Hydrogen and the Brain: What the Research Reports on Getting Past the Skull

Red light therapy and molecular hydrogen are sometimes discussed together, and the pairing sits on top of a real research question — one closer to physics than to biology. Red light and molecular hydrogen are aimed at roughly the same place inside a brain cell. They arrive there by completely different routes — and one of those routes runs straight through the skull.

Light Has to Get Through Bone First

Transcranial photobiomodulation asks a lot of a photon. It has to leave the device, cross hair and scalp, pass through the frontal bone, and still have enough energy left to do something at the tissue underneath.

What Tedford and colleagues measured

Tedford and colleagues published a quantitative analysis of transcranial and intraparenchymal light penetration in human cadaver brain tissue in Frontiers in Neurology in 2024. They measured how much energy from consumer-style LED devices and from laser sources actually survives the trip inward, rather than modeling it.

The number that reframes the conversation

The researchers reported that more than 99.99% of the energy from 50 mW and 200 mW LED devices was absorbed by scalp, skull and tissue before reaching 3 cm of depth. At the frontal bone specifically, they reported roughly 2% transmission for 1064 nm laser light and about 3.7% for 810 nm LED light. That is a delivery constraint, not a mechanism failure. Light works. Getting it where you want it is the hard part.

Molecular Hydrogen Has the Opposite Delivery Story

Which brings us to the reason hydrogen keeps showing up in the same conversations as red light.

The sentence in Ohsawa's own abstract

Ohsawa, Ishikawa, Takahashi and colleagues published their foundational paper in Nature Medicine in 2007. The abstract contains a line that is easy to skim past: hydrogen works, they wrote, "owing to its ability to rapidly diffuse across membranes, it can reach and react with cytotoxic ROS." Diffusion, not delivery engineering.

Small, neutral, uncharged

H2 is the smallest molecule there is. It carries no charge, it is not water-soluble in the way that keeps larger compounds trapped in circulation, and the research literature treats its movement across membranes as a given rather than as an obstacle to be solved. No bone to cross. No absorption curve to fight.

What the Blood-Brain Barrier Actually Screens For

The blood-brain barrier is the reason most promising compounds never become brain compounds.

Why so little gets through

Endothelial cells lining the brain's capillaries are packed together far more tightly than they are elsewhere in the body, and the pharmacology literature has long described the resulting filter as favoring molecules that are small, lipid-soluble and electrically neutral. Most drug candidates fail that screen. Whole categories of otherwise promising compounds have been abandoned not because they did not work in a dish, but because nobody could get them past those tight junctions in a living animal at a dose that made sense. Hydrogen was never screened out of it.

Where the search query comes from

People who type hydrogen water blood brain barrier into a search bar are asking a delivery question, whether or not they would phrase it that way. Can this thing actually get to brain tissue? Ohsawa and colleagues answered the diffusion half of that question in 2007, and researchers have been building on it since.

Photobiomodulation's Mechanism, Reported Fairly

None of this makes red light therapy a lesser idea. The mechanism researchers describe for it is elegant, and it is worth stating properly before pivoting away.

Cytochrome c oxidase and ATP

The working model in the photobiomodulation literature is that red and near-infrared light is absorbed by cytochrome c oxidase in the mitochondrial respiratory chain, which researchers have associated with increased ATP production and downstream changes in cellular signaling.

What Salehpour and colleagues described

Salehpour and colleagues, in a 2018 narrative review of brain photobiomodulation therapy, catalogued the range of neurological function and behavioral outcomes that investigators had explored with transcranial light, and described the mitochondrial absorption model as the field's central hypothesis. Their review is a fair, generous account of a young field — and it is the account we would point a curious reader toward.

The Byproduct That Connects the Two Modalities

Here is the hinge of the whole argument, and it did not come from us.

Wang and colleagues on mitochondrial dynamics

Wang and colleagues, writing in 2023 on photobiomodulation for global cerebral ischemia, framed the intervention in terms of mitochondrial dynamics and function — fission, fusion, and the energetics of stressed brain tissue. Push mitochondrial output up and reactive oxygen species come along with it. That is not a criticism of light therapy; it is a description of how mitochondria behave when you ask them to work harder.

Hong 2021: Researchers Studied Both Together

In 2021, a group of researchers stopped treating these as two separate topics.

Their stated rationale

Hong, Hu, Lin and Wu published a pilot study in Medicine on the concomitant use of hydrogen water and photobiomodulation. Their reasoning, in their own abstract: photobiomodulation may enhance mitochondrial function and boost ATP, "however, this process can cause increased reactive oxygen species (ROS) production. Molecular hydrogen (H2) is a potent and possibly therapeutic antioxidant that can mitigate the effect of ROS," and so, in their words, the concomitant hydrogen "may clear additional ROS" that the light exposure itself generates.

