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Research · Food and drink

Water intake

Single trials found extra water cut repeat kidney stones and bladder infections in women who drank little, and added about 2 kg to weight loss on a diet. It did not slow kidney decline or cut headache days. No study supports eight glasses a day for everyone. Overdrinking in a marathon was linked to low blood sodium.

Add to Bio HackChecked 2026-11-13
Status
A food, not a regulated health product. The Dietary Guidelines for Americans, 2025-2030 call hydration a key factor in overall health, say to choose water (still or sparkling) and unsweetened beverages, and say to avoid sugar-sweetened beverages [1]. The National Academies set an Adequate Intake for total water, from drinking water, other beverages and food, of 3.7 L a day for men and 2.7 L for women aged 19 to 30, of which beverages supplied about 3.0 L (about 13 cups) and 2.2 L (about 9 cups). They state this is not a requirement, and was set from median US intakes [2].
Also called
drinking water, water, plain water, tap water, bottled water, sparkling water, hydration, fluid intake, 8 glasses a day, 8x8 rule, eight glasses of water
Studied for
weight loss, appetite, kidney stones, kidney health, urinary tract infections, headache, focus, concentration, hydration, eight glasses a day

Strong several reviews agree · Moderate one review or consistent trials · Limited one or a few small studies · Untested no study in people, listed with what it rests on

Shown to help3

Reported risks2

What it was studied for, and what was found

Shown to help3

Limited

Kidney stones coming back

One trial found drinking enough water to pass at least 2 L of urine a day cut kidney stone recurrence over 5 years, about 12% against 27%.

Compared with standard water intake [3]

The numbers and sources
Effect
In people who had had a stone, increased water intake lowered stone recurrence (RR 0.45, 95% CI 0.24 to 0.84), 149 fewer per 1,000 against 270 per 1,000 over 5 years with standard intake, and lengthened the time to recurrence (HR 0.40, 95% CI 0.20 to 0.79). The Cochrane review found no randomized trials in people who had never had a stone, and no evidence on adverse events [3].
Population
220 people with a history of urinary stones, 199 analyzed, in one randomized trial [3].
Sources
[3]
Caveats
One trial, rated low certainty for study limitations and imprecision [3]. Nothing is known from trials about preventing a first stone.
LimitedStudied in women

Bladder infections in women

One trial found 1.5 L of extra water a day cut recurrent bladder infections over 12 months, 1.7 against 3.2 episodes, in premenopausal women who drank little.

Compared with no additional fluids [4]

The numbers and sources
Effect
Mean cystitis episodes over 12 months were 1.7 in the water group and 3.2 in the control group (difference 1.5, 95% CI 1.2 to 1.8). Antibiotic courses were 1.9 and 3.6, and the time between episodes 142.8 and 84.4 days. Urine volume rose 1.4 L a day with water against 0.1 L in controls [4].
Population
140 healthy premenopausal women, mean age 35.7, with 3 or more episodes of cystitis in the past year and under 1.5 L of daily fluid [4].
Sources
[4]
Caveats
One open-label trial. It was funded by Danone Research, which sells bottled water and supplied the water used, and four authors were its employees [4].
Limited

Focus and attention

Small trials found drinking water improved some attention and short-term memory tasks in adults and children, with no change on other tasks.

Compared with no drink [11]; school classes without the water program [12]

The numbers and sources
Effect
In adults, a drink of water improved a letter cancellation task and thirst ratings, whether or not people were told to expect a benefit, with no change in digit span or reaction time [11]. In a cluster-randomized school trial, the amount of water children drank had a nonlinear relation to executive function scores, with benefits on memory tasks up to 1,000 mL [12].
Population
47 adults [11]. 250 children in grades 5 and 6 in 14 classes in Germany [12].
Sources
[11], [12]
Caveats
Small, short trials. The school trial result relates the amount drunk to scores, not only the randomized comparison [12].

Mixed results3

Limited

Weight loss alongside a diet

Limited evidence: 500 mL of water before meals added about 2 kg of weight loss to a 12-week diet in one trial. Trials outside dieting found none.

Compared with the same calorie-reduced diet without the water [5]; weight-loss or maintenance programs alone, and no added water [6]

The numbers and sources
Effect
On a calorie-reduced diet, adults who drank 500 mL of water before each main meal lost about 2 kg more over 12 weeks than those on the diet alone, and ate less at a test meal after a water preload at the start of the study [5]. A systematic review found that in dieters, more water plus a weight program reduced weight over 3 to 12 months against the program alone, while in people not dieting, 2 short trials found no effect on weight and 6 cross-sectional studies were inconsistent [6].
Population
48 adults aged 55 to 75 with a BMI of 25 to 40 [5]. 11 original studies and 2 systematic reviews in adults [6].
Sources
[5], [6]
Caveats
One small trial. The review rated the evidence low, mostly for lack of good-quality studies [6].
Limited

Water in place of sugary drinks

Mixed results: one trial found swapping caloric drinks for water did not change weight loss against an attention control. Cohorts linked the swap to less weight gain.

