Effects of Hydrogen Water (HW) on Tissue Inflammation: A Systematic Review

Introduction

In the United States, over 50% of all deaths are linked to inflammation-related diseases (e.g., heart disease, cancer, Alzheimer's disease, chronic obstructive pulmonary disease (COPD), and diabetes).¹ Conditions like arthritis affect more than 54 million adults and lead to chronic pain.² One promising area of research is the potential anti-inflammatory properties of hydrogen-rich water. H₂ is a small, non-reactive molecule that has been shown to possess antioxidant and anti- inflammatory properties. Preclinical studies suggest that hydrogen-rich water may reduce oxidative stress and modulate inflammatory pathways, making it a potential therapeutic option for managing chronic inflammation.³⁻⁸ The goal of this study is to investigate the effect of HW on tissue inflammation, with the main focus on mice models.

Literature review

Inflammation is a fundamental biological process essential for tissue repair and immune defense.⁹ It is a complex response involving the activation of immune cells, the release of cytokines, and changes in blood flow, among other complex biological mechanisms, such as the recognition of harmful stimuli by pathogen recognition receptors (PRRs), the recruitment of immune cells such as neutrophils and macrophages, the production of inflammatory mediators (e.g., histamines, prostaglandins), and finally the resolution of inflammation through anti- inflammatory signals and tissue repair processes.¹⁰ While inflammation is protective and crucial for the healing processes,¹¹^,¹² its chronic manifestation can lead to a host of pathological conditions.¹³ Chronic inflammation is increasingly recognized as a key contributor to the development and progression of various diseases, including cardiovascular disease,¹⁴ diabetes,¹⁵ cancer, and neurodegenerative disorders.¹⁶^,¹⁷ These conditions collectively account for a significant burden of morbidity and mortality worldwide.¹⁸⁻²¹

The impact of chronic inflammation highlights the urgent need for effective management strategies. Current therapeutic approaches, including nonsteroidal anti-inflammatory drugs (NSAIDs)²² and corticosteroids,²³^,²⁴ offer symptom relief but are often associated with significant side effects such as ulcers, kidney damage, liver damage, weight gain, and mood swings, and do not address the underlying causes of inflammation.²⁵⁻²⁷

Corticosteroids mimic the effects of cortisol, a hormone produced by the adrenal glands. They work by inhibiting the enzyme phospholipase A2, which blocks the release of arachidonic acid, a precursor to pro-inflammatory molecules.²⁸ Corticosteroids also suppress the immune response by reducing pro-inflammatory cytokines and inhibiting transcription factors like NF-κB.²⁹ As a result, they reduce inflammation and immune system activity, making them effective for treating autoimmune conditions, allergies, and inflammatory diseases.³⁰^,³¹ NSAIDs reduce pain, inflammation, and fever by inhibiting cyclooxygenase (COX) enzymes, particularly COX-1 and COX-2.³² These enzymes are responsible for converting arachidonic acid into prostaglandins, which are chemicals that mediate inflammation and pain.³³ By blocking this process, NSAIDs reduce prostaglandin production, leading to less inflammation, pain relief, and lower fever, commonly used for conditions like arthritis, headaches, and muscle pain.³⁴^,³⁵

This has sparked interest in alternative and complementary therapies that might offer more sustainable and less harmful ways to manage inflammation, such as herbal supplements (e.g., turmeric, ginger), dietary changes (like the Carnivore diet), physical therapy, and mindfulness practices (such as yoga and meditation).³⁶^,³⁷ Hydrogen water is gaining attention for various health benefits beyond inflammation management. It can reduce oxidative stress, enhance exercise performance and recovery, and support metabolic health.

Oxidative stress occurs when there is an imbalance between harmful reactive oxygen species (ROS) and the body's antioxidant defenses. While ROS are naturally produced during normal metabolism and serve important functions, excessive levels can damage cellular components like proteins, lipids, and DNA. This damage triggers inflammation and contributes to various diseases, similar to how metal rusts when exposed to oxygen. Environmental factors and lifestyle choices can increase oxidative stress, while antioxidants help maintain the balance needed for cellular health.³⁸⁻⁴⁰ The mechanism of hydrogen water involves the antioxidant properties of H₂, which reacts with harmful reactive species such as hydroxyl free radicals (•OH), neutralizing them to produce water.⁴¹ This action reduces oxidative stress, a major contributor to inflammation and tissue damage.⁴² In the body, hydrogen functions without generating harmful byproducts and diffuses easily, making it particularly effective in neutralizing radicals during inflammatory conditions.⁴³

Review Objective

To the best of the authors’ knowledge, this will be the first systematic review investigating the effect of hydrogen water specifically on tissue inflammation. A couple of other reviews were found related to other health aspects.³ Dhillon et al.¹ systematically reviewed hydrogen water’s general health possible benefits, including its antioxidant and metabolic effects. While their review highlighted various health outcomes, it did not specifically address tissue inflammation, hinting at the need for more targeted research.³ Hu et al.⁴ conducted a review on the antioxidant and anti-inflammatory effects of electrolyzed hydrogen water, focusing on its effects in living organisms. The review was investigating hydrogen water’s impact on oxidative stress and inflammation in broader terms. While they explored anti-inflammatory mechanisms, they did not focus on tissue-level inflammation.⁴

In contrast, this systematic review will be the first to specifically assess the effect of hydrogen water on tissue inflammation, filling an important gap in the current literature and advancing the understanding of its therapeutic potential. The aim of this systematic review is to critically evaluate the effects of hydrogen-rich water on tissue inflammation. By analyzing the existing results from primary studies, this review aims to guide future research and support the development of innovative therapeutic strategies for managing inflammation in humans.

Methods

 A systematic literature search was conducted in two databases: PubMed and Google Scholar. The search was carried out in June 2024 using the following keywords: "Hydrogen Water" and "Tissue Inflammation." A total of 74 studies were identified across both databases, 1 from PubMed and 73 from Google Scholar. Inclusion criteria were defined as studies containing both keywords, "hydrogen water" and "tissue inflammation," involving human or animal subjects, organs, or cells. Human research results were few but were also considered to evaluate potential impact on humans. Only randomized controlled trials (RCTs) were considered, to better investigate cause and effect, and also reduce bias. The eligible published studies identified ranged in publication year from 2007 to 2024. Exclusion criteria included studies not related to both hydrogen water and tissue inflammation, mainly to ensure that the articles relate to the research question, and studies with religious context, such as the use of Zamzam water. The religious exclusion criteria were to avoid biases such as a placebo effect associated with religion.

The initial search yielded 74 studies. After removing duplicates (n=2) and those that could not be retrieved (n=2), a total of 70 articles were screened for eligibility. During this screening process, 1 study was excluded due to religious context (Zamzam water), 60 studies were removed because they were not related to both hydrogen water and inflammation, and 1 study was excluded for only assessing anemia. 1 study was excluded for relating to PCOS and not inflammation specifically.

In total, 7 studies met the inclusion criteria and were included in the final analysis. To expand the comprehensiveness of this review, the reference sections were used to look for complementing articles. Through this process, 25 more articles were found that met the exclusion and inclusion criteria, but one article was removed due the lack of the focus on inflammation, resulting in a total of 31 articles in this review.

The Quality assessment was performed using a SYRCLE (Systematic Review Centre for Laboratory Animal Experimentation) criteria for the mice and cell studies,⁴⁴ and Cochrane for the Human Study.⁴⁵ Data were extracted from the included studies based on study design, sample size, type of tissue or organ assessed, and outcomes related to tissue inflammation. Key information from each study was synthesized to evaluate the impact of hydrogen water on tissue inflammation in different experimental models. To extract the data, statistics and explanations were compiled into Table 1. To analyze the data, the trends in the data were investigated descriptively and qualitatively.⁴⁶

The quality of the included studies was assessed to evaluate the risk of bias and methodological rigor using a structured point-based system. Of the 31 included articles, 25 were animal studies involving in vivo mouse or rat models. These were assessed using the SYRCLE (Systematic Review Centre for Laboratory Animal Experimentation) risk of bias tool, which covers 10 domains: sequence generation (SG), baseline characteristics (BC), allocation concealment (AC), random housing (RH), blinding of caregivers or investigators (BCg), random outcome assessment (ROA), blinding of outcome assessors (BOA), incomplete outcome data (IOD), selective outcome reporting (SOR), and unit of analysis errors (UoA). Funding or conflict of interest (FCOI) was also evaluated as an additional domain.

For human-related studies, only one randomized controlled trial (Sim et al., 2020) met the criteria for assessment using the Cochrane Collaboration’s Risk of Bias tool. This tool evaluates five domains: bias arising from the randomization process (R), deviations from intended interventions (DI), missing outcome data (MD), measurement of the outcome (MO), and selection of the reported result (SR). Two additional studies that used human or combined human and mouse cell models (Xiao et al., 2017; Xiao et al., 2021) were excluded from risk of bias scoring because standard quality assessment tools are not applicable to in vitro cell-based experiments. Each domain was scored as "+" (1 point for low risk), "?" (0 points for unclear risk), or "−" (0 points for high risk). The quality assessment was conducted by a single reviewer. Results are summarized in the Results section, with SYRCLE scores detailed in Table 3 and Cochrane scores in Table 4.

Figure 1. Prisma Chart* 

*PRISMA 2020 flow diagram is used with permission from the PRISMA 2020 statement. The diagram is available under the terms of the Creative Commons Attribution (CC BY 4.0) license. For more information, visit PRISMA 2020 flow diagram.⁴⁷

Results

 To better understand the result of the studies, they were categorized into different categories based on their type of tissue being examined. eight categories were created, including nervous system, circulatory system, respiratory system, gastrointestinal system, renal system, integumentary system, immune system and other systems.

Table 1. Summary of Studies

Table 2. Results Showing the Therapeutic Benefit of HW

Note: *All results indicated significant results (p<0.05), Adenosine Triphosphate (ATP), Hydrogen Rich Saline (HRS), Hydrogen Water (HW), Reactive Oxygen Species (ROS), Melanopsin-expressing Retinal Ganglion Cells (mRGCs), Pattern Electroretinogram (pERG), human keratinocyte cell line (HaCaT)

Nervous System

 Six studies assessed the effects of HW and hydrogen-rich saline (HRS) on neural inflammation and recovery in various mice species. Across all studies, HW/HRS consistently reduced oxidative stress, inflammation, and apoptosis, while improving neuronal survival and function. The most significant results are discussed below.

Cai et al.⁴⁸ found that HW improved spatial memory and neuronal survival in a hypoxia- ischemia model by reducing oxidative stress. Similarly, Liu et al.⁴⁹ demonstrated improved locomotor function and reduced inflammation following spinal cord injury.

In the most recent study, Wang et al.⁵², HRS was shown to accelerate recovery from retinal damage caused by blue light exposure. The treatment increased melanopsin expression in intrinsically photosensitive retinal ganglion cells (ipRGCs) and improved retinal function within two weeks. The study highlighted HRS’s role in reducing oxidative stress and promoting retinal recovery.

Overall, these studies agree that HW is effective in reducing oxidative stress and inflammation, leading to improved outcomes in various models of neural injury and disease.

Circulatory System

 Five studies explored the impact of HW and HRS on cardiovascular health, consistently showing protective effects against oxidative stress and inflammation in various mice species. The most significant results are discussed below.

Ohsawa et al.⁵⁴ demonstrated that HW prevented atherosclerotic lesions and oxidative stress in apolipoprotein E-knockout mice, highlighting its role in atherosclerosis prevention. Sun et al. found that HW reduced neointima formation and improved endothelial integrity in a vascular graft model, while Zhao et al.⁵⁷ showed that HRS improved flap survival and perfusion in ischemia-reperfusion injury. Wang et al.⁵⁸ reported that HRS reduced mortality and systemic inflammation in severe burns with delayed resuscitation.

Noda et al.⁵⁶ provided a more in-depth exploration of HW's effects on cardiac allografts. In a heterotopic heart transplantation model, HW significantly prolonged allograft survival, reduced intimal hyperplasia in aortic grafts, and improved mitochondrial function by enhancing citrate synthase activity. HW restored mitochondrial Complex I, II/III, and V activities in cardiac grafts, which were otherwise compromised. Furthermore, HW decreased inflammatory markers such as T cell infiltration and the expression of pro-inflammatory cytokines (e.g., IFN-γ, IL-2). These findings suggest HW may enhance graft longevity by mitigating oxidative damage and modulating immune responses.

Together, these studies underscore HW’s capacity to reduce oxidative stress, improve mitochondrial function, and decrease inflammation in cardiovascular contexts.

Respiratory System

 Two studies examined the effects of HW on respiratory health, showing protective effects against oxidative stress and inflammation in lung injury models various mice species. Sato et al.⁵⁹ focused on paraquat-induced pulmonary fibrosis, a condition driven by oxidative stress. In this study, 10-week-old C57BL/6 mice were exposed to paraquat, and HW was administered orally. After 3 weeks, the HW group showed significant improvements in lung function compared to the control group, with reduced elastance (E) (P = 0.010) and hysteresivity (P = 0.048). Histologically, the HW-treated mice exhibited less pulmonary inflammation and fibrosis, with lower levels of inflammatory cell infiltration and alveolar damage. These results suggest that HW mitigates oxidative damage and prevents the progression of pulmonary fibrosis.

Wu et al.⁶⁰ found that HW reduced oxidative stress and inflammation in a one-lung ventilation injury model. HW protected lung tissue during mechanical ventilation, reducing markers of lung injury and improving overall lung function.

Both studies demonstrate that HW has significant potential in reducing oxidative stress and preventing lung injury, particularly in chemically or mechanically induced respiratory conditions.

Gastrointestinal System

Seven studies investigated the effects of HW and HRS on gastrointestinal health, particularly in reducing inflammation and oxidative stress in models of colitis and liver injury. The most significant results are discussed below.

Kajiya et al.⁶¹ focused on a Dextran sodium sulfate (DSS)-induced colitis model in BALB/c mice, demonstrating that HRS reduced colitis symptoms and pro-inflammatory cytokine levels, particularly interleukin-1β (IL-1β) and tumor necrosis factor-alpha (TNF-α). HRS administration, either orally or via injection, effectively suppressed inflammation and oxidative damage in the colon, improving overall gastrointestinal health. Xu⁶² investigated hydrogen's effect on lipopolysaccharide (LPS)-induced acute liver dysfunction in Sprague-Dawley rats, finding that the administration of hydrogen saline via the caudal vein significantly reduced liver dysfunction markers and prolonged survival. Their results suggested that hydrogen mitigates oxidative stress and offers hepatoprotective benefits.

Liu et al.⁶⁶ examined the role of HRS in reducing postsurgical peritoneal adhesions (PPAs) using a cecum rubbing model in C57BL/6 mice. Intraperitoneal HRS administration significantly reduced PPA formation at days 3 and 7 post-surgery. It also decreased oxidative stress markers (MDA and MPO) and inflammatory cytokines (TNF-α, IL-6), which contributed to reducing adhesion severity. The ability of hydrogen to attenuate both oxidative stress and inflammation appeared key in preventing the fibrous bands characteristic of PPAs.

Hu D et al.⁶⁷ focused on electrolyzed hydrogen water (EHW) in a rat model of inflammatory bowel disease (IBD) induced by 2,4,6-trinitrobenzene sulfonic acid (TNBS). EHW alleviated abdominal pain and colonic inflammation by reducing oxidative stress and suppressing pro- inflammatory cytokines such as IL-1β, TNF-α, and IL-6. Hydrogen treatment also improved antioxidant enzyme activity, enhancing superoxide dismutase (SOD) levels in the colon and helping maintain redox balance in inflamed tissues.

Together, these studies suggest that hydrogen therapy consistently reduces inflammation, oxidative stress, and tissue damage in gastrointestinal models, highlighting its therapeutic potential for conditions such as colitis and liver injury.

Renal System

 Four studies investigated the protective effects of HW and HRS on renal function, highlighting their ability to reduce oxidative stress, inflammation, and kidney damage.

Cardinal et al.⁶⁸ found that HW improved kidney function and survival in a model of chronic allograft nephropathy, reducing proteinuria and inflammation. Wang et al.⁶⁹ demonstrated that HW decreased oxidative stress and inflammation, protecting against renal injury. Xu et al.⁷⁰ observed reduced renal apoptosis and injury, with increased antioxidant activity following HW administration.

Guo et al.⁷¹ focused on the impact of HRS on kidney damage in severely burned rats. HRS reduced the severity of renal tubular damage and apoptosis while alleviating oxidative stress. The study also showed improved kidney function, with lower levels of malondialdehyde (MDA) and increased antioxidant enzyme activity. Additionally, HRS reduced pro-inflammatory cytokines (TNF-α, IL-1β, IL-6) and increased the anti-inflammatory cytokine IL-10, which helped mitigate kidney damage and inflammation following severe burn injuries.

Together, these studies suggest that hydrogen-based therapies provide renal protection by reducing oxidative stress and inflammation, improving overall kidney function.

Integumentary System

Three studies evaluated the effects of HW on the skin, particularly in reducing allergic responses, oxidative stress, and promoting skin health.

Yoon et al.⁷² focused on the effects of HW on atopic dermatitis (AD) induced by 2,4- dinitrochlorobenzene (DNCB) in NC/Nga mice. The study showed that HW ameliorated AD-like symptoms by reducing skin inflammation and scratching behavior. HW also significantly decreased reactive oxygen species (ROS) and increased glutathione peroxidase (GPx) activity, indicating enhanced antioxidant defense. Additionally, the levels of pro-inflammatory cytokines, such as tumor necrosis factor-alpha (TNF-α), interleukin-5 (IL-5), and interleukin-6 (IL-6), were significantly reduced in HW-treated mice, along with a marked reduction in total serum immunoglobulin E (IgE), suggesting that HW modulated immune responses and redox balance to alleviate AD.

Xiao et al. (2021)⁷³ demonstrated that HW protected against oxidative stress and cell death, promoting wound healing by maintaining skin integrity. Similarly, Xiao et al. (2017)⁷⁴ showed that HW protected skin from oxidative damage, maintained collagen levels, and prevented skin degradation.

These studies suggest that HW has strong potential for improving skin health by reducing oxidative stress, inflammation, and enhancing healing in various skin conditions.

Immune System

In a single study, Itoh et al.⁷⁵ examined the effects of HW on allergic reactions and inflammation, focusing on type I hypersensitivity responses. The study demonstrated that oral intake of HW significantly attenuated allergic reactions in a mouse model of passive cutaneous anaphylaxis. This reduction in allergic response was linked to a decrease in histamine levels and a suppression of mast cell degranulation. HW also inhibited the phosphorylation of key proteins involved in the FcεRI-mediated signal transduction pathway, such as Lyn and Syk, thereby preventing the release of inflammatory mediators. Furthermore, HW reduced the activity of NADPH oxidase, leading to lower production of reactive oxygen species (ROS), including hydrogen peroxide (H₂O₂), which contributed to the attenuation of allergic inflammation. These findings suggest that HW can effectively reduce oxidative stress and modulate immune responses in allergic conditions.

Other Systems

All three were found to have a significant impact on inflammation, two studies were mainly focused on oxidative stress and inflammation, while the third article investigated these factors in relation to cell death. The third study⁷⁸ was focusing on individual human (HaCaT) and mice (OP9) cells. in this study, oxidative stress and inflammation were induced using hydrogen peroxide (H2O2) and phorbol 12-myristate 13-acetate (PMA), and the use of hydrogen water significantly reduced H2O2-induced cell death, cellular reactive oxygen species (ROS) generation, intracellular ROS, and lipid accumulation.

Quality Assessment of Included Studies

Among the 31 included studies, 25 animal studies were evaluated using the SYRCLE risk of bias tool. The average score across these studies was approximately 5.69 out of 11, indicating possible considerable methodological limitations. High potential risk of bias was frequently observed in allocation concealment, random housing, and blinding of caregivers, as reflected by consistent “−” ratings. Domains such as random outcome assessment and blinding of outcome assessors showed marked variability, with many studies receiving “−” or “?” scores. In contrast, most studies scored well in incomplete outcome data and selective outcome reporting, which predominantly received “+” ratings. Unit of analysis errors and funding or conflict of interest reporting showed mixed results across the dataset.

One human clinical trial⁷⁷ was assessed using the Cochrane tool and achieved a score of 4 out of 5, with low risk (“+”) ratings in most domains and a “?” in selective reporting due to retrospective trial registration.

These results indicate substantial variation in methodological quality. Animal studies were particularly affected by inadequate reporting and high risk in key bias domains, while the human clinical study demonstrated relatively low risk. The cell-based studies were notably weaker, with limited detail and elevated risk across most assessment criteria.

Table 3. SYRCLE Quality Assessment for Animal Studies

Abbreviations: SG = Selection Bias (Sequence Generation), BC = Selection Bias (Baseline Characteristics), AC = Allocation Concealment, RH = Random Housing, BCg = Blinding (Caregivers/Investigators), ROA = Random Outcome Assessment, BOA = Blinding (Outcome Assessors), IOD = Incomplete Outcome Data, SOR = Selective Outcome Reporting, UoA = Unit of Analysis, FCOI = Funding Conflict of Interest. Symbols: + (low risk, 1 point), ? (unclear risk, 0 points), − (high risk, 0 points). Total score out of 11.

Table 3 Cochrane Quality Assessment for Human Study(s)

Discussion

This systematic review analyzed 31 studies investigating the effects of HW and HRS on tissue inflammation across multiple biological systems. The findings consistently demonstrate anti-inflammatory and antioxidant properties across diverse tissue types and inflammatory conditions.

To best of authors’ knowledge, no other review was found investigating the impact of hydrogen water on a variety of body systems. Two other studies^79,80 were found investigating similar themes, but only this study had its focus on inflammation.

Dhillon et al. (2024)⁷⁹ conducted a broad systematic review examining hydrogen-rich water's effects across multiple health domains, analyzing 25 studies. While their review covered various physiological systems including exercise and physical endurance, cardiovascular system, mental health, anti-cancer pathways, liver function, renal function in dialysis patients, oxidative stress response systems, and aging-related systems, it focused more broadly on general health benefits rather than specifically on inflammatory responses. Their findings showed positive effects on exercise capacity, cardiovascular health, and oxidative stress markers, which aligned with some aspects of our inflammatory findings. However, their analysis emphasized the need for larger clinical trials to establish definitive therapeutic benefits.⁷⁹

Jamialahmadi et al. (2024)⁸⁰ performed a systematic review specifically examining hydrogen- rich water's effects on blood lipid profiles in metabolic disorders including type 2 diabetes mellitus, impaired glucose tolerance, non-alcoholic fatty liver disease, metabolic syndrome, hypercholesterolemia, and obesity. Their meta-analysis of 8 randomized controlled trials showed modest lipid-lowering effects and demonstrated some anti-inflammatory benefits, though this was not their primary focus. They observed similar limitations regarding study heterogeneity and emphasized the need for more standardized research protocols to better understand hydrogen water's therapeutic mechanisms.⁸⁰

Mechanism of Action

The current understanding of hydrogen water's mechanism of action, based on the reviewed studies, reveals several key insights into its therapeutic potential. Hydrogen functions as a selective antioxidant, specifically reacting with harmful reactive species such as hydroxyl free radicals (•OH), transforming them into harmless water molecules through electron donation. ^81,82 Due to its status as the smallest and lightest molecule in existence, hydrogen can readily diffuse through biological membranes, including the blood-brain barrier and cell membranes, allowing it to reach intracellular compartments where oxidative damage often occurs.^81,82

The neutralization process involves hydrogen molecules directly interacting with and eliminating various reactive oxygen species beyond hydroxyl radicals, including peroxynitrite (ONOO-), hypochlorite ion (OCl-), and singlet oxygen (1O2).^81,82 This process effectively reduces oxidative stress, which is a major contributor to inflammation and tissue damage across multiple pathological conditions. Importantly, this neutralization occurs without generating harmful byproducts, making it a particularly safe intervention. ^81,82

Hydrogen appears especially effective during inflammatory conditions, where these harmful radicals are produced in excess. However, a significant limitation noted across studies is that the exact molecular pathways and signaling cascades involved in hydrogen's anti-inflammatory effects remain unclear and require further investigation. This includes understanding how hydrogen molecules interact with cellular signaling pathways and whether there are specific cellular receptors or targets involved in its therapeutic effects. ^81,82

Limitations

Several key limitations were identified in this systematic review that warrant careful consideration. The database search strategy was restricted to PubMed and Google Scholar, potentially missing relevant studies indexed in other scientific databases such as Scopus, Web of Science, or specialized Asian databases where hydrogen water research is prominent.

A significant geographic and population bias was observed, with the majority of studies being conducted in Asian populations. This limited demographic diversity raises questions about the generalizability of findings across different ethnic groups and geographical regions. The research landscape showed a clear predominance of animal models with limited human studies, highlighting a critical gap in clinical evidence. The human research was particularly sparse, with only one population study by Sim et al.⁷⁷ and three studies examining human ^73,74,78, constraining our ability to draw robust conclusions about effectiveness in human populations.

The methodological quality assessment revealed significant challenges due to incomplete adherence to SYRCLE (Systematic Review Centre for Laboratory Animal Experimentation) criteria. Many studies failed to report essential elements of the SYRCLE framework, such as proper randomization methods, allocation concealment, random housing of animals, blinding of caregivers/investigators, random selection for outcome assessment, and blinding of outcome assessors. This inconsistency in reporting made it difficult to conduct standardized quality analyses across studies.

The mechanistic understanding of hydrogen water's effects remains incomplete, with variations in hydrogen administration methods and concentrations complicating cross-study comparisons. For instance, studies reported hydrogen concentrations in different units (ppm, mmol/L, mL/L) and used various delivery methods (drinking water, injection, inhalation), making it challenging to establish standardized protocols or draw conclusive comparisons between studies.

The quality assessment highlighted substantial methodological constraints, with animal studies averaging a SYRCLE score of 5.69 out of 11, driven by consistent high-risk ratings in allocation concealment, random housing, and blinding, suggesting elevated bias potential (Table 3).⁴⁴ The human trial’s Cochrane score of 4 out of 5 indicated lower risk, though limited by selective reporting concerns (Table 4),⁴⁵ while the exclusion of cell-based studies from scoring due to tool inapplicability further restricts evidence evaluation, collectively undermining the reliability and generalizability of the findings.

Future Directions

Based on the identified limitations, future research directions should focus on several interconnected areas to advance our understanding of hydrogen water's therapeutic potential. Comprehensive mechanistic studies should investigate the precise molecular mechanisms underlying hydrogen water's anti-inflammatory effects, particularly in tissues where free radical damage is most prominent, such as in neurological tissue, cardiovascular system, and skeletal muscle during intense exercise. These studies should examine mitochondrial function, oxidative stress markers, and inflammatory cytokine cascades. Researchers should standardize hydrogen concentration and delivery methods through direct consumption or inhalation methods to ensure consistent research protocols.

To ensure research quality, studies should implement rigorous methodology reporting using the SYRCLE risk of bias tool for animal studies, with explicit documentation of randomization procedures, blinding methods, and sample size calculations in their methods sections. Human clinical research needs substantial expansion, with a particular focus on diverse populations including different age groups, ethnicities, and health conditions. Long-term safety studies and follow-up periods of at least 12-24 months are crucial to establish the sustained benefits and potential side effects.

Geographic diversification of research is essential, as current findings predominantly come from Asian populations. Implementation of multi-center trials across North America, Europe, and other regions would help validate the cross-cultural applicability of hydrogen water therapy.

Clinical applications should be systematically explored through standardized protocols, particularly in conditions where oxidative stress plays a key role, such as burns, sports-related inflammation, neurodegenerative disorders, cardiovascular diseases, and metabolic conditions. These investigations should include comparative effectiveness studies against current standard treatments like conventional antioxidants and anti-inflammatory medications.

Addressing the quality assessment outcomes, future animal studies must enhance methodological rigor to achieve higher SYRCLE scores, targeting consistent low-risk ratings through detailed reporting of randomization, allocation concealment, and blinding practices, as current averages of 5.69 out of 11 indicate significant deficiencies (Table 3).⁴⁴ Human trials should build on the 4 out of 5 Cochrane benchmark by ensuring preregistered protocols to eliminate selective reporting risks (Table 4),⁴⁵ while developing validated quality tools for cell-based studies will enable comprehensive assessment of in vitro evidence, strengthening the overall research framework.

Conclusion

The reviewed studies provide evidence supporting the effectiveness of hydrogen water in reducing tissue inflammation across multiple biological systems. However, significant gaps remain in our understanding of its precise mechanisms and its efficacy in diverse human populations. Moving forward, priority should be given to conducting well-designed human clinical trials, expanding research to more diverse populations, and elucidating the exact molecular mechanisms of action. These steps are crucial for establishing hydrogen water as a potential clinical treatment option for inflammatory conditions.

Future research and clinical practice should focus on implementing more rigorous methodological approaches in animal studies while significantly expanding human clinical trials. Specific therapeutic applications need thorough investigation, alongside the development of standardized treatment protocols. Given that most research has been conducted in Asian populations, there is a pressing need to explore the use of hydrogen water and other high-dose free radical antioxidants in Western populations. While current evidence suggests promising anti-inflammatory effects of hydrogen water, more comprehensive research is needed to fully understand its therapeutic potential and establish its role in clinical practice.

References

1. Slavich GM. Understanding inflammation, its regulation, and relevance for health: A top scientific and public priority. Brain Behav Immun. 2015;45:13-14. doi:10.1016/j.bbi.2014.10.012
2. Greiner B, Checketts J, Fishbeck K, Hartwell M. Clinical characteristics and lifestyle behaviors among individuals with arthritis: an analysis of 2017 Behavioral Risk Factor Surveillance System data. J Osteopath Med. 2021;121(1):113-119. doi:10.1515/jom- 2020-0123
3. Dhillon G, Buddhavarapu V, Grewal H, et al. Hydrogen water: extra healthy or a hoax? —a systematic review.Int J Mol Sci. 2024;25(2):973. doi:10.3390/ijms25020973
4. Maruyama Y, Nakayama M, Ueda A, Miyazaki M, Yokoo T. Comparisons of fatigue between dialysis modalities: A cross-sectional study. PLoS One. 2017;12(9):e0184535. doi:10.1371/journal.pone.0184535.
5. Hu D, Li D, Shigeta M, et al. Alleviation of the chronic stress response attributed to the antioxidant and anti-inflammatory effects of electrolyzed hydrogen water. Biochem Biophys Res Commun. 2021;535:1-5. doi:10.1016/j.bbrc.2020.12.035
6. Ohta S. Molecular hydrogen as a novel antioxidant. In: Methods in Enzymology. Vol 555. Elsevier; 2015:289-317. doi:10.1016/bs.mie.2014.11.038
7. Noda K, Tanaka Y, Shigemura N, et al. Hydrogen-supplemented drinking water protects cardiac allografts from inflammation-associated deterioration: Hydrogen water preserves cardiac/aortic allografts. Transpl Int.2012;25(12):1213-1222. doi:10.1111/j.1432- 2277.2012.01542.x
8. Hu D, Kabayama S, Watanabe Y, Cui Y. Health benefits of electrolyzed hydrogen water: antioxidant and anti-inflammatory effects in living organisms. Antioxidants (Basel). 2024;13(3):313. doi:10.3390/antiox13030313
9. Steven JL. Metchnikoff on the comparative pathology of inflammation. Glasgow MedJ. 1892;38(3):195-205.
10. Libby P. Inflammatory mechanisms: the molecular basis of inflammation and disease. Nutr Rev. 2008;65. doi:10.1111/j.1753-4887.2007.tb00352.x
11. Daniel V. Anti-inflammatory activity. In: Hock FJ, ed. Drug Discovery and Evaluation: Pharmacological Assays.Springer International Publishing; 2016:1905-2024. doi:10.1007/978-3-319-05392-9_42
12. Calder PC, Albers R, Antoine JM, et al. Inflammatory disease processes and interactions with nutrition. Br J Nutr. 2009;101(S1):1-45. doi:10.1017/S0007114509377867
13. Pahwa R, Goyal A, Jialal I. Chronic inflammation. In: StatPearls. StatPearls Publishing; 2024. Accessed August 23, 2024. http://www.ncbi.nlm.nih.gov/books/NBK493173/
14. Ferrucci L, Fabbri E. Inflammageing: chronic inflammation in ageing, cardiovascular disease, and frailty. Nat Rev Cardiol. 2018;15(9):505-522. doi:10.1038/s41569-018- 0064-2
15. Tsalamandris S, Antonopoulos AS, Oikonomou E, et al. The role of inflammation in diabetes: current concepts and future perspectives. Eur Cardiol. 2019;14(1):50-59. doi:10.15420/ecr.2018.33.1
16. Missiroli S, Genovese I, Perrone M, et al. The role of mitochondria in inflammation: from cancer to neurodegenerative disorders. J Clin Med. 2020;9(3):740. doi:10.3390/jcm9030740
17. Chen WW, Zhang X, Huang WJ. Role of neuroinflammation in neurodegenerative diseases (Review). Mol Med Rep. 2016;13(4):3391-3396. doi:10.3892/mmr.2016.4948
18. Ferlay J, Colombet M, Soerjomataram I, et al. Cancer statistics for the year 2020: an overview. Int J Cancer.2021;149(4):778-789. doi:10.1002/ijc.33588
19. Lippi G, Mattiuzzi C, Sanchis-Gomar F, Bovo C. Cardiovascular risk factors: updated worldwide population statistics. J Hosp Manag Health Policy. 2020;4:3-3. doi:10.21037/jhmhp.2019.12.03
20. Roglic G, Unwin N. Mortality attributable to diabetes: estimates for the year 2010. Diabetes Res Clin Pract.2010;87(1):15-19. doi:10.1016/j.diabres.2009.10.006

21. Moses VS, Bertone AL. Nonsteroidal anti-inflammatory drugs. Vet Clin North Am Equine Pract. 2002;18(1):21-37. doi:10.1016/S0749-0739(01)00002-5

22. Barnes PJ. How corticosteroids control inflammation: Quintiles Prize Lecture 2005. Br J Pharmacol.2006;148(3):245-254. doi:10.1038/sj.bjp.0706736

23. Brooks P. Use and benefits of nonsteroidal anti-inflammatory drugs. Am J Med. 1998;104(3):9S-13S. doi:10.1016/S0002-9343(97)00204-0

24. Lea S, Higham A, Beech A, Singh D. How inhaled corticosteroids target inflammation in COPD. Eur Respir Rev. 2023;32(170):230084. doi:10.1183/16000617.0084-2023

25. Wynne HA, Long A. Patient awareness of the adverse effects of non-steroidal anti- inflammatory drugs (NSAIDs). Br J Clin Pharmacol. 1996;41(4):421-426. doi:10.1046/j.1365-2125.1996.41420.x

26. Buchman AL. Side effects of corticosteroid therapy. J Clin Gastroenterol. 2001;33(4):289-294.

27. Rainsford KD. Profile and mechanisms of gastrointestinal and other side effects of nonsteroidal anti-inflammatory drugs (NSAIDs). Am J Med. 1999;107(6):27-35. doi:10.1016/S0002-9343(99)00365-4

28. Claman H. **How corticosteroids work1, 2. J Allergy Clin Immunol. 1975;55(3):145-151. doi:10.1016/0091-6749(75)90010-X

29. Greaves MW. Anti-inflammatory action of corticosteroids. Postgrad Med J. 1976;52(612):631-633. doi:10.1136/pgmj.52.612.631

30. Barnes PJ. Corticosteroids. In: Asthma and COPD. Elsevier; 2002:547-564. doi:10.1016/B978-012079028-9/50127-7

31. Vane JR, Botting RM. Mechanism of action of anti-inflammatory drugs. Scand J Rheumatol.1996;25(sup102):9-21. doi:10.3109/03009749609097226

32. Yaksh TL, Dirig DM, Malmberg AB. Mechanism of action of nonsteroidal anti- inflammatory drugs. Cancer Invest. 1998;16(7):509-527. doi:10.3109/07357909809011705

33. Vane JR, Botting RM. Mechanism of action of nonsteroidal anti-inflammatory drugs. Am J Med.1998;104(3):2S-8S. doi:10.1016/S0002-9343(97)00203-9

34. Langhorst J, Wulfert H, Lauche R, et al. Systematic review of complementary and alternative medicine treatments in inflammatory bowel diseases. J Crohns Colitis. 2015;9(1):86-106. doi:10.1093/ecco-jcc/jju007

35. Liu X, Lin YJ, Cheng Y. Complementary and alternative therapies for inflammatory diseases. Evid Based Complement Alternat Med. 2016;2016(1):8324815. doi:10.1155/2016/8324815

