1. Introduction
Gas molecules are increasingly being used for medical purposes and have developed into a separate field of medicine. The gases most widely used in medicine include oxygen (O
2), nitric oxide (NO), methane (CH
4). carbon monoxide (CO), hydrogen sulfide (H
2S) and hydrogen (H
2). As shown in
Figure 1, the number of articles related to medical gas molecules has grown substantially from 1998 to 2022, especially O
2 and H
2.
O
2 and NO are the first two medical gas molecules that attract the researchers’ attention, with tens of thousands of studies focus on these two gases as early as the 1990s. O
2 is the most crucial gas for all living organisms on earth which accounts for around 1/5 of the volume of air. As an important gas to maintain human respiration, O
2 is mainly used to provide supplemental respiration for the sick, astronauts traveling in space, and mountaineers, etc. In addition, it has the function of destroying bacteria. Due to the importance of O
2, the 2019 Nobel Prize in Physiology or Medicine was awarded to Willianm G. Kaelin Jr., Sir Peter J. Ratcliffe and Gregg L. Semenza who discovered how cells sense and adapt to the availability of O
2 [
1]. NO, a free-radical gas named "laughing gas". At the end of the last century, NO was found to work as a mediator of cell-to-cell communication in vasodilatation, inflammation, and neurotransmission. Robert F. Furchgott, Louis J. Ignarro, and Ferid Murad et al. demonstrated that NO is an important signaling molecule in the cardiovascular system, and this discovery won the 1998 Nobel Prize in Physics or Medicine [
2].
CH
4 is the simplest of the organic compounds. For decades, CH
4 was thought to have almost no physiological role, while in the last few years, scientists have realized that CH
4 can play important biological roles such as anti-inflammatory, antioxidant, and anti-apoptotic. As a result, CH
4 has been used as a gastric decontaminant in emergency clinical settings of poisoning or drug overdose, and also serves as a passive indicator of colonic function [
3].
CO and H
2S have long been known as hazardous factors. Long-term exposure to environments which rich with CO may be fatal. However, a growing amount of research suggests that CO is an important gaseous mediator along with NO and H
2S. Endogenously produced or inhaled CO has important physiological functions in regulating vascular function, inflammation, apoptosis, cell proliferation, and signaling pathways. Studies have shown that inhaled CO suppresses chronic inflammation in patients with stable chronic obstructive pulmonary disease (COPD) [
4]. Same as CO, Scientists simply regard H
2S as a harmful gas initially since exposure to H
2S may irritate the eyes and respiratory system. However, scientists have now shown that H
2S is an essential physiological factor as it is produced by bacteria in the human oral cavity and gastrointestinal tract. Being the least appreciated of the three gaseous mediators (gas transport mediators), it is now considered to be an important gas transport mediator after NO and CO. H
2S has been shown to modulate many physiological processes such as vasodilation, anti-inflammation, resistance to oxidative stress, and protection against ischemia-reperfusion injury, etc. [
5]
As the smallest of all molecules, the functions of H
2 have also caught the eye of scientists in the field of biomedicine. As early as the beginning of the last century, H
2 was first tested as a diving gas, proving that it is the best breathing medium for medium and deep diving, and is safe for the organism, with no toxic side effects found. Up to now, H
2 biomedicine has investigated the effect and mechanism by which H
2 molecules, including H
2 positive and negative ions and heavy H
2 (deuterium and tritium), act in various diseases [
6].
2. History of H2 medicine
H
2 is known to be a colorless, odorless, and tasteless gas with chemically stable [
7]. In general, around 35 mL to 321 mL of H
2 is produced and released through bacterial fermentation by the human digestive system per day [
8]. Several ways are used to ingest or consume H
2, such as drinking or injecting H
2 water (HW), inhalation of H
2, bathing in HW, dropping H
2 saline into the eyes, etc. H
2 plays an anti-inflammatory and anti-apoptotic effect through its selective antioxidant properties and has become a unique cytoprotective agent [
9].
