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Reference topic 17: The Water You Forgot to Drink

How much water should you drink a day? What thirst evolved for

ProbableEvidence grade: Probable. Good observational evidence with a plausible mechanism, or small or few trials.42 sources8 graded recommendationsTopic hub: Food

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In April 2002 Cynthia Lucero ran the Boston Marathon. She was 28, from Guayaquil, Ecuador, had finished her doctoral dissertation days earlier and was running for a blood-cancer charity.12 Friends said she drank a lot of sports drink and looked well at the foot of Heartbreak Hill. Near Cleveland Circle she faltered. A friend jumped into the race beside her, and Lucero said she felt "dehydrated and rubber-legged." She fell, reached the hospital in a coma and died soon afterward.12

In August the state medical examiner's office ruled. In the Boston Globe's summary, she had been "felled by a precipitous cascade of medical events brought on in part by drinking too much fluid." The cause was hyponatremic encephalopathy, a swelling of the brain driven by a critical imbalance of sodium. She was only the second runner to die in more than a century of the race.1

Mismatch card

The system

Water balance: thirst, kidneys and vasopressin.

What it expects

Wet food and drinking to thirst, in an ape that turns over 30% to 50% less water than its cousins, size for size.

What it gets

Taps, climate control, a 1945 allowance stripped of its caveats, and a market selling hydration.

What happens in the gap

Little, for most healthy people. In long events, overdrinking can dilute blood sodium and swell the brain; thirst lags in older people and in heat.

The handrail

Drink to thirst; plan a little for heat and long exercise (●●○)

Strength of evidence

●●○ Thirst and overdrinking are well measured; trials of extra water against disease are few.

What this topic covers

  • ·The Water-Saving Ape
  • ·Wet Food
  • ·The Thirst Machine
  • ·The Sentence Everyone Forgot
  • ·More Must Be Better

The practical version is in the book: Chapter 5, Eat.

The handrail

Each recommendation carries the strength of the evidence behind it. How the grades work.

  1. Drink when thirsty and with meals

    ProbableEvidence grade: Probable. Good observational evidence with a plausible mechanism, or small or few trials.

    Coffee, tea, milk and water-rich foods such as fruit and vegetables all count; healthy adults in mild conditions need no glass count (Topic 40).2037319

  2. Choose water over sugary drinks

    ProbableEvidence grade: Probable. Good observational evidence with a plausible mechanism, or small or few trials.

    The swap may modestly help weight; trials of simply drinking more are mixed.3828

  3. Plan a little for heat and long exercise

    ProbableEvidence grade: Probable. Good observational evidence with a plausible mechanism, or small or few trials.

    Weigh yourself before and after long sessions. Losing more than about 2% suggests drinking more next time; any gain means you drank too much.2227 For moderate work in the heat lasting under 2 hours, US workplace guidance is about 240 milliliters (1 cup, 8 ounces) every 15 to 20 minutes, and generally no more than about 1.4 liters (6 cups) an hour.39

  4. Treat suspected overdrinking as an emergency

    StrongEvidence grade: Strong. Multiple randomized controlled trials, or consistent large cohort studies with a well-understood mechanism.

    Headache, vomiting, confusion or seizures after heavy drinking during a long event, especially with weight gain, need urgent care, not more fluid.244

  5. Offer older relatives drinks in hot weather

    ProbableEvidence grade: Probable. Good observational evidence with a plausible mechanism, or small or few trials.

    Thirst fades with age, and urine color is an unreliable guide.1340

  6. Drink more for specific conditions, with a clinician

    ProbableEvidence grade: Probable. Good observational evidence with a plausible mechanism, or small or few trials.

    After a first calcium kidney stone, drinking more cut recurrences over 5 years from 27 in 100 people to 12 in 99.41 A year-long trial enrolled premenopausal women with recurrent bladder infections who drank little. Those who added 1.5 liters of water a day averaged 1.7 infections, against 3.2 in those who did not.26

  7. For diarrhea, use oral rehydration solution

    StrongEvidence grade: Strong. Multiple randomized controlled trials, or consistent large cohort studies with a well-understood mechanism.

    a mixture of clean water, salt and sugar. Severe dehydration needs medical care, sometimes with intravenous fluids.42

  8. With heart failure, kidney disease or a prescribed fluid limit

    follow your clinician's advice over any general rule.

The skeptic's corner

The case against this topic, from the research itself.

Much hydration science is funded by drink companies, and the papers say so. Four authors of the bladder-infection trial in the Handrail worked for Danone Research, and two occasional Danone consultants wrote the editorial on the eight-glass mix-up. A review favoring planned drinking in long, hot exercise appeared in a supplement funded by the Gatorade Sports Science Institute, a division of PepsiCo.261727 Funding does not make results wrong, but ask who gains from "drink more."

Trials of drinking more are few: one review found only 18 randomized trials of changing water intake. Extra water went with fewer kidney stones and, in some trials, more weight loss, but eight of the trials reported negative results.28

"Dehydration" figures depend on cut-offs. By one urine threshold, more than half of American children and teenagers were inadequately hydrated.29 By a comparable one, so were about half of Tsimane adults,15 who have the lowest coronary artery calcium (calcium buildup in the heart's arteries) of any population yet measured (Topic 8).30

Electrolyte products promise more than trials show. In a trial in resting men, a sports drink kept water in the body no better than plain water.31 One powder lists 1,000 milligrams of sodium per serving, on top of American diets that already exceed the recommended limit.3233

Nor is water much of a mineral source. Food supplies more than four-fifths of the calcium and magnesium people take in,34 though a liter of some European mineral waters holds a fifth or more of daily calcium needs.35 Ordinary municipal treatment does not strip minerals; reverse osmosis, distillation and deionization remove nearly all of them, and home softeners swap calcium and magnesium for sodium.34 In one national survey, nearly half of Americans got less magnesium from food than the estimated average requirement: a shortfall in intake, not a diagnosed deficiency.36

Studies that start from deaths and look back (case-control studies) link magnesium-rich water to fewer cardiovascular deaths; studies that follow people forward (cohort studies) do not. The World Health Organization says the evidence "does not prove causality."34 Trials showing that extra water prevents disease in healthy people would change the picture.

Terms this topic defines

Related topics

Sources cited above

  1. 1.Smith, S. (2002, 13 August). "Marathon runner's death linked to excessive fluid intake." The Boston Globe. Text consulted via a verbatim repost: https://freerepublic.com/focus/news/732350/posts
  2. 2.William James College (2019, 4 April). "Making a Difference: The Legacies of Dr. Cynthia Lucero." https://www.williamjames.edu/news/Lucero-Legacy.html (Her dissertation examined how marathon training affected the families and friends of cancer patients.)
  3. 3.Almond, C.S., Shin, A.Y., Fortescue, E.B., Mannix, R.C., et al. (2005). "Hyponatremia among runners in the Boston Marathon." New England Journal of Medicine, 352(15), 1550-1556. https://doi.org/10.1056/NEJMoa043901 (766 runners enrolled and 488 gave blood at the finish; hyponatremia was defined as a blood sodium level of 135 millimoles per liter or less, and 0.6% of runners were critically low; the odds of about seven times compared finishing in over 4 hours with under 3 hours 30.)
  4. 4.Hew-Butler, T., Loi, V., Pani, A., and Rosner, M.H. (2017). "Exercise-Associated Hyponatremia: 2017 Update." Frontiers in Medicine, 4, 21. https://doi.org/10.3389/fmed.2017.00021
  5. 26.Hooton, T.M., Vecchio, M., Iroz, A., Tack, I., et al. (2018). "Effect of Increased Daily Water Intