Introduction
Molecular hydrogen (i.e. H2 gas) is gaining significant attention from academic researchers, medical doctors, and physicians around the world for its recently reported therapeutic potential 1. One of the earliest publications on hydrogen as a medical gas was in 1975, by Dole and colleagues from Baylor University and Texas A&M 2. They reported in the journal Science that hyperbaric (8 atm) hydrogen therapy was effective at reducing melanoma tumors in mice. However, the interest in hydrogen therapy only recently began after 2007, when it was demonstrated that administration of hydrogen gas via inhalation (at levels below the flammability limit of 4.6%) or ingestion of an aqueous-solution containing dissolved hydrogen, could also exert therapeutic biological effects 3. These findings suggest hydrogen has immediate medical and clinical applications 4.
In 2007, Dr. Ohta’s team reported in Nature Medicine 3 that inhalation of 2-4% hydrogen gas significantly reduced the cerebral infarct volumes in a rat model of ischemia-reperfusion injury induced by middle cerebral artery occlusion. 
Hydrogen was more effective than edaravone, an approved clinical drug for cerebral infarction, but with no toxic effects (See figure on left). The authors further demonstrated that dissolved hydrogen in the media of cultured cells, at biologically relevant concentrations, reduces the level of toxic hydroxyl radicals (*OH), but does not react with other physiologically important reactive oxygen species (e.g. superoxide, nitric oxide, hydrogen peroxide).
This biomedical research on hydrogen was still in its infancy 2 years with only around 500 articles and 1,600 researchers, but these publications have now doubled to over 1,000 articles and researchers suggest that hydrogen has therapeutic potential in over 170 different human and animal disease models, and in essentially every organ of the human body 5. Hydrogen appears to provide these benefits via modulating signal transduction, protein phosphorylation, and gene expressions (See section Pharmacodynamics) 4.
The idea of therapeutic gaseous molecules is not a new concept. For example, carbon monoxide (CO), hydrogen sulfide (H2S), and of course nitric oxide (NO*), which was initially ridiculed by skeptics, but later was subject to a Nobel Prize, are all biologically active gases 6. However, it may still be difficult to believe that H2 can exert any biological effect, because in contrast to these other gases, hydrogen is a non-radical, non-reactive, non-polar, highly diffusible neutral gas, thus it is unlikely to have specific binding sites, or interact with specificity on a specific receptor 7.
From an evolutionary perspective it may not be strange that hydrogen exerts a biological effect 8. In addition to its role in the origins of the universe, hydrogen was also involved in the genesis of life and played an active role in the evolution of eukaryotes 9. Over the millions of years of evolution, plants and animals have developed a mutualistic relationship with hydrogen-producing bacteria resulting in basal levels of molecular hydrogen in eukaryotic systems. This constant exposure to molecular hydrogen may have conserved the original targets of hydrogen, as can be extrapolated by genetic remnants of hydrogenase enzymes in higher eukaryotes. Alternatively, but not exclusively, eukaryotes may have developed sensitivity to molecular hydrogen over the millions of years of evolution 7, 10.
Methods of Administration
Molecular hydrogen can be administered via inhalation 11, ingestion of dissolved hydrogen-rich solutions (e.g. water, flavored beverages, etc.) 12, hydrogen-rich kidney dialysis solution 13, intravenous injection of hydrogen-rich saline 14, topical administration of hydrogen-rich media (e.g. bath, shower, and creams) 15 hyperbaric treatment 2, ingestion of hydrogen-producing material upon reaction with gastric acid 15, ingestion of non-digestible carbohydrates as pre-biotic to hydrogen-producing intestinal bacteria 16, rectal insufflation 17, and other methods 15.
Hydrogen’s unique physicochemical properties of hydrophobicity, neutrality, size, mass, etc. afford it with superior distribution properties allowing it to rapidly penetrate biomembranes (e.g. cell membranes, blood-brain, placental, and testis barrier) and reach subcellular compartments (e.g. mitochondria, nucleus, etc.) where it can exert its therapeutic effects 15.
Although various medical clinics in Japan use intravenous injection of hydrogen-rich saline, the most common methods are inhalation and drinking hydrogen-rich water. The pharmacokinetics of each method are still under investigation, but are dependent on dosage, route, and timing. An article published in Nature’s Scientific Reports 18 compared inhalation, injection and drinking with different hydrogen concentrations and found helpful insights for clinical use. Based on this and various studies, we briefly summarize the pharmacokinetics of inhalation and drinking.
Inhalation of Hydrogen
For inhalation, a 2-4% hydrogen gas mixture is common because it is below the flammability level; however, some studies use 66.7% H2 and 33.3% O2, which is nontoxic and effective, but flammable. Inhalation of hydrogen reaches a peak plasma level (i.e. equilibrium based on Henry’s Law) in about 30 min, and upon cessation of inhalation the return to baseline occurs in about 60 min.
Drinking Dissolved Hydrogen
The concentration/solubility of hydrogen in water at standard ambient temperature and pressure (SATP) is 0.8 mM or 1.6 ppm (1.6 mg/L). For reference, conventional water (e.g. tap, filtered, bottled, etc.) contains less than 0.0000002 ppm of H2, which is well below the therapeutic level . The concentration of 1.6 ppm is easily achieved by many methods, such as simply bubbling hydrogen gas into water. Because of molecular hydrogen’s low molar mass (i.e. 2.02 g/mol H2 vs. 176.12 g/mol vitamin C), there are more hydrogen molecules in a 1.6-mg dose of H2 than there are vitamin C molecules in a 100-mg dose of pure vitamin C (i.e. 1.6 mg H2 has 0.8 millimoles of H2 vs. 100 mg vitamin C has 0.57 millimoles of vitamin C).
The half-life of hydrogen-rich water is shorter than other gaseous drinks (e.g. carbonated or oxygenated water), but therapeutic levels can remain for a sufficiently long enough time for easy consumption. Ingestion of hydrogen-rich water results in a peak rise in plasma and breath concentration in 5-15 min in a dose-dependent manner (see figure). The rise in breath hydrogen is an indication that hydrogen diffuses through the submucosa and enters systemic circulation where it is expelled out the lungs. This increase in blood and breath concentration returns to baseline in 45-90 min depending on the ingested dosage.
Source: Molecular Hydrogen Institute