H
2 used to be considered an inert gas, not involved in any life activity. It was not until 1975 that Dole et al. found significant regression of mouse skin tumors in squamous cell carcinoma mice exposed for a fortnight into a mixture of 97.5% H
2 and 2.5% O
2 at a total pressure of eight atmospheres, confirming the medical usefulness of H
2 firstly [
10]. Unfortunately, this study has not attracted academic attention due to the technical difficulties of applying hyperbaric H
2 therapy in a clinical application.
Until 1996, Chinese scientist Yuanwei Du noticed the significance of H
2 for life [
11]. Dr. Du believes that excessive accumulation of peroxides produced in the metabolic process is the root cause of various diseases and aging, the organism must have a certain mechanism to fight against these peroxides. H
2 is a reducing agent, which can eliminate peroxides naturally without side effects, making creatures achieve a balance in the sense of redox balance. In Du's experiment, tritium gas was produced by electrolysis of tritium water, the tritium gas was then fed instead of H
2 into the mouse living environment. He found that tritium was present in all tissues and organs of mice, which means that tritium gas is involved in the life activities of living organisms by transforming into tritium ions prevalent in living organisms, indirectly proving that the air H
2 is both a constituent substance and an energetic substance of life. This experiment also proves the basic mechanism of H
2 metabolism. A number of H
2 medicine-related papers published by Yuanwei Du at the end of the 20
th century further confirmed that H
2 produced by water electrolysis has a pronounced effect on the vital activities of plants (lilac branches), animals (mice), as well as humans [
12]. Du’s work creatively combines the physiological effects of H
2 with the free radical aging theory, explains the antioxidant activity of H
2 molecules, as well as confirms that H
2 may have an immeasurable effect on a wide range of diseases.
In 2007, Ohsawa et al. from the Nippon Medical School published an important article on H
2 medicine in the journal Nature Medicine [
13]. This study used a low concentration of H
2 (1-4%) for inhalation over a short period (35 minutes) to mice and found positive effects in the treatment of the cerebral ischemia-reperfusion injury, showing that short-term inhalation of a low concentration of H
2 for the treatment of the disease is feasible. They proposed a mechanism that H
2 could act as a therapeutic antioxidant, selectively reducing cytotoxic oxygen radicals (
•OH and ONOO
-), leading to inhibition of cerebral ischemia-reperfusion injury. Because this study was published in the top journal of Nature Medicine, it provides a broad prospect for both basic and clinical research on H
2 and has brought H
2 medicine to the attention of a wide range of academic cycles. Since then, more and more scholars have joined the research on H
2 medicine to explore its effects on various diseases such as inflammation, drug toxicity, and obesity. More than a thousand peer-reviewed research papers have been published to date.
In the beginning, scientists focused mainly on acute and chronic organ injuries related to oxidative stress, such as the animal experiment of drug toxic injury or ischemia-reperfusion injury in vital organs such as the heart and liver. During this period, researchers mostly used diverse injury models to validate the therapeutical effects of H
2 inhalation. Between 2009 and 2012, more research began to appear on drinking H
2-enrich water (HRW) [
14] and injecting H
2-enrich saline (HRS) [
15], as well as studies on boosting H
2 replenishment through gut bacteria [
16]. Meanwhile, a number of clinical studies have used HRW in the treatment of diseases including metabolic syndrome, Parkinson's disease, hemodialysis, sports injuries, and rheumatoid arthritis [
9]. For the past few years, on the foundation of previous studies, H
2 medicine research has studied the molecular mechanisms, especially focusing on the molecular pathways of inflammation and oxidative stress mediated by H
2. However, regarding the molecular mechanism of H
2, most scholars have followed the view of Ohsawa et al. in Nature Medicine paper that H
2 is a selective hydroxyl radical (•OH) scavenger, and focused on the antioxidant mechanisms of H
2 mainly based on it [
17,
18]. Nevertheless, some scholars have proposed that H
2 plays a signaling role that may be involved in metabolic processes and may even provide energy for cells, which is subversive to the development of H
2 research [
19,
20].
A number of Chinese researchers have devoted themselves to developing H2 medicine, which has received more than 80 grants from the National Natural Science Foundation of China and has published hundreds of basic and clinical academic papers. Prof. Xuejun Sun of the Second Military Medical University is one of the leading figures in H2 medicine in China. Prof. Sun's group engaged in the research of diving hyperbaric medicine for a long time, the most important research object of diving hyperbaric medicine is all kinds of gases that can be breathed by human beings, with H2 being one of the key types of gases in the field of diving hyperbaric medicine. Sun’s group focuses on the biological effects of H2 and its application in medicine for the first time revealing the value of H2 in medicine in China. Moreover, Prof. Sun participated in organizing several international symposiums on H2 medicine, inviting experts from all over the world to discuss the future of H2 medicine. His team collaborates with medical organizations around the world to carry out research on the application of H2 medicine and to expand the scope of H2 applications in the medical field.
Prof. Shucun Qin of Shandong First Medical University is another key promoter of H
2 molecular medicine in China. Prof. Qin established the first H
2 Biomedical Research Institute at the university in 2015, training a number of key researchers in H
2 medicine. He established the standardized laboratory for H
2 molecular biology that has published multiple placebo-controlled population trials, providing important clinical evidence for the translation of H
2 into medicine. Qin's recent review summarizes 51 clinical trials involving 1,213 subjects in four areas of H
2 biomedicine: basic research, exercise, dermatology and healthcare [
21]. The results showed that H
2 can reduce oxidative stress damage caused by strenuous exercise, reduce lactic acid build-up after exercise, prevent exercise acidosis, and reduce exercise fatigue. In addition, H
2 intervention can play a positive role in skin beauty, and improve cardiovascular health.
Prof. Xuemei Ma's team at the Beijing Institute of Technology is also an early group of H
2 medicine researchers in China. Prof. Ma is committed to elucidating the biological basis of H
2 medicine at the molecular, cellular and holistic levels, conducting in-depth basic research and clinical translational research, especially in the mechanism of H
2 molecules on tumor prevention. Her team has verified that H
2 can inhibit the proliferation of gliomas (Gliomas) by inducing glial stem cell differentiation in vitro and in vivo experiments [
22].
Besides these key researchers, there are hundreds of scientists are doing work on H2 medicine, including the Chinese academicians of Prof. Nanshan Zhong, Zhaofen Xia, Hongyang Wang, and young scientists like Prof. Qianjun He who proposed the concept of H2 nanomedicine to address the issues of H2 medicine by using functional micro/nanomaterials for augmented H2 therapy in cancer, and Wenbiao Shen who is devoted in the application of H2 in agriculture. An academic association of H2 medicine with more than 400 members has been formed. As of today, current clinical studies on H2 are still continuously emerging and the scale of the studies is gradually expanding. With its favorable biosafety and the convenience of safe low-dose use, the H2 inhalation device has been included in the Chinese Food and Drug Administration's new medical device development process. Moreover, in Japan, H2 has been approved as a food supplement.
3. H2: a mitochondria-targeting molecule/nutrient, rather than a selective •OH scavenger
Sustained oxidative stress leads to the onset and progression of many common diseases. Up to now, little achievement has been gained, although a large number of studies have attempted to develop an effective antioxidant without side effects. Mitochondria, as a major source of oxidative stress, is considered a new therapeutic target for small molecule interventions [
23]. H
2 suppresses reactive oxygen species (ROS) accumulation, inhibits the cell death program, maintains the mitochondrial structure and function [
24,
25]. Preliminary clinical trials suggest that drinking H
2-dissolved water appears to improve the pathology of mitochondrial disease [
26,
27].
Mitochondria have a double membrane structure that forms the difference in potentials between the inner and outer membranes and controls the movement of diverse molecules and factors (e.g., ions) in and out of the organelle while affecting mitochondrial stability. Although the outer membrane is comparatively permeable to the small molecules and large proteins (which are transported by diffusion or transposases), the inner mitochondrial membrane is highly impermeable to most molecules [
28]. Nearly special membrane transport proteins (e.g., TIM-TOM (preprotein translocase of the inner membrane of mitochondria-preprotein translocase of the outer membrane of mitochondria) complex, etc.) are needed for all ions and molecules to enter or leave the mitochondrial matrix. This makes most antioxidants cannot enter the mitochondria to effectively scavenge •OH [
29,
30]. The difference with other antioxidants is as the smallest molecule in nature, H
2 can easily spread and penetrate into the cell membrane to react with organelles such as mitochondria and the nucleus [
31].
While the idea that H
2 is a selective antioxidant is popularized [
7], it is still not known whether the effects of H
2 are from the direct reaction with •OH or from the inhibition of •OH production. Let’s first give a little basic information on free radicals.
As we know, •OH is generated by the Haber−Weiss reaction:
This reaction is thermodynamically feasible but kinetically too slow. So, •OH is mainly generated by the Fenton reaction:
The three main properties of •OH are below: 1. SHORT LIFE: •OH has a very short half-life (10−9 s, or 1 ns whereas the half-life of superoxide is 15 sec), no time to diffuse (no more than 50 molecular diameters from the site of formation), so the reaction is local with antioxidant at where •OH is produced; 2. HIGH REACTIVITY: •OH is the most ROS with high reduction potential, compared to other oxygen species, it reacts with extremely high rate constants (high reactivity) that approach diffusion-limited, with rate constants of 109–1010 M−1 s−1. So, •OH is the strongest (most powerful) oxidant of the oxyradicals; 3. UNSELECTIVE and INDISCRIMINATE: •OH reacts unselectively and indiscriminately with almost every type of molecule found in living cells, including lipids, proteins, amino acids, DNA, RNA and sugars. Therefore, the best antioxidant is not a •OH scavenger, but rather an iron chelate to prevent the generation of •OH.
The reaction with many substances in the body at a rate that exceeds that of H
2, which means H
2 is difficult to compete with these molecules effectively in the body, especially when H
2 is at a relatively lower concentration than other endogenous substances. Biokinetic analyses of the intracellular reactions of •OH/ONOO
- show that intracellular molecules, such as nucleic acids and amino acids, react with •OH more readily at a significantly faster rate than H
2 [
32,
33], which implies that H
2 can hardly act as an •OH scavenger or barely direct react with •OH.
In 2005, we first proposed the new concept of "mitochondrial nutrient". The so-called "Mitochondrial nutrients" refer to any compound that can protect mitochondria from damage, repair mitochondria injury, and promote mitochondrial function. Their mechanisms of action may include (1) protect mitochondrial enzymes and/or stimulate enzyme activity by increasing the levels of substrate and cofactors; (2) induce the activation of endogenous antioxidant systems such as phase II enzymes to enhance antioxidant defense; (3) prevent mitochondria from producing ROS and removing ROS in mitochondria, and (4) protect and repair mitochondrial damage, including energy promoters [
34,
35,
36].
Researchers in our lab reported that in the LPS-induced lung injury mouse model, hyperoxic HRS effectively reduced mitochondrial swelling and cristae breaks, as well as reversed the reduction of mitochondrial complex I, IV, and V activities significantly [
37,
38]. Not coincidentally, in the high-fat diet (HFD)-induced liver injury model, coral calcium hydride (CCH, a solid form of molecular H
2 carrier made from coral calcium) treatment improved glucose and lipid metabolism, ameliorated hepatic mitochondria abnormalities, restored the protein expression and the activity of complex II, while also activated phase II enzymes [
37,
38]. These studies imply that H
2 is able to target mitochondria, as a highly promising mitochondrial nutrient.