36. Aoki K, Nakao A, Adachi T, Matsui Y, Miyakawa S. Pilot study: Effects of drinking hydrogen-rich water on muscle fatigue caused by acute exercise in elite athletes. Med Gas Res. 2012;2(1):12. doi:10.1186/2045-9912-2-12

37. Nakao A, Toyoda Y, Sharma P, Evans M, Guthrie N. Effectiveness of hydrogen rich water on antioxidant status of subjects with potential metabolic syndrome—an open label pilot study. J Clin Biochem Nutr.2010;46(2):140-149. doi:10.3164/jcbn.09-100

38. Ohta S. Molecular hydrogen is a novel antioxidant to efficiently reduce oxidative stress with potential for the improvement of mitochondrial diseases. Biochim Biophys Acta Gen Subj. 2012;1820(5):586-594. doi:10.1016/j.bbagen.2011.05.006

39. Halliwell B, Gutteridge JMC. Free Radicals in Biology and Medicine. 5th ed. Oxford University Press; 2015.

40. Gharib B, Hanna S, Abdallahi OMS, Lepidi H, Gardette B, De Reggi M. Anti- inflammatory properties of molecular hydrogen: investigation on parasite-induced liver inflammation. C R Acad Sci III. 2001;324(8):719-724. doi:10.1016/S0764- 4469(01)01350-6

41. Hooijmans CR, Rovers MM, De Vries RB, et al. SYRCLE’s risk of bias tool for animal studies. BMC Med Res Methodol. 2014;14(1):43. doi:10.1186/1471-2288-14-43

42. Higgins JPT, Altman DG, Gotzsche PC, et al. The Cochrane Collaboration’s tool for assessing risk of bias in randomised trials. BMJ. 2011;343(oct18 2). doi:10.1136/bmj.d5928

43. Cumpston M, Li T, Page MJ, et al. Updated guidance for trusted systematic reviews: a new edition of the Cochrane Handbook for Systematic Reviews of Interventions. Cochrane Database Syst Rev. 2019;2019(10). doi:10.1002/14651858.ED000142

44. Vane JR, Botting RM. Mechanism of action of anti-inflammatory drugs. Scand J Rheumatol.1996;25(sup102):9-21. doi:10.3109/03009749609097226

45. Yaksh TL, Dirig DM, Malmberg AB. Mechanism of action of nonsteroidal anti- inflammatory drugs. Cancer Invest. 1998;16(7):509-527. doi:10.3109/07357909809011705

46. Vane JR, Botting RM. Mechanism of action of nonsteroidal anti-inflammatory drugs. Am J Med.1998;104(3):2S-8S. doi:10.1016/S0002-9343(97)00203-9

47. Page MJ, McKenzie JE, Bossuyt PM, et al. The PRISMA 2020 statement: an updated guideline for reporting systematic reviews. BMJ. Published online March 29, 2021:n71. doi:10.1136/bmj.n71

48. Cai J, Kang Z, Liu K, et al. Neuroprotective effects of hydrogen saline in neonatal hypoxia–ischemia rat model. Brain Research. 2009;1256:129-137. doi:10.1016/j.brainres.2008.11.048

49. Liu F, Xu S, Xiang Z, et al. Molecular hydrogen suppresses reactive astrogliosis related to oxidative injury during spinal cord injury in rats. CNS Neurosci Ther. 2014;20(8):778- 786. doi:10.1111/cns.12258

50. Tian R, Hou Z, Hao S, et al. Hydrogen-rich water attenuates brain damage and inflammation after traumatic brain injury in rats. Brain Research. 2016;1637:1-13. doi:10.1016/j.brainres.2016.01.029

51. Jiang B, Li Y, Dai W, Wu A, Wu H, Mao D. Hydrogen-rich saline alleviates early brain injury through regulating of ER stress and autophagy after experimental subarachnoid hemorrhage. Acta Cir Bras. 2021;36(8):e360804. doi:10.1590/acb360804

52. Wang X, Sun Y, Luan C, et al. Effect of hydrogen‐rich saline on melanopsin after acute blue light‐induced retinal damage in rats. Photochem & Photobiology. Published online April 18, 2024:php.13952. doi:10.1111/php.13952

53. Qi LS, Yao L, Liu W, et al. Sirtuin type 1 mediates the retinal protective effect of hydrogen-rich saline against light-induced damage in rats. Invest Ophthalmol Vis Sci. 2015;56(13):8268. doi:10.1167/iovs.15-17034

54. Ohsawa I, Nishimaki K, Yamagata K, Ishikawa M, Ohta S. Consumption of hydrogen water prevents atherosclerosis in apolipoprotein E knockout mice. Biochemical and Biophysical Research Communications. 2008;377(4):1195-1198. doi:10.1016/j.bbrc.2008.10.156

55. Sun Q, Kawamura T, Masutani K, et al. Oral intake of hydrogen-rich water inhibits intimal hyperplasia in arterialized vein grafts in rats. Cardiovascular Research. 2012;94(1):144-153. doi:10.1093/cvr/cvs024

56. Noda K, Tanaka Y, Shigemura N, et al. Hydrogen-supplemented drinking water protects cardiac allografts from inflammation-associated deterioration: Hydrogen water preserves cardiac/aortic allografts. Transplant International. 2012;25(12):1213-1222. doi:10.1111/j.1432-2277.2012.01542.x

57. Zhao L, Wang Y bin, Qin S rui, et al. Protective effect of hydrogen-rich saline on ischemia/reperfusion injury in rat skin flap. J Zhejiang Univ Sci B. 2013;14(5):382-391. doi:10.1631/jzus.B1200317

58. Wang X, Yu P, YongYang, et al. Hydrogen-rich saline resuscitation alleviates inflammation induced by severe burn with delayed resuscitation. Burns. 2015;41(2):379- 385. doi:10.1016/j.burns.2014.07.012

59. Sato C, Kamijo Y, Yoshimura K, et al. Effects of hydrogen water on paraquat-induced pulmonary fibrosis in mice. Kitasato Med J. 2015;45:9-16.

