Water: one proven job

There is exactly one thing water has been randomised for with a hard outcome, and it is kidney stones. Among people who had already passed one, drinking more than two litres a day cut the chance of another by about 61 percent. The plain numbers behind that are 12 percent against 27 percent over five years in one trial, and 8 percent against 56 percent over two to three years in the other. A 2026 review looked at the same question across 31 studies and rated the strength of that evidence low, which is the part usually dropped.

The numbers, per 100 g

What your body absorbs

• Plain water is not always the fastest way to take water in: Europe allows drinks with carbohydrate and electrolytes to say they speed water absorption during exercise. — — Evidence E source
  Status Authorised

Safety, limits and interactions

• Sports bodies frame fluid during exercise as balancing losses, and they name low blood sodium as a risk in the same sentence as dehydration. — athletes and physically active people Evidence E source
  The position says athletes should be well hydrated before exercise and drink enough during and after to balance fluid losses, and that sports beverages with carbohydrate and electrolytes may decrease the risk of both dehydration and hyponatremia

What people believe that the data does not support

• The two things Europe lets anyone claim about water are maintenance, not improvement: normal physical and cognitive function, and normal body temperature. — — Evidence E source
  Status Authorised; across the register 46 entries mention water and 3 are authorised
• The second authorised wording covers temperature regulation and nothing wider. — — Evidence E source
  Status Authorised

Who this works differently for

• For people who have already had a kidney stone, drinking more than two litres a day cut the chance of another one by about six tenths. — 1855 adults with a history of nephrolithiasis across 8 eligible trials (n = 1855) Evidence A source
  Water intake above 2 L a day, or fluids enough for urine output above 2.5 L a day, reduced stone recurrence with a relative risk of 0.39 (95% CI 0.19-0.80) in two trials; reducing soft drinks helped heavy soft-drink consumers (34% versus 41% with renal colic, p=0.023); supplements showed no benefit in two trials
• Behind that relative risk sit two trials with plain numbers: 12 percent of stones came back with extra water against 27 percent without, over five years. — one trial randomised people to more than 2 L a day of water or no treatment for 5 years; the second randomised people after shockwave lithotripsy to fluid enough for urine output above 2.5 L a day or no treatment for 2 to 3 years (n = two trials) Evidence A source
  12% recurrence against 27% (p = 0.008) in the five-year trial; 8% against 56% (p < 0.01) among stone-free participants in the lithotripsy trial; 46% against 18% became stone free among those with residual fragments; pooled reduction 61% (RR 0.39; 95% CI 0.19-0.80)

What is still unknown

• A 2026 review looked again at the same question and rated the strength of that evidence low, which is the part usually left out. — 31 studies, 26 randomised and 5 non-randomised, in non-pregnant adults and children; all but 3 included adults only (n = 31 studies) Evidence A source
  Increased water intake, a diet with normal to high calcium, low protein and low sodium, thiazides, alkali treatment and allopurinol may reduce recurrence, all at low strength of evidence; no study evaluated imaging strategies; evidence in children was limited

The bottom line

Everything else is smaller than its reputation. Of 46 entries mentioning water in the European claims register, three are authorised, and all three are about keeping a normal function going: normal physical and cognitive function, and normal body temperature. Nothing about weight, skin or clearing anything out. The one place where plain water is beaten is exercise, where drinks containing carbohydrate and electrolytes are allowed to say they speed water absorption. As for eight glasses a day, we went looking for where that number comes from in the agency layers, the Daily Value documents and the WHO guidelines, and it is in none of them. Sports bodies describe fluid during exercise as balancing what you lose, and they name low blood sodium in the same breath as dehydration.

Sources

1. wat-dr-04 · regulatory register entry
   POL-HC-6349 Carbohydrate-electrolyte solutions Carbohydrate-electrolyte solutions enhance the absorption of water during physical exercise Authorised
   EU Register on nutrition and health claims, заява POL-HC-6349, знімок 2026-09-21 · checked 2026-09-22
2. wat-dr-07 · position stand
   Athletes should be well hydrated before exercise and drink enough fluid during and after exercise to balance fluid losses.
   Med Sci Sports Exerc, 2009 · checked 2026-09-22
3. wat-dr-05 · regulatory register entry
   POL-HC-6526 Water Water contributes to the maintenance of normal physical and cognitive functions Authorised
   EU Register on nutrition and health claims, заява POL-HC-6526, знімок 2026-09-21 · checked 2026-09-22
4. wat-dr-06 · regulatory register entry
   POL-HC-6527 Water Water contributes to the maintenance of normal regulation of the body's temperature Authorised
   EU Register on nutrition and health claims, заява POL-HC-6527, знімок 2026-09-21 · checked 2026-09-22
5. wat-dr-01 · systematic review and meta-analysis of randomised trials
   In two trials, water intake > 2 l/d or fluids to achieve urine output > 2.5 l/d reduced stone recurrence (relative risk: 0.39; 95% confidence interval: 0.19-0.80).
   Eur Urol, 2009 · checked 2026-09-22
6. wat-dr-02 · systematic review and meta-analysis of randomised trials
   In one study, participants were randomized to >2 l/d of water intake versus no treatment for 5 yr [13]. Those allocated to high water intake were significantly less likely to have stone recurrence (12% vs 27%, p = 0.008).
   Eur Urol, 2009 · checked 2026-09-22
7. wat-dr-03 · systematic review of randomised and non-randomised studies
   For adults with calcium oxalate or phosphate stones, increased water intake; a diet with normal to high calcium, low protein, and low sodium; thiazides; alkali treatment; and allopurinol may reduce stone recurrence (low SOE).
   Ann Intern Med, 2026 · checked 2026-09-22