What the pilot recorded

Eighteen participants with Parkinson disease at Hoehn and Yahr stages II to III used daily photobiomodulation plus hydrogen water for two weeks. The researchers reported that UPDRS scores began significantly decreasing from the first week, and that no adverse event was recorded during the study.

What a proof-of-concept study is

The authors called it exactly that — a proof-of-concept, hypothesis-generating result warranting a larger trial. We are reporting their framing, not upgrading it. Eighteen people, two weeks, no control arm. What makes it worth reading is not the effect size. It is that the delivery logic came from the investigators themselves.

Selective, Not Sweeping

The second half of Ohsawa's 2007 paper is the part that explains why researchers reached for hydrogen rather than a conventional antioxidant.

The hydroxyl radical, specifically

Ohsawa and colleagues reported that hydrogen selectively reduced the hydroxyl radical — the most cytotoxic reactive oxygen species — and did not react with other reactive oxygen species that possess physiological roles. They also reported that inhaled hydrogen suppressed brain injury in a rat model of focal ischemia and reperfusion. Selectivity is the whole point. Blunting every oxidative signal in a cell is not obviously desirable, which is a distinction we pulled apart at length in our piece on hydrogen water as a selective antioxidant.

The Year-Long Trial in Mild Cognitive Impairment

Human data in this space is thinner than the preclinical literature, which makes the trials that do exist worth reading closely.

How Nishimaki and colleagues built the study

Nishimaki, Asada, Ohsawa and colleagues published a randomized, double-blind, placebo-controlled study in Current Alzheimer Research in 2018. Seventy-three subjects with mild cognitive impairment drank roughly 300 mL of hydrogen-rich water per day for a full year — an unusually long run for this field.

The APOE4 carriers

Across the whole group, the researchers reported no significant difference on ADAS-cog. Within the APOE4-genotype carriers, though, the hydrogen group improved significantly on total ADAS-cog and on word recall, which is the finding that sent other investigators looking at genotype as a variable. A null main result and a real subgroup signal, in the same paper, reported honestly by the people who ran it. That is what an evidence base looks like while it is still being built.

What the Animal Research Adds

Two animal studies carry a lot of the mechanistic weight here, and both deserve their species label stated out loud.

Gu and colleagues' mice

Gu, Huang, Inoue and colleagues reported in the Journal of Clinical Biochemistry and Nutrition in 2010 that drinking hydrogen water ameliorated cognitive impairment in senescence-accelerated mice. Mice, not people. The value of the SAMP8 model is that it compresses a slow process into a timeframe a study can actually observe.

Tian and colleagues on barrier permeability

Tian and colleagues, publishing in Brain Research in 2016, reported that hydrogen-rich water attenuated brain damage and inflammation after traumatic brain injury in rats, with reduced blood-brain barrier permeability, decreased brain edema, and activation of the Nrf2 pathway. Rats, again — and the barrier result is the one that matters for this article, because it puts the blood-brain barrier on both sides of the hydrogen question at once.

The Safety Record Investigators Keep Reporting

One thing runs consistently through this literature, and it tends to get buried under the mechanism discussion.

Hong and colleagues recorded no adverse event across their two-week pilot. Nishimaki and colleagues kept 73 people drinking hydrogen-rich water every day for a full year in a randomized, placebo-controlled design, and the reason that trial could run that long at all is that nothing about the intervention required them to stop it early. Ohsawa's group worked with inhaled hydrogen in animals at concentrations far above anything a consumer device produces. Across the published human work on molecular hydrogen, the tolerability reporting has been consistently unremarkable.

That is a meaningful part of why hydrogen therapy has stayed on researchers' radar for nearly two decades while flashier candidates have come and gone.

Where the Research Goes Next

Hong and colleagues asked for a larger trial of the combination, and that is the obvious next step. Nishimaki's APOE4 result points at genotype-stratified designs. The animal work on blood-brain barrier permeability points at injury models. Three open threads, all pulling in the same direction, all with published starting points already in the literature. What none of them require is a new mechanism — the diffusion argument and the selective-antioxidant argument were both laid down in 2007, and the work since has mostly been a matter of picking populations, endpoints and durations. That is a healthier position for a research field than it sounds. The shared-mechanism argument between these two modalities is one we laid out separately in hydrogen water and red light therapy, and the broader cognitive literature gets its own treatment in our piece on hydrogen water and brain health.

The Short Version

Red light therapy has a mechanism researchers find compelling and a delivery constraint researchers have now measured precisely. Molecular hydrogen has a mechanism aimed at the same mitochondrial target and a diffusion profile that the foundational paper treats as its defining feature. One fights bone. The other does not. Hong and colleagues were the first to run them together and report what happened, and their rationale — light raises ROS, hydrogen may clear it — is the hypothesis they published. It remains untested beyond an 18-person, two-week pilot with no control arm.