60. Wu Q, Zhang J, Wan Y, et al. Hydrogen water alleviates lung injury induced by one-lung ventilation. Journal of Surgical Research. 2015;199(2):664-670. doi:10.1016/j.jss.2015.06.017

61. Kajiya M, Sato K, Silva MJB, et al. Hydrogen from intestinal bacteria is protective for Concanavalin A-induced hepatitis. Biochemical and Biophysical Research Communications. 2009;386(2):316-321. doi:10.1016/j.bbrc.2009.06.024

62. Xu XF, Zhang J. Saturated hydrogen saline attenuates endotoxin-induced acute liver dysfunction in rats. Physiol Res. Published online August 31, 2013:395-403. doi:10.33549/physiolres.932515

63. Koyama Y, Taura K, Hatano E, et al. Effects of oral intake of hydrogen water on liver fibrogenesis in mice. Hepatology Research. 2014;44(6):663-677. doi:10.1111/hepr.12165

64. Xue J, Shang G, Tanaka Y, et al. Dose-dependent inhibition of gastric injury by hydrogen in alkaline electrolyzed drinking water. BMC Complement Altern Med. 2014;14(1):81. doi:10.1186/1472-6882-14-81

65. Zhang JY. Hydrogen-rich water protects against acetaminophen-induced hepatotoxicity in mice. WJG. 2015;21(14):4195. doi:10.3748/wjg.v21.i14.4195

66. Liu Z, Cheng S, Gu C, Pei H, Hong X. Effect of hydrogen-rich saline on postoperative intra-abdominal adhesion bands formation in mice. Med Sci Monit. 2017;23:5363-5373. doi:10.12659/MSM.904669

67. Hu D, Huang T, Shigeta M, et al. Electrolyzed hydrogen water alleviates abdominal pain through suppression of colonic tissue inflammation in a rat model of inflammatory bowel disease. Nutrients. 2022;14(21):4451. doi:10.3390/nu14214451

68. Cardinal JS, Zhan J, Wang Y, et al. Oral hydrogen water prevents chronic allograft nephropathy in rats. Kidney International. 2010;77(2):101-109. doi:10.1038/ki.2009.421

69. Wang F, Yu G, Liu SY, et al. Hydrogen-rich saline protects against renal ischemia/reperfusion injury in rats. Journal of Surgical Research. 2011;167(2):e339- e344. doi:10.1016/j.jss.2010.11.005

70. Xu B, Zhang Y bo, Li Z zhu, Yang M wen, Wang S, Jiang D peng. Hydrogen-rich saline ameliorates renal injury induced by unilateral ureteral obstruction in rats. International Immunopharmacology. 2013;17(2):447-452. doi:10.1016/j.intimp.2013.06.033

71. Guo SX, Jin YY, Fang Q, et al. Beneficial effects of hydrogen-rich saline on early burn- wound progression in rats. Shimosawa T, ed. PLoS ONE. 2015;10(4):e0124897. doi:10.1371/journal.pone.0124897

72. Yoon YS, Sajo MaEJV, Ignacio RMC, Kim SK, Kim CS, Lee KJ. Positive effects of hydrogen water on 2,4-dinitrochlorobenzene-induced atopic dermatitis in nc/nga mice. Biological & Pharmaceutical Bulletin. 2014;37(9):1480-1485. doi:10.1248/bpb.b14- 00220

73. Xiao L, Miwa N. Hydrogen-rich water achieves cytoprotection from oxidative stress injury in human gingival fibroblasts in culture or 3D-tissue equivalents, and wound- healing promotion, together with ROS-scavenging and relief from glutathione diminishment. Human Cell. 2017;30(2):72-87. doi:10.1007/s13577-016-0150-x

74. Xiao L, Mochizuki M, Nakahara T, Miwa N. Hydrogen-generating silica material prevents uva-ray-induced cellular oxidative stress, cell death, collagen loss and melanogenesis in human cells and 3d skin equivalents. Antioxidants. 2021;10(1):76. doi:10.3390/antiox10010076

75. Itoh T, Fujita Y, Ito M, et al. Molecular hydrogen suppresses FcεRI-mediated signal transduction and prevents degranulation of mast cells. Biochemical and Biophysical Research Communications. 2009;389(4):651-656. doi:10.1016/j.bbrc.2009.09.047

76. Zhang J, Wu Q, Song S, et al. Effect of hydrogen-rich water on acute peritonitis of rat models. International Immunopharmacology. 2014;21(1):94-101. doi:10.1016/j.intimp.2014.04.011

77. Sim M, Kim CS, Shon WJ, Lee YK, Choi EY, Shin DM. Hydrogen-rich water reduces inflammatory responses and prevents apoptosis of peripheral blood cells in healthy adults: a randomized, double-blind, controlled trial. Sci Rep. 2020;10(1):12130. doi:10.1038/s41598-020-68930-2

78. Xiao L, Miwa N. Hydrogen nano-bubble water suppresses ros generation, adipogenesis, and interleukin-6 secretion in hydrogen-peroxide- or pma-stimulated adipocytes and three-dimensional subcutaneous adipose equivalents. Cells. 2021;10(3):626. doi:10.3390/cells10030626

79. Dhillon G, Buddhavarapu V, Grewal H, et al. Hydrogen water: extra healthy or a hoax? — a systematic review. IJMS. 2024;25(2):973. doi:10.3390/ijms25020973

80. Jamialahmadi H, Khalili-Tanha G, Rezaei-Tavirani M, Nazari E. The effects of hydrogen-rich water on blood lipid profiles in metabolic disorders clinical trials: a systematic review and meta-analysis. Int J Endocrinol Metab. 2024;22(3). doi:10.5812/ijem-148600

81. Morita M, Naito Y, Yoshikawa T, Niki E. Plasma lipid oxidation induced by peroxynitrite, hypochlorite, lipoxygenase and peroxyl radicals and its inhibition by antioxidants as assessed by diphenyl-1-pyrenylphosphine. Redox Biology. 2016;8:127-135. doi:10.1016/j.redox.2016.01.005

 82. Morita M, Naito Y, Yoshikawa T, Niki E. Inhibition of plasma lipid oxidation induced by peroxyl radicals, peroxynitrite, hypochlorite, 15-lipoxygenase, and singlet oxygen by clinical drugs. Bioorganic & Medicinal Chemistry Letters. 2016;26(22):5411-5417. doi:10.1016/j.bmcl.2016.10.033.