The effects of ingestion of hydrogen-dissolved alkaline electrolyzed water on stool consistency and gut microbiota: a double-blind randomized trial.
Tanaka Y, Kiuchi M, Higashimura Y, Naito Y, Koyama K. · Medical Gas Research. 2021;11:318858.
Study at a glance
Human
Healthy men aged 30 to 59 years.
500 mL/day hydrogen-dissolved alkaline electrolyzed water. · Approximately 0.3 mg/L dissolved H2; intervention water pH 9.5.
Two weeks.
Bifidobacterium abundance and stool-consistency distribution differed between groups, while alpha-diversity measures did not.
Very small exploratory trial; hydrogen concentration and alkalinity differed together, preventing clean attribution to H2.
What kind of evidence is this?
Human
Double-blind randomized parallel trial
Gastrointestinal health
Other forms
Mixed exploratory human signal.
Functional stool measure and microbiome outcomes.
Methods
Healthy men aged 30 to 59 years.
20 participants; 10 per group.
Two weeks.
500 mL/day hydrogen-dissolved alkaline electrolyzed water.
H2 dissolved in alkaline electrolyzed water; not Brown's gas.
Approximately 0.3 mg/L dissolved H2; intervention water pH 9.5.
Purified water with 0 mg/L H2 and pH 7.6.
Outcomes and reported result
Stool consistency and gut-microbiome composition and diversity.
Bifidobacterium abundance and stool-consistency distribution differed between groups, while alpha-diversity measures did not.
The full article, methods, microbiome analyses and significant and null outcomes were checked.
A reported association, difference or mechanism is not automatically a clinical benefit.
Limitations and applicability
Very small exploratory trial; hydrogen concentration and alkalinity differed together, preventing clean attribution to H2.
Healthy middle-aged men over two weeks.
Cochrane RoB 2 — preliminary
Some concerns
Randomization and blinding were reported, but the small sample, short follow-up and co-varying alkalinity create concerns.
Randomization reported; concealment not fully established · Deviations cannot be excluded · Available-case reporting checked · Outcome measurement varies by endpoint · No outcome-level protocol comparison completed
No single-study GRADE certainty rating is assigned. Read how records and evidence assessments are prepared.
Sources and record status
PMID: 34213495 · DOI: 10.4103/2045-9912.318858
Free full article in PubMed Central.
PubMed record and PubMed Central full text; extraction checked 9 August 2026.
10 August 2026
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