Compared with attention controls who made dietary changes of their own choosing [7]; one serving a day of sugar-sweetened drinks, in a substitution model [8]

The numbers and sources
Effect
Over 6 months, weight fell 2.03% in the group replacing caloric drinks with water and 1.76% in the attention control group, not significantly different. Fasting glucose fell more in the water group than in controls (P = 0.019). Those assigned to water or diet drinks combined were twice as likely to lose 5% of their weight (OR 2.07, 95% CI 1.02 to 4.22) [7]. In cohorts, replacing one daily serving of sugar-sweetened drinks with a cup of water was linked to 0.49 kg less weight gain every 4 years (95% CI 0.32 to 0.65), and replacing fruit juice to 0.35 kg less [8].
Population
318 adults with overweight or obesity, 84% female, who drank at least 200 kcal a day of caloric drinks [7]. 124,988 US women and men followed in 4-year periods for up to 20 years [8].
Sources
[7], [8]
Caveats
One trial. The cohort estimate is a statistical substitution, observational, with confounding by other habits [8]. Sugary drinks are on the added sugar page.
Limited

Headache

Mixed results: in one trial, 1.5 L of extra water a day improved headache-related quality of life but did not reduce days with moderate headache.

Compared with written advice on stress and sleep alone [9]; placebo medication [10]

The numbers and sources
Effect
Over 3 months, drinking more water raised migraine-specific quality of life by 4.5 points (95% CI 1.3 to 7.8), and 47% in the water group reported much improvement against 25% of controls. Days with at least moderate headache did not change meaningfully [9]. In a pilot trial, extra water lowered total headache hours by 21 in 2 weeks (95% CI -48 to 5) and intensity by 13 mm on a 100 mm scale (95% CI -32 to 5), both with intervals crossing zero [10].
Population
102 primary care patients with recurrent headache drinking under 2.5 L a day [9]. 18 people with migraine, followed 12 weeks [10].
Sources
[9], [10]
Caveats
Small trials in which participants knew their group, with quality of life self-reported.

Not shown to help1

Limited

Kidney function in chronic kidney disease

One trial found coaching adults with chronic kidney disease to drink more water did not slow the fall in kidney function over 1 year.

Compared with coaching to keep usual water intake [13]

The numbers and sources
Effect
Urine volume rose 0.6 L a day more with coaching. eGFR fell 2.2 mL/min/1.73 m² with more water and 1.9 in controls (adjusted difference -0.3, 95% CI -1.8 to 1.2). Creatinine clearance was 3.6 higher and plasma copeptin 2.2 pmol/L lower, and urine albumin did not differ significantly [13].
Population
631 adults with stage 3 chronic kidney disease, mean age 65, 63% men, in Ontario, Canada [13].
Sources
[13]
Caveats
One trial, which the authors say may have been too small to detect a clinically important difference [13].

Amounts studied

The National Academies' Adequate Intake is 3.7 L of total water a day for men and 2.7 L for women, about 81% from drinks and 19% from food in US survey data [2]. Trials added 1.5 L of water a day [4][9], 500 mL before each main meal [5], or enough to pass at least 2 L of urine a day [3]. Coaching in kidney disease raised urine volume by 0.6 L a day [13]. The drink-swap trial replaced at least 200 kcal a day of caloric drinks [7].

Risks

Reported risks2

LimitedVery common

Low blood sodium from overdrinking

Linked to low blood sodium in a marathon cohort: 13% of runners tested at the finish had it, and weight gain during the race was a risk factor.

The numbers and sources
How common
Very common: more than 1 in 10 people.
Effect
At the finish, 13% of runners had hyponatremia (sodium 135 mmol/L or less) and 0.6% had critical hyponatremia (120 mmol/L or less). Weight gain during the race (OR 4.2, 95% CI 2.2 to 8.2), a race time over 4 hours, and very low or high BMI were linked to it. Drinking more than 3 liters during the race was linked in single-factor analyses. The type of fluid drunk and use of NSAIDs were not linked [14].
Population
488 runners with usable blood samples, of 766 enrolled, in the 2002 Boston Marathon [14].
Sources
[14]
Caveats
One race, with fluid intake self-reported. Sodium drinks and sports hyponatremia are on the electrolytes page.
ModerateHow often is not known

Dehydration and thinking

Shown in pooled experiments to impair attention, executive function and motor coordination when people were dehydrated, more so above a 2% loss of body weight.

The numbers and sources
How common
How often is not known: the sources give no rate.
Effect
Across studies, dehydration impaired cognitive performance (effect size -0.21, 95% CI -0.31 to -0.11), most for attention (-0.52), motor coordination (-0.40) and executive function (-0.24), and not significantly for reaction time (-0.10). Impairment was larger above 2% body weight loss (-0.28) than at 2% or less (-0.14) [15].
Population
413 people in 33 studies, dehydrated by 1% to 6% of body weight [15].
Sources
[15]
Caveats
Studies used different ways to dehydrate people, such as heat, exercise or fluid restriction [15].

Claims tested and not found

  1. That everyone needs eight 8-ounce glasses of water a day. A review searching for the origin and evidence of the advice found no scientific studies supporting it [16]. The National Academies set intake values from median US intakes, counting water from all drinks and food, and state that normal hydration is kept over a wide range of intakes [2].
  2. That drinking more water causes weight loss without a diet. In people not dieting, two short trials found no effect on weight [6].
  3. That more water slows kidney decline in chronic kidney disease. Coaching to drink more did not slow the fall in eGFR over 1 year [13].
  4. That more water cuts the number of headache days. Days with at least moderate headache did not change meaningfully over 3 months [9].

Related

ElectrolytesAdded sugar

Sources

  1. US Department of Agriculture and US Department of Health and Human Services. Dietary Guidelines for Americans, 2025-2030. realfood.gov. 2026. https://cdn.realfood.gov/DGA.pdf
  2. Institute of Medicine. Dietary Reference Intakes for Water, Potassium, Sodium, Chloride, and Sulfate. The National Academies Press. 2005. https://doi.org/10.17226/10925
  3. Bao Y, Tu X, Wei Q. Water for preventing urinary stones. Cochrane Database of Systematic Reviews. 2020. https://doi.org/10.1002/14651858.cd004292.pub4
  4. Hooton TM, Vecchio M, Iroz A, et al. Effect of Increased Daily Water Intake in Premenopausal Women With Recurrent Urinary Tract Infections: A Randomized Clinical Trial. JAMA Internal Medicine. 2018. https://doi.org/10.1001/jamainternmed.2018.4204
  5. Dennis EA, Dengo AL, Comber DL, et al. Water consumption increases weight loss during a hypocaloric diet intervention in middle-aged and older adults. Obesity. 2010. https://doi.org/10.1038/oby.2009.235
  6. Muckelbauer R, Sarganas G, Grüneis A, Müller-Nordhorn J. Association between water consumption and body weight outcomes: a systematic review. The American Journal of Clinical Nutrition. 2013. https://doi.org/10.3945/ajcn.112.055061
  7. Tate DF, Turner-McGrievy G, Lyons E, et al. Replacing caloric beverages with water or diet beverages for weight loss in adults: main results of the Choose Healthy Options Consciously Everyday (CHOICE) randomized clinical trial. The American Journal of Clinical Nutrition. 2012. https://doi.org/10.3945/ajcn.111.026278
  8. Pan A, Malik VS, Hao T, et al. Changes in water and beverage intake and long-term weight changes: results from three prospective cohort studies. International Journal of Obesity. 2013. https://doi.org/10.1038/ijo.2012.225
  9. Spigt M, Weerkamp N, Troost J, et al. A randomized trial on the effects of regular water intake in patients with recurrent headaches. Family Practice. 2012. https://doi.org/10.1093/fampra/cmr112
  10. Spigt MG, Kuijper EC, Schayck CP, et al. Increasing the daily water intake for the prophylactic treatment of headache: a pilot trial. European Journal of Neurology. 2005. https://doi.org/10.1111/j.1468-1331.2005.01081.x
  11. Edmonds CJ, Crombie R, Ballieux H, et al. Water consumption, not expectancies about water consumption, affects cognitive performance in adults. Appetite. 2013. https://doi.org/10.1016/j.appet.2012.10.016
  12. Drozdowska A, Falkenstein M, Jendrusch G, et al. Water Consumption during a School Day and Children's Short-Term Cognitive Performance: The CogniDROP Randomized Intervention Trial. Nutrients. 2020. https://doi.org/10.3390/nu12051297
  13. Clark WF, Sontrop JM, Huang SH, et al. Effect of Coaching to Increase Water Intake on Kidney Function Decline in Adults With Chronic Kidney Disease: The CKD WIT Randomized Clinical Trial. JAMA. 2018. https://doi.org/10.1001/jama.2018.4930
  14. Almond CS, Shin AY, Fortescue EB, et al. Hyponatremia among runners in the Boston Marathon. The New England Journal of Medicine. 2005. https://doi.org/10.1056/nejmoa043901
  15. Wittbrodt MT, Millard-Stafford M. Dehydration Impairs Cognitive Performance: A Meta-analysis. Medicine and Science in Sports and Exercise. 2018. https://doi.org/10.1249/mss.0000000000001682
  16. Valtin H. "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. 2002. https://doi.org/10.1152/ajpregu.00365.2002

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