Ohsawa et al. [
13] used antimycin A (an inhibitor of mitochondrial respiratory complex III) to induce excess O
2•− production. In this model, O
2•− rapidly converted to H
2O
2, which was further converted to •OH. Their result showed that H
2 treatment prevented the decrease in mitochondrial membrane potential caused by antimycin A treatment, believing that H
2 protects mitochondria from •OH damage. The researchers hypothesized that H
2 enters the mitochondria and acts on the mitochondrial respiratory chain, weakening the Fenton reaction by inhibiting transition metal activity and ultimately inhibiting •OH production but not scavenging •OH directly [
39]. Accordingly, H
2 is considered as a potential and promising mitochondria-targeting molecule or nutrient that acts as a redox homeostasis regulator [
40].
As is well known, H
2 is a moderate/mild reducing agent (The standard reduction potential of H
+/H
2 at PH7 is -0.42, stronger than NAD
+/NADH -0.32 but weaker than acetate/acetaldehyde -0.60), barely able to scavenge •OH directly in a living body (
Figure 2). Because mitochondria are the main sites of ROS generation and the targets of ROS, we suggest that the more important mechanism of H
2 molecule maybe that can easily enter cells and subcellular organelles, including mitochondria, to play a protective role through their strong penetration ability, then activating the Keap1-Nrf2 (Kelch-1ike ECH-associated protein l, NF-E2-related factor 2) antioxidant defense system, to inhibit oxidative damage and improve the mitochondrial function, finally prevent and improve a various of disease. H
2 has been shown to significantly activate the Keap1-Nrf2 system, regulate the activities of endogenous antioxidants, and enhance the ability of cells to fight against damage [
41].
4. The mechanisms of H2 as an Nrf2 activator
Nrf2 is a key factor in the regulation of oxidative stress which belongs to the CNC-BZIP transcription factor family. Upon normal physiological conditions, Nrf2 binds to Keap1 to form a complex present in the cytoplasm in a low-activity state [
42]. When the organism is stimulated by oxidative stress or other pathological conditions, the cysteine residue of Keap1 is modified or Nrf2 is phosphorylated, then, Nrf2 is released from the complex and translocated to the nucleus where it binds to the antioxidant response elements (AREs) sequence in the nucleus, initiating NRF2-mediated transcriptional processes to activate a series of phase II antioxidant enzymes to generate antioxidants to scavenge ROS and other harmful substances.
It is reported that various ways can activate Nrf2, among which the Keap1-Nrf2 pathway is the most classical Nrf2 activation pathway. Keap1 contains multiple oxidative stress response sensor proteins which have different physiological functions in response to different forms of stress. Up to now, several studies have demonstrated that H
2 activated Nrf2 through the Keap1-Nrf2 system [
43,
44], but the clear mechanism of the activation is not known.
Nrf2 inducers are diverse, of which most are electrophilic and readily react with Keap1 by the cysteine thiol groups. Among them, Cys151/Cys273/Cys288 plays a fundamental role in the perception of electrophilic Nrf2-inducing chemicals. Therefore, the Nrf2 inducer has been divided into different categories based on the different cysteine residues of Keap1 they react with (
Table 1). The first class specifically targets the Cys151 sensor, such as medically relevant bardoxolone methyl. Bardoxolone methyl acts as an electrophilic inducer of Nrf2 that forms a covalent interaction with the Cys151 residue of Keap1, thereby inhibiting Nrf2 ubiquitination. In mice, the Cys151 point mutation in Keap1 eliminated Nrf2 signaling and hepatoprotective effect of bardoxolone methyl in vivo [
45]. The second class of inducer targets Cys288, and 15-deoxy-prostaglandin J2 (15d-PGJ2) has been identified in this group. 15d-PGJ2, one of the endogenous Nrf2 inducers synthesized from arachidonic acid, forms a covalent compound with Keap1 to compete for the Keap1-Nrf2 binding. Class III targets Cys151/Cys273/Cys288, such as 4-hydroxynonenal (4-HNE). Mass spectrometry analysis revealed that 4-HNE directly modifies cysteine residues on Keap1 and deregulates its inhibition of Nrf2 by inhibiting Keap1, further increasing the expression levels of Nrf2 target genes (e.g. TXNRD1, thioredoxin reductase-1) [
46]. Indeed, Nrf2 activation was significantly reduced when Cys151 was mutated, whereas Nrf2-induced target gene activation was only slightly affected when Cys273 and Cys288 residues were mutated [
47,
48].