Frequently Asked Questions

Does hydrogen water reach the brain?

Ohsawa and colleagues wrote in their 2007 Nature Medicine abstract that hydrogen's effect owes to "its ability to rapidly diffuse across membranes," and they reported that inhaled hydrogen suppressed brain injury in a rat model of focal ischemia and reperfusion. Tian and colleagues separately reported reduced blood-brain barrier permeability in rats after traumatic brain injury. The animal evidence on distribution is the strongest part of this picture; human neurological outcome data remains a smaller literature.

Has hydrogen water been studied in people with cognitive impairment?

Nishimaki, Asada, Ohsawa and colleagues ran a randomized, double-blind, placebo-controlled study of 73 subjects with mild cognitive impairment who drank roughly 300 mL of hydrogen-rich water daily for one year. They reported no significant difference across the whole group on ADAS-cog, and significant improvement on total ADAS-cog and word recall among APOE4-genotype carriers in the hydrogen group.

Did anyone study hydrogen water and red light therapy together?

Yes. Hong, Hu, Lin and Wu published an 18-person pilot study in Medicine in 2021 using daily photobiomodulation plus hydrogen water for two weeks in Parkinson disease. They reported decreasing UPDRS scores from the first week with no adverse event recorded, and described the work as proof-of-concept warranting a larger trial.

Further Reading

• PMC7954968 — a critical review pulling the neuroprotective effects of molecular hydrogen into one place, and the fastest way to see how wide the preclinical literature has become.
• PMC6041198 — Salehpour's narrative review of brain photobiomodulation, useful if you want the light side of this story told generously and in detail.
• PMC7356229 — a systematic review of controlled animal studies on photobiomodulation and neuroinflammation, which shows how much of the field still rests on laboratory models.
• PMC9945713 — Wang's 2023 paper on light and mitochondrial dynamics in cerebral ischemia, the clearest statement of why ROS enters the photobiomodulation conversation at all.
• PMC9514540 — Qu and colleagues on repeated transcranial photobiomodulation and working memory in healthy older adults, a rare study in people rather than in mice.
• PMID 20490324 — the senescence-accelerated mouse work, worth reading for how the researchers set up a cognitive decline model you can study in months.
• PMID 26826009 — the rat traumatic brain injury study, notable for measuring barrier permeability and edema rather than behavior alone.

References

1. Ohsawa I, Ishikawa M, Takahashi K, et al. Hydrogen acts as a therapeutic antioxidant by selectively reducing cytotoxic oxygen radicals. Nature Medicine. 2007;13(6):688-694. PMID: 17486089. DOI: 10.1038/nm1577
2. Hong CT, Hu CJ, Lin HY, Wu D. Effects of concomitant use of hydrogen water and photobiomodulation on Parkinson disease: A pilot study. Medicine (Baltimore). 2021;100(4):e24191. PMID: 33530211. PMC7850666. DOI: 10.1097/MD.0000000000024191
3. Nishimaki K, Asada T, Ohsawa I, et al. Effects of Molecular Hydrogen Assessed by an Animal Model and a Randomized Clinical Study on Mild Cognitive Impairment. Current Alzheimer Research. 2018;15(5):482-492. PMID: 29110615. PMC5872374. DOI: 10.2174/1567205014666171106145017
4. Gu Y, Huang CS, Inoue T, et al. Drinking hydrogen water ameliorated cognitive impairment in senescence-accelerated mice. Journal of Clinical Biochemistry and Nutrition. 2010;46(3):269-276. PMID: 20490324. PMC2872234
5. Tian R, et al. Hydrogen-rich water attenuates brain damage and inflammation after traumatic brain injury in rats. Brain Research. 2016. PMID: 26826009
6. Tedford C, et al. Quantitative analysis of transcranial and intraparenchymal light penetration in human cadaver brain tissue. Frontiers in Neurology. 2024.
7. Salehpour F, Mahmoudi J, Kamari F, et al. Brain Photobiomodulation Therapy: a Narrative Review. Molecular Neurobiology. 2018;55(8):6601-6636. PMID: 29327206. PMC6041198
8. Wang R, et al. Photobiomodulation for Global Cerebral Ischemia: Targeting Mitochondrial Dynamics and Functions. 2023. PMC9945713

The information in this article is provided for educational purposes only and should not be considered medical advice. Nothing here is intended to diagnose, treat, cure, or prevent any disease. Always consult a qualified healthcare provider before beginning any new wellness practice, especially if you have a medical condition, are pregnant or nursing, or take prescription medications.