The 8 Glasses Myth
Myth: "You need to drink 8 glasses of water a day"
Reality: There is no scientific evidence for the 8x8 recommendation[1]. This widely repeated advice has no solid scientific foundation.
Reality: There is no scientific evidence for the 8x8 recommendation[1]. This widely repeated advice has no solid scientific foundation.
The "8 glasses a day" rule appears to come from a misinterpretation of a 1945 US Food and Nutrition Board recommendation that said people need about 2.5 litres of water daily – but this included water from all food and beverages, not just plain water.
What Science Actually Says
Evidence on Increased Water Intake
Clinical trials on water intake show mixed results[2]. Some benefits have been suggested for:
- Weight loss – Some studies found increased water consumption helped with weight loss
- Kidney stones – Higher fluid intake reduces recurrence of kidney stones
- Urinary tract infections – Increasing water intake halved UTI recurrence[3]
But for most other health claims about drinking more water (better skin, "detoxification", improved energy), evidence is weak or absent.
Overhydration Risks
Warning: Drinking too much water can be dangerous – Exercise-associated hyponatremia[4] is a serious condition caused by excessive fluid intake.
Hyponatremiaⓘ
At-Risk Groups for Overhydration
- Marathon runners and endurance athletes – Particularly those who drink on a schedule rather than by thirst
- People taking MDMA/ecstasy – The drug can cause excessive thirst and water retention
- Elderly people with certain medications – Some medications affect water balance
- People with psychiatric conditions – Psychogenic polydipsia (compulsive water drinking) is rare but serious
Practical Guidelines
When to Drink More
- During vigorous exercise or physical labour
- In hot or humid weather
- When ill with fever, vomiting, or diarrhoea
- When pregnant or breastfeeding
- If you have kidney stones or recurrent UTIs
Signs of Adequate Hydration
- Urine is pale yellow (not clear, not dark)
- You rarely feel very thirsty
- No symptoms of dehydration (dizziness, fatigue, dry mouth)
Signs of Dehydration
Significant dehydration impairs cognitive performance[6].
- Dark yellow urine
- Infrequent urination
- Thirst
- Dry mouth and lips
- Headache, dizziness, fatigue
Caffeine and Alcohol
Myth: "Coffee and tea don't count toward hydration"
Reality: Caffeinated beverages do contribute to daily fluid intake[7]. While caffeine has a mild diuretic effect, the water in coffee and tea more than compensates.
Reality: Caffeinated beverages do contribute to daily fluid intake[7]. While caffeine has a mild diuretic effect, the water in coffee and tea more than compensates.
Alcohol is different – it does have a significant diuretic effect, especially at higher concentrations. Beer in moderation provides some hydration, but spirits cause net fluid loss.
Bottom Line
- Trust your thirst – For most healthy people, this is sufficient
- Don't force yourself to drink – There's no benefit to drinking beyond thirst
- Adjust for circumstances – Exercise, heat, and illness increase needs
- Urine colour is a useful guide – Pale yellow indicates adequate hydration
- Overhydration is real – Especially during prolonged exercise
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References
- Valtin H (2002). Drink at least eight glasses of water a day - Really? Is there scientific evidence for 8 x 8?. American Journal of Physiology - Regulatory, Integrative and Comparative Physiology. [DOI]
- Hakam N, Guzman Fuentes JL, Nabavizadeh B et al. (2024). Outcomes in Randomized Clinical Trials Testing Changes in Daily Water Intake: A Systematic Review. JAMA Network Open. [DOI]
- Hooton TM et al. (2018). Effect of Increased Daily Water Intake in Premenopausal Women With Recurrent Urinary Tract Infections: A Randomized Clinical Trial. JAMA Internal Medicine. [DOI]
- Klingert M, Nikolaidis PT, Weiss K et al. (2022). Exercise-Associated Hyponatremia in Marathon Runners. Journal of Clinical Medicine. [DOI]
- Wittbrodt MT et al. (2018). Dehydration Impairs Cognitive Performance: A Meta-analysis. Medicine & Science in Sports & Exercise. [DOI]
- Valtin H (2002). Drink at least eight glasses of water a day - Really? Is there scientific evidence for 8 x 8?. American Journal of Physiology - Regulatory, Integrative and Comparative Physiology. [DOI]