In addition, the electrophilic compound that activated Nrf2 on the cysteine residues other than Cys151/Cys273/Cys288, we classify as Class IV. The compounds of this group include, for example, Pubescenoside A, which acts on Cys77/Cys434.
Moreover, several inducers activate Nrf2 in a more complex way than previously identified electrophilic sensors that bind to Cys226, Cys613, Cys622, and Cys624. We classify them as Class V. Hydrogen peroxide (H
2O
2), a key ROS molecule important in cellular physiology, is representative of this classification. Suzuki et al. revealed that Keap1 uses cysteine residues to create a special mechanism to make a disulfide bond between any combination of Cys226, Cys613, Cys622 and Cys624 to sense H
2O
2 [
49]. This sensing mechanism is different from that used by the electrophilic Nrf2 inducer.
There is also a kind of inducers that do not act through the cysteine of Keap1 which have been classified as a Class VI, they directly inhibit the interaction between Keap1 and Nrf2, such as non-electrophilic protein-protein interaction inhibitors (PPIs) [
50]. Horie et al. suggested that Keap1 binding to Nrf2 is a ‘hinge and latch model’, with PPIs actively using a hinge-locking mechanism, whereas electrophilic Nrf2 activators do not use this mechanism when activating Nrf2 [
51].
As the smallest and one of the simplest molecules, H
2 molecules have the capacity to pass through the Keap1 and Nrf2 binding structure and play the role of an activator, the mechanism of Nrf2 activation by H
2 seems different from the mechanism of perception of electrophilic Nrf2 inducers but may be closer to the mechanism of class V and VI to inhibit the interaction between Keap1 and Nrf2 (
Figure 3) [
49]. Up to now, the activation of Nrf2 and its mediated antioxidant enzyme system by H
2 has been reported in a variety of tissue-associated diseases, including brain, lung, liver, heart, ovary and kidney [
43,
52,
53].
A result in neuroblastoma cells showed that exposure of SH-SY5Y cells to H
2 increased the production of mitochondrial superoxide. This process was accompanied by Nrf2 nucleus translocation, as well as increased expression of Nrf2-regulated antioxidant enzymes, suggesting that H
2 alleviates mitochondrial oxidative stress through activating Nrf2 [
54]. Inhaled H
2 also reduces neuroinflammation in memory-related regions through increasing Nrf2 protein expression in a sepsis-induced blood-brain barrier impairment and memory dysfunction [
55,
56]. Interestingly, one of the studies we were involved in reported that H
2 (2%-4%) protected against delayed encephalopathy after acute carbon monoxide poisoning and this protective effect was related to the involvement of Nrf2 and its mediated phase II enzyme system [
57].
