H
2 has been demonstrated to possess anti-inflammatory, antioxidant, and anti-endoplasmic reticulum (ER) stress properties, and its involvement in the regulation of apoptosis, autophagy, and pyroptosis has been elucidated
[12][13][14][15][16][17][18]. In this regard, H
2 can be considered a unique molecule that influences fundamental biological responses. Although mechanisms for the multifaceted effects of H
2 have been proposed, the fundamental processes still remain unclear. Ohsawa et al. suggested the possibility of H
2 directly scavenging hydroxyl radicals
[7]; however, subsequent studies revealed the effectiveness of H
2 preconditioning in organ protection. As this phenomenon cannot be solely explained as due to chemical reactions of H
2, it is speculated that H
2 may impact the body’s inflammatory and antioxidant systems, activating the body’s defense mechanisms
[11]. In this context, recent attention has been drawn to the relationship between H
2, the redox system in the body, and the mitochondria
[14][15][16].
Accumulated reports from animal experiments indicate that H
2 administration enhances Nrf2 expression
[21][22][23][24][25][26][27]. However, Nrf2 expression is supposed to be triggered by oxidative stress stimuli. From this perspective, there is a possibility that H
2 induces a so-called hormesis phenomenon
[28]. Indeed, the potential of H
2 to induce mild oxidative stress has been reported
[28][29][30], and activation of antioxidant systems via oxidative stimulation cannot be denied. This is analogous to the body’s response to exercise
[31]. The hypothesis regarding the mechanism of H
2 to activate Nrf2 is outlined in
Figure 1b. The core of the matter lies in the fact demonstrated by Ohsawa et al.
[7], who reported that H
2 directly scavenges hydroxyl radicals in the form of electrophilic hydrogen radicals (H:H → H + H, H + OH → H
2O). However, in general, a single electrophile could have both protective and toxic effects on cells. It is known that electrophiles react with nucleophiles, including protein thiols (-SH), such as those found in reduced glutathione (GSH) or guanine bases in DNA
[32]. Each of them has an unshared pair of electrons, and the reaction of an electrophile with the -SH of a cysteine residue results in alkylation
[33], leading to the decrease of the reductive capacity of cells, i.e., depletion of GSH. If we consider this phenomenon in terms of hydrogen radicals, theoretically it is possible that hydrogen radicals could react with GSH, resulting in a reduced reductive capacity of cells. Hydrogen radicals may also react with the -SH of cysteine residues in Keap1, leading to the generation of H
2 and modification of the cysteine residue of Keap1 (disulfide reaction, -S-S-), which may trigger the activation of Nrf2 action. However, the responses of electrophiles are generally characterized by dose-response
[34]; therefore, different doses of H
2 and accompanying levels of hydrogen radicals may induce different responses. At present, the fate of hydrogen radicals in cells is completely unknown and requires further investigation.
Mitochondria, the energy production mechanism, are the major source of reactive oxygen species in cells. It is assumed that the small molecule H
2 is easily distributed within cells, and therefore, it is expected to be directly involved with mitochondria. It has been demonstrated that H
2 supplementation is related to the preservation and maintenance of mitochondria
[35][36]. The proposed mechanism suggests that H
2 captures excess reactive oxygen species in mitochondria, preserving them from oxidative stress damag, and ultimately exhibiting organ protection effects
[14][15][16]. Recently, a connection between the gut microbiota and mitochondrial function has been suggested
[37][38][39][40]. H
2 is involved in preserving mitochondrial function, while the gut microbiota serves as a source of H
2 production in the body
[41]. Future investigations are expected to explore whether H
2 acts as a missing link between the gut microbiota and mitochondrial function.
In summary, recent findings have been summarized, but many aspects of the fundamental mechanisms and starting points of H2’s actions on cells and the body remain unknown. However, considering that no adverse effects of H2 on the body have been confirmed, the clinical application of H2 has become a realistic challenge. Within this context, establishing methods of H2 administration that can demonstrate the effectiveness of H2, taking into account the characteristics of the disease, is considered a challenge.
3. H2 Intervention for CKD and Hemodialysis
Given the multifaceted involvement of oxidative stress in various pathologies, antioxidant therapy is considered extremely significant. However, the expected results are not always obtained in interventions employing antioxidants, including for CKD
[43]. Reactive oxygen species are a double-edged sword, having both detrimental effects on the body and being crucial for the body’s defense. In this sense, excessive oxidative stress should be suppressed, but the degree of suppression should be at a level that does not compromise the benefits of reactive oxygen species to the body
[15]. To date, while many preclinical studies using H
2 have confirmed organ-protective effects and correction of metabolic abnormalities through its antioxidant and anti-inflammatory effects
[12][13][14][15][16][17][18], no severe side effects of H
2 loading have been observed. Therefore, the clinical application of antioxidant therapy with H
2 is considered a realistic challenge. The following summarizes the research on CKD and dialysis-related topics.
