It was initially suggested that the beneficial effect of hydrogen was due to an antioxidant as hydrogen selectively neutralized cytotoxic hydroxyl radicals 1 in vitro. However, although H2 reduces *OH radicals 2, as has been shown in various systems 1, 4, 5, it may not occur via direct scavenging, and it also cannot fully explain all the benefits of hydrogen 6. For example, in a double-blinded placebo controlled trial in rheumatoid arthritis 3, hydrogen had a residual effect that continued improving the disease symptoms for four weeks after hydrogen administration was terminated 3. Many cell studies also show that pre-treatment with hydrogen has marked beneficial effects even when the assault (e.g. toxin, radiation, injury, etc.) is administered long after all the hydrogen has dissipated out of the system 7, 8, 9. Additionally, the rate constants of hydrogen against the hydroxyl radical are relatively slow (4.2 x 107 M-1 sec-1) 2, and the concentration of hydrogen at the cellular level is also quite low (micromolar levels), thus making it unlikely that H2 could effectively compete with the numerous other nucleophilic targets of the cell 10. Lastly, if the mechanism were primarily associated with scavenging of hydroxyl radicals, then we should see a greater effect from inhalation compared to drinking, but this is not always the case 11, 12. In short, we consider it inaccurate or at least incomplete to claim that the benefits of hydrogen are due to its acting directly as a powerful antioxidant. Indeed, hydrogen is selective because it is a very weak antioxidant and thus does not neutralize important ROS or disturb important biological signaling molecules. Nevertheless, a metabolic tracer study 13 using deuterium gas demonstrated that, under physiological conditions, deuterium gas is oxidized, and the oxidation rate of hydrogen increases with an increasing amount of oxidative stress 14, but the physicochemical mechanism for this may still not be direct radical scavenging 13. However, not all studies show that hydrogen is oxidized via mammalian tissues 15, and it has also been reported that deuterium gas did not exert a therapeutic effect in the model studied whereas 1H did (unpublished data).
Unlike conventional antioxidants 16, hydrogen does have the ability to reduce excessive oxidative stress 6, but only under conditions where the cell is experiencing abnormally high levels of oxidative stress that would be harmful and not hormetic.
One mechanism that hydrogen uses to protect against oxidative damage is by the activation of the Nrf2-Keap1 system and subsequent induction of the antioxidant response element (ARE) pathway, which leads to the production of various cyto-protective proteins like glutathione, catalase, superoxide dismutase, glutathione per-oxidase, heme-1 oxygenase, etc. 17, 18, 19. In some disease models, the benefits of hydrogen are negated by using Nrf2 gene knockouts 20, 21, Nrf2 genetic silencing using iRNA 22, or pharmacologically blocking the Nrf2 pathway 23, 24. Importantly, hydrogen only activates the Nrf2 pathway when there is an assault (e.g. toxin, injury, etc.) 23 as opposed to constituently acting as a promoter, which could be harmful 25, 26. The method that hydrogen activates the Nrf2 pathway remains unclear 17.
Besides the potential scavenging of hydroxyl radicals and/or activation of the Nrf2 pathway, hydrogen may ameliorate oxidative stress via a cell modulating effect 17 and reduce the formation of free radicals 27, such as down regulating the NADPH oxidase system 28. The various cell modulating effects of hydrogen are responsible for mediating the anti-inflammatory, anti-allergy, and anti-obesity effects of hydrogen. Hydrogen has been shown to downregulate pro-inflammatory cytokines (e.g. IL-1, IL-6, IL-8, etc.) 29, attenuate the activation of TNF-a 3, NF-?B 30, NFAT 12, 31, NLRP3 32, 33, HMGB1 34, and other inflammatory mediators 17. Additionally, hydrogen has beneficial effects on obesity and metabolism by increasing the expression of FGF21 35, PGC-1a 36, PPARa 36, and more. 37. Additional 2nd messenger molecules or transcription factors affected by hydrogen include ghrelin 38, JNK-1 28, ERK1/2 39, PKC 40, GSK 41, TXNIP 32, STAT3 42, ASK1 43, MEK 44, SIRT1 45, and many more. Over 200 bio-molecules are altered by hydrogen administration including over 1000 gene expressions.
However, the primary targets and master regulators responsible for these changes are still elusive 29. There are many feedback systems and loops to consider, which makes it difficult to determine if we are detecting the cause or the effect of hydrogen administration.
The exact mechanism of how hydrogen modulates signal transduction, gene expression, and protein phosphorylation is still being investigated 17. A recent publication 46 in Scientific Reports provides good evidence to suggest that one of the mechanisms through which hydrogen accomplishes the various cell-modulating effects is by modifying lipid per-oxidation in the cell membrane. In cultured cells, at biologically relevant concentrations, hydrogen suppressed the free radical chain reaction-dependent per-oxidation and recovered Ca2+-induced gene expressions, as determined by comprehensive micro-array analysis (see figure 6) 46.
Scientific Recognition of Hydrogen
Although the primary targets or exact biochemical mechanisms of hydrogen are still not fully understood, the therapeutic effect in cells, tissues, animals, humans and even plants 47 is becoming widely accepted due to the now over 500 peer-reviewed articles and the 1,600 researchers on the medical effects of hydrogen. The quality of the publications is also improving with an average impact factor (IF) of the journals publishing hydrogen is about 3. The table below shows a few of the studies published in the higher IF journals, which range from six to 27.
The goal of the Molecular Hydrogen Institute (MHI) is to help advance the research, education, and awareness of hydrogen as a therapeutic medical gas. It is uncommon to find a treatment that has both a high therapeutic potential and a high safety profile; hydrogen appears to fit this combination 6. Some researchers become interested in hydrogen simply due to its unforeseen ability to have a biological effect; with the realization that hydrogen is both safe and effective, a moral obligation develops to advance the research, education, and awareness of hydrogen as a medical gas.
Source: Molecular Hydrogen Institute