Molecular hydrogen (H2) has been accepted to be an inert and nonfunctional molecule in our body. Mammalian cells cannot metabolize H2 because of the lack of hydrogenase enzymes. We have overturned this concept by demonstrating that H2 reacts with strong oxidants such as hydroxyl radicals in cells, and proposed its potential for preventive and therapeutic applications. There are several methods to ingest or consume H2; inhaling H2 gas, drinking H2-dissolved water (H2-water), injecting H2-dissolved saline (H2-saline), or taking an H2 bath. H2 has a number of advantages exhibiting extensive effects: H2 rapidly diffuses into tissues and cells, and it is mild enough neither to disturb metabolic redox reactions nor to affect signaling reactive oxygen species; therefore, there should be no or little adverse effects of H2. The numerous publications on its biological and medical benefits revealed that H2 functions as an anti-inflammatory and anti-apoptotic, and stimulates energy metabolism by regulating the gene expressions. In addition to growing evidence obtained by model animal experiments, extensive clinical studies were performed or are under investigation. Owing to its great efficacy and lack of adverse effects, H2 has promising potential for clinical use against many diseases.
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