Similar results were obtained in the lung from seawater instillation-induced acute lung injury rabbit or cecal ligation and puncture-induced sepsis mice, which proved that H
2 could regulate the expression of heme oxygenase-1 (HO-1), the Nrf2 downstream antioxidant protein [
58,
59]. Inhaled H
2 significantly alleviated the drop in blood O
2 during hyperoxic exposure, remitted lung inflammation, and upregulated the HO-1 expression. However, H
2 could not attenuate hyperoxic lung injury or induce HO-1 in Nrf2-KO mice, suggesting that H
2 could improve the hyperoxic lung injury through the Nrf2-HO-1 pathway [
60]. In the sepsis-induced acute lung injury model, H
2 molecules inhibited high mobility group protein1 (HMGB1) expression by activating the Nrf2-HO-1 pathway [
61,
62]. The latest research has revealed that H
2 also affected COVID-19-induced lung injury via Nrf2 [
63].
Sun et al. [
44] demonstrated that H
2 administration reduced oxidative stress in the LPS-treated mice livers through activation of the Keap1-Nrf2 system. Moreover, Liu et al. [
52] reported that H
2 improved lipid accumulation by modulating the miR-136/MEG3/Nrf2 pathway in non-alcoholic fatty liver disease.
In the ischemia model induced in the H9C2 cell line, the H
2 gas-rich medium reduced the production of •OH, promoted Nrf2 nuclear translocation and regulated the Nrf2-HO-1 pathway, suggesting that H
2 can preserve ischemic cardiomyocytes by stimulating the Nrf2 pathway [
64]. H
2 amelioration of LPS-injured HUVECs injured and inflammatory responses through Nrf2 and its downstream protein HO-1 [
65].
In the long-term cyclosporine A (CsA) induced nephrotoxicity model, HRW reduced the ROS and MDA levels, increased the activities of GSH and SOD, then improved the vascular and renal functions of rats with renal damage. Meanwhile, HRW significantly decreased the level of Keap1 while increasing the expression of Nrf2, NADPH dehydrogenase quinone1, and HO-1. Suggesting that HRW restored the balance of the redox state and improved CsA-induced renal function suggesting that HRW restored the balance of the redox state and improved CsA-induced renal function by activating the Keap1-Nrf2 signaling pathway [
43].
In the ovarian injury rat model induced by cisplatin, HRS recovered the activity of SOD and catalase, reduced MDA levels in serum and ovarian tissues, as well as increased ovarian Nrf2 expression [
66]. Inhalation of 2% H
2 also attenuated severe sepsis-induced intestinal injury by modulating HO-1 and HMGB1 release mice [
67].
4. Conclusions and perspectives
In conclusion, H2 medicine has risen as a bright star in gas medicine, but it faces a few problems. Firstly, in H2 basic research area, although a large number of H2 medicine-related studies have been carried out, the mechanisms of H2 effects are quite controversial. People do not have a high level of awareness of H2, doubts still exist about the efficacy and safety of H2. Therefore, more specific and clear mechanisms need to be clarified. This requires more outstanding scientists to join and make more efforts. This review attempts to challenge the view that H2 is a selective •OH scavenger by proposing that H2 is a mitochondria-targeting molecule/nutrient via activating the Keap1-Nrf2 antioxidant system. Of course, this is a quite premature idea and needs more and further investigations to test and challenge.
Secondly, in H2 industry, the market demand for H2 health products is insufficient. There are still many technical bottlenecks in the H2 medicine industry, such as low efficiency of H2 preparation, high storage and transport costs. In addition, the industrial chain of H2 medicine is incomplete, as well as lacks the development of relevant standards. The H2 health industry involves a number of links, such as H2 preparation, storage and transport, H2 generators, H2 testing, and so on. At present, these links have not formed a complete industrial chain, the connection between the links is not smooth enough. Due to the lack of complete and well-defined standards, the H2 industry chain is difficult to regulate with high quality.
Thirdly, insufficient policy support for H2 medicine. While the H2 health industry has a great potential for development, the current government support for the H2 health industry is insufficient and there are some deficiencies in the policy support, in terms of there is a lack of clear policy planning and support measures. Therefore, the market prospect of the H2 medicine industry is promising, which urgently needs to be promoted.