3.1. Pre-Clinical Studies of H2 in CKD Models
The reno-protective effects of H
2 have been reported in various models of kidney diseases
[44][45] via H
2 administration through drinking water, intraperitoneal administration, and inhalation. These studies involve acute models such as acute kidney injury via ischemia–reperfusion of renal artery clamp
[46][47][48][49][50][51][52][53][54][55][56], allograft rejection
[57][58], drug-induced nephrotoxicity
[59][60], renal calculi
[61][62], and renal fibrosis via ureteral ligation
[63][64][65]. However, reports on CKD are limited
[21][66][67] (
Table 1).
Table 1. Effect of H2 water ad libitum drinking for CKD model rats.
In a study of spontaneously hypertensive rats, the effects of drinking hydrogen water (1.2 mg/L) freely for 3 months under salt reduction were investigated
[66]. The results showed no difference in the onset of high blood pressure compared to the control group. However, histologically, kidney damage was suppressed, and a significant increase in antioxidant substances and a significant decrease in proinflammatory substances were confirmed in rats on hydrogen water compared to the control group. Although hydrogen water drinking did not suppress the onset of hypertension itself, it was suggested that H
2 could inhibit the process of oxidative kidney damage associated with hypertension.
In Dahl salt-sensitive rats, which exhibit high blood pressure, kidney damage, and cardiac hypertrophy in response to salt loading
[21], the effects of drinking EHW (0.4 mg/L) freely for 12 months under salt reduction were investigated. The results showed delayed onset of high blood pressure, and histologically, kidney damage and myocardial hypertrophy were suppressed compared to the control group. In the hydrogen water-drinking group, Nrf2 expression in cardiac tissues was enhanced, suggesting increased resistance to oxidative stress in the body.
It is known that episodes of renal ischemia in CKD accelerate the progression of subsequent kidney damage. This pathophysiology is thought to involve not only localized renal ischemia but also the induction of inflammation in non-ischemic areas initiated by localized renal ischemia
[68]. In experiments using Dahl salt-sensitive rats raised drinking EHW freely, the effects of contralateral kidney effects following unilateral renal ischemia were investigated
[67]. The degree of renal tissue damage in the EHW drinking group was significantly lower than that in the control group, and oxidative stress in the same site was suppressed compared to the control group.
Summarizing these considerations, free drinking of hydrogen-containing water suggests a suppression of renal tissue damage in a model of hypertensive rats. The mechanism is presumed to be related to the alleviation of renal stress due to microcirculatory changes in the kidneys, which are mediated through systemic hypertension, as hydrogen-containing water did not directly suppress the onset of hypertension itself
[21][66]. On the other hand, inhalation of H
2 has been reported to ameliorate hypertension, implying involvement of changes in the balance of the sympathetic and parasympathetic nervous systems
[69]. Considering the reported ability of parasympathetic nerve stimulation to suppress acute renal injury
[70], it may be possible to hypothesize an influence of H
2 on the autonomic nervous system balance in the mechanism of inhibiting renal injury in CKD rats.
3.2. Clinical Studies of H2 Intervention in Related to CKD Pathologies
Up to now, no intervention studies with H
2 have been reported in CKD patients. However, regarding CKD risk factors, such as diabetes mellitus (DM), metabolic syndrome, and hypertension, six clinical trials were reported to explore the potentials of H
2 [71]. Among them, three studies may indicate the potential reno-protective effect, i.e., decreased oxidative stress marker and/or increased antioxidants in urine, by drinking hydrogen-rich water. Kajiyama et al.
[72] conducted a double-blind cross-over trial of drinking 900 mL/d of hydrogen-rich pure water (1.2 mg/L) for 8 weeks in 30 patients with type 2 DM and 6 with impaired glucose intolerance. Intake of hydrogen-rich water was associated with significant decreases in urinary 8-isoprostanes, and there was a trend of decreased serum oxidized LDL and increased plasma levels of extracellular superoxide dismutase, which may indicate the amelioration of decreased oxidative stress in the body including the kidneys. Ogawa et al.
[73] conducted a double-blind trial in which type 2 diabetes patients were given EHW for free consumption (1.5~2.0 L/day) for three months (23 patients on EHW, 20 on filtered water), and they found significant improvements in insulin resistance in those with high insulin resistance, and an amelioration of enhanced serum d-ROM, an oxidative stress marker, in the EHW group. In a secondary analysis of this study, drinking EHW significantly increased eGFR at 3 months as compared to the basal level, and significantly decreased the change in urinary 8-OHdG excretion (ng/mgCr), an oxidative stress marker (oral presentation by Ogawa et al. at the 62nd annual meeting of the Japanese Society of Nephrology, Nagoya, 2019). Nakao et al.
[74] performed an open-label one-arm pilot study of drinking HRW (~1 mmol H
2/day) for 8 weeks in 20 subjects with metabolic syndrome, and they reported a significant increase of SOD in urine; a significant decrease of TBARS, an oxidative stress marker in urine; and a significant decrease of serum creatinine levels, which indicate the reno-protective action of drinking HRW.
Regarding the impact of H
2 on hypertension, Liu et al.
[75] examined the effect of a mixture of H
2–oxygen (O
2) gas inhalation on middle-aged and elderly hypertensive patients for four hours daily over a two-week period (20 cases with the H
2 + O
2 gas, 29 with placebo air). As a result, the group inhaling the mixed gas showed a significant reduction in brachial systolic blood pressure and nighttime blood pressure measured via ambulatory blood pressure monitoring (ABPM). This effect was more pronounced in the elderly. Additionally, levels of angiotensin II and a certain aldosterone value were significantly decreased. The improvement of BP control accompanying decreased levels of plasma angiotensin II and aldosterone may well indicate the risk reduction for kidney damage.
Given that diabetes and hypertension are deeply involved in the progression of CKD, measures to correct these conditions are expected to contribute to the management of CKD. In clinical studies, the administration of hydrogen-containing water was within the scope of daily life without causing inconvenience, suggesting the potential for societal implementation. It is anticipated that future research will involve long-term investigations of a larger number of cases.
In addition, as mentioned in the introduction, the improvement in gastrointestinal symptoms (such as constipation) through the consumption of EHW has been confirmed
[1][9]. Constipation is a risk factor for the presence of CKD
[76], and it is hypothesized that this may be influenced by substances, such as indoxyl sulfate, produced within the intestinal tract
[77]. It should be considered whether the improvement of constipation through the consumption of EHW affects the pathophysiology related to CKD progression, and this is an aspect that should be investigated in the future.
3.3. Clinical Studies Using EHW for Hemodialysis
The clinical application of hydrogen-rich water, specifically in hemodialysis, has been a focus of investigation. The initial report by Huang et al. demonstrated a reduction in inflammatory markers in hemodialysis using electrolyzed water
[78][79]. Furthermore, it was reported that ERW enhances dissociation of indoxyl sulfate from albumin as an underlying mechanism
[80]. However, widespread recognition of this treatment system did not occur. Subsequently, Nakayama et al. focused on the H
2 concentration in electrolyzed dialysis fluid and proposed a treatment system based on this feature, leading to increased recognition of the system
[81]. Currently, in Japan, it is estimated that over 30 facilities have introduced the hemodialysis system, with over 3000 treated patients.
Table 2 summarizes these reports
[30][82][83][84][85][86][87][88].
Table 2. Clinical trials of chronic HD employing electrolyzed hydrogen water.
In hemodialysis, the dialysis solution required for a standard hemodialysis treatment exceeds 140 L per person per session. Dialysis solution itself has chemical oxidative stimuli (bio-incompatibility), but its character is suppressed in the solution created with EHW. As a result, in hemodialysis using EHW, a decrease in oxidative stress and inflammatory markers of patients has been confirmed. Furthermore, with long-term continuation of this treatment, the redox state of patients approaches that of healthy individuals
[84].
Next, what does such a change in the internal environment bring clinically? One is the improvement in the overall prognosis of patients. In a prospective observational study comparing the outcomes of an EHW treatment group and a conventional dialysis group, a 41% significantly lower hazard ratio for the composite endpoint (total death, new stroke, new cardiovascular disease, lower limb amputation) was observed in the EHW group over a 5-year observation period
[86]. Another potential improvement is in the most important patient-reported outcome, dialysis-related fatigue. Dialysis-related fatigue is both an inhibitory factor for quality of life and an independent risk factor for life prognosis. In EHW hemodialysis, the reduction of this dialysis-related fatigue has been reported in several observational studies
[30][85][87][88]. The mechanism may involve the suppression of oxidative stress stimulation initiated by MPO induced by dialysis
[30], changes in the balance of the sympathetic and parasympathetic nervous systems
[87]. The H
2 concentration in the dialysis solution influenced the patients’ reported outcome, i.e., more anti-fatigue effects at 150 ppb than an average of 50 ppb
[88]. Future studies applying a novel device, which could simultaneously monitor H
2 concentration in the dialysate
[89], are expected to investigate the optimal H
2 levels for the comprehensive patients’ outcome via randomized controlled study.