Narrative reviewindependent cross-checkabstract only checked I-FAST-08 standard review
Do we need eight glasses of water a day?
"Drink at least eight glasses of water a day." Really? Is there scientific evidence for "8 x 8"?
Overview
Valtin H. Am J Physiol Regul Integr Comp Physiol 2002 · PMID 12376390 · DOI 10.1152/ajpregu.00365.2002
No proof for 8 glasses in healthy adults in mild climates, but more is needed in heat, hard work and some diseases.
Does the study support the specific claim? Partly · Methodological quality: moderate
Plain-language summary
Study explained simply
- What did the researchers want to find out?
Is there evidence for '8 x 8' glasses of water a day?
- How was the study done?
Narrative review
- How many people, animals or samples were studied?
not applicable; Healthy adults
- What was actually found?
No evidence for 8 x 8.
- How large was the observed effect?
not applicable
- What do the results mean in everyday life?
Thirst usually suffices if healthy and in mild climate.
- What are the study's limitations?
Narrative; Old
- What does it say about Elawady's specific claim?
Supports C-FAST-09; partly C-FAST-10 (no fixed high intake), with stated exceptions.
- Do the results apply to Germany and Europe?
Temperate climate applies to Germany.
- What can explicitly not be concluded from this study?
Advice for hot climates or patients.
Scientific in-depth review
1. Full reference
- Authors
- Valtin H
- Journal
- Am J Physiol Regul Integr Comp Physiol
- Year
- 2002
- Identifier
- PMID 12376390 DOI 10.1152/ajpregu.00365.2002
- Verification
- verified – PubMed efetch 2026-10-25 (this agent)
- Retracted
- no
- Corrections
- none
- Extent checked
- abstract only
- Conflicts of interest and funding
not stated in abstract
2. Research question
Is there evidence for '8 x 8' glasses of water a day?
3. Study design and population
- Study design
- Narrative review – Narrative review
- Population
Healthy adults
- Country
USA
- Period
To 2002
- Sample size
not applicable
- Inclusion and exclusion criteria
As described in the abstract; details not available in abstract.
4. Methods and endpoints
- Exposure / intervention
Water intake
- Comparator
not applicable
- Endpoints
not applicable
- Statistical methods
not applicable
5. Actual results
- Results
No scientific studies found supporting 8 x 8; surveys suggest such amounts not needed in healthy adults in temperate climate; caffeinated drinks count; osmoregulation precise; large intakes advisable for some diseases and in vigorous work, especially hot climates.
- Absolute risks
not reported
- Authors' conclusion
Conclusion limited to healthy sedentary adults in temperate climates.
6. Strengths
- Careful scope
7. Weaknesses
- Narrative
- Old
8. Risk of bias
- Orienting tool
- none applicable
- Type of assessment
- simplified assessment (abstract only)
- Full formal assessment
- no
- Overall risk of bias
- not assessable
- Rationale
Review.
9. Assessment of the statistical interpretation
not applicable
10. Relevance to the specific claim
Supports C-FAST-09; partly C-FAST-10 (no fixed high intake), with stated exceptions.
11. Independent contrary findings
12. Transferability to Germany and Europe
Temperate climate applies to Germany.
13. Overall methodological judgement
Reasonable for healthy adults.
| Multidimensional assessment | |
|---|---|
| Authenticity of the source | verified |
| Methodological quality | moderate |
| Risk of bias | not assessable |
| Precision of results | not applicable |
| Directness | direct |
| Reproducibility | partly replicated |
| Consistency with the overall evidence | consistent |
| Regional transferability | moderate |
| Adequacy of the interpretation (by the archive) | not applicable |
14. Open questions
None beyond those stated.
15. Linked fact checks
- Drink water only when thirsty; there is no need to force a fixed daily amount. – Studies supporting the claim
- Do not drink much water because it distends the abdomen and raises abdominal pressure; even when thirsty during long fasting hours just wet the lips; fasting three days in a row each week without food and drink is not harsh. – Studies supporting the claim
- Drinking a lot of water harms the liver; one should drink only when thirsty and not 'flood' the body with water. – Studies with mixed or unclear results