmacological and lifestyle interventions.^(64,65,67)

Safety, Dosage, and Practical Use

Molecular H₂ has a favourable safety profile and is recognized by the U.S. Food and Drug Administration (FDA) as Generally Recognized as Safe (GRAS) for food and beverage applications. Evidence supporting the safety of inhalational H₂ exposure derives from toxicological and clinical studies rather than regulatory GRAS.⁽⁷⁰⁾ Preclinical safety assessments further support this profile; rodents administered molecular H₂ at doses up to 10 ‍mg H₂/kg/day exhibited no mortality, organ toxicity, behavioural abnormalities, or histopathological alterations, indicating an exceptionally wide safety margin for systemic exposure.⁽⁷¹⁾ Consistent findings have been reported in human studies, where daily intake of 0.3–1.2 ‍L of HRW containing approximately 0.6–1.6 ‍ppm dissolved H₂ over periods of 4–12 weeks resulted in no clinically meaningful changes in liver enzymes, renal function markers, hematologic indices, or electrocardiographic parameters.⁽¹³⁾

A practical consideration for clinical and consumer use is H₂’s rapid diffusion and volatility, which leads to measurable decline in dissolved concentration when water is left unsealed or exposed to agitation.^(44,47) Therefore, fresh preparation, sealed pressurized packaging, or certified storage vessels are required to maintain therapeutic concentrations.⁽⁴⁴⁾ Standardized HRW production methods such as electrolytic generators, pressurized filling systems, and magnesium-based effervescent tablet reactions have been shown to yield reproducible H₂ concentrations when validated with dissolved H₂ meters and controlled preparation protocols.^(42,44,45) Both continuous daily intake and intermittent consumption schedules have demonstrated physiological effects across clinical studies, suggesting that H₂’s biological activity is mediated via transient redox signalling modulation rather than cumulative tissue accumulation, with benefits generally sustained during regular intake and diminishing gradually after discontinuation.^(13,64,65)

Challenges and Limitations

Despite expanding clinical interest in HRW, several limitations continue to restrict wider therapeutic adoption. Considerable variability in HRW preparation, whether through electrolysis, magnesium-based generation, or direct gas infusion, results in inconsistent dissolved H₂ concentrations, differing dissolution kinetics, and uncertain bioavailability across studies.^(44,65,67) Additionally, many clinical investigations to date have enrolled relatively small sample sizes and involved short intervention periods, limiting statistical robustness, effect-size interpretation, and comprehensive long-term safety evaluation.^{(13,65,66,68)} Interpretation is further complicated by heterogeneous outcome measures, ranging from biochemical oxidative stress biomarkers to exercise performance indices and subjective quality of life scales, reducing comparability across trials.^(64–68) A further methodological concern is the lack of standardized reporting practices, with key parameters such as H₂ concentration, measurement method, temperature control, container type, and exposure time often insufficiently documented, thereby hindering reproducibility and meta-analytic synthesis.^(65,67) To overcome these limitations, the field would benefit from internationally accepted consensus guidelines defining HRW preparation standards, validated concentration measurement protocols, and uniform reporting criteria to ensure methodological transparency and facilitate rigorous clinical evaluation.^(64,65,67)

Despite its practical advantages, oral administration of HRW has inherent limitations related to the physicochemical properties of molecular H₂. Following ingestion, dissolved H₂ is rapidly absorbed and eliminated primarily via pulmonary exhalation, resulting in transient systemic exposure with a short biological half-life.^(44,47) Consequently, tissue H₂ concentrations achieved through oral HRW intake are brief and may vary depending on gastrointestinal absorption, circulation dynamics, and organ perfusion.⁽⁴⁷⁾

These characteristics suggest that the suitability of oral HRW may be disease- and organ-dependent. While intermittent redox modulation via oral HRW may be sufficient to influence systemic metabolic, inflammatory, or mitochondrial signaling pathways, conditions requiring sustained or localized H₂ delivery such as acute ischemic injury or focal neurological damage may benefit more from alternative administration routes.^(11,44,47) Accordingly, oral HRW should be viewed as a convenient and low-risk modality with specific strengths and limitations, rather than a universally optimal H₂ delivery strategy.^(44,64,65)

Future Directions

Future research directions increasingly point toward the need for well-powered, multicentre, double-blind RCTs assessing HRW in chronic diseases in which oxidative stress and mitochondrial dysfunction play central pathogenic roles, most notably T2DM, cardiovascular disorders, and neurodegenerative conditions.^(64,65,67) Such trials should incorporate standardized HRW preparation methods, validate H₂ concentration measurements, and harmonize clinical endpoints to permit comparative interpretation across studies.^(65,67) Parallel mechanistic investigations are warranted to delineate how molecular H₂ influences key cellular redox-sensing pathways including Keap1–Nrf2 antioxidant signalling, sirtuin-mediated metabolic regulation, and AMPK-dependent mitochondrial homeostasis, as well as its potential role in modulating neuroimmune activation, glial signalling, and systemic inflammatory tone.^(10,41,50)

There is growing rationale to explore combination strategies, as H₂ may provide complementary effects when used alongside established pharmacotherapies such as SGLT2 inhibitors, statins, antihypertensives, or anti-inflammatory agents, potentially enhancing metabolic resilience, endothelial protection, and oxidative stress reduction.^{(64,65,67,72)} In parallel, technological innovation is rapidly advancing H₂ delivery systems, including nanobubble formulations with prolonged dissolution stability, encapsulated effervescent tablets, pressurized HRW systems, and H₂-releasing biomaterials, all of which aim to improve concentration retention, dosing precision, and controlled biological exposure.^(44,45,73)

Conclusion

Growing preclinical and clinical evidence shows that HRW provides antioxidant, anti-inflammatory, anti-apoptotic, and cytoprotective benefits across multiple organ systems. By selectively reducing hydroxyl radicals and influencing redox-sensitive pathways such as Nrf2 and NF-κB, HRW helps restore oxidative balance and limit tissue injury. With its strong safety profile, low cost, and ease of use, HRW is a promising adjunct for conditions driven by oxidative stress, though its broader clinical adoption will depend on larger randomized trials and clearer mechanistic validation.

Author Contributions

IL conceived the study, designed the conceptual framework, and contributed to data interpretation and manuscript writing. WHM read and approved the final version of the manuscript.

Funding

This work was supported by an Industry Grant from ANYGEN HOLDING SDN. BHD (Company No. 221784-U; Grant No. 304/PPSK/6150290/A173). The funder had no role in the study design, data collection, data analysis, interpretation of results, manuscript preparation, or decision to publish.

Availability of Data and Material

The datasets generated and/or analyzed during the current study are available from the corresponding author on reasonable request.

Code Availability

Not applicable. No custom code or software was used in this study.

Conflict of Interest

No potential conflicts of interest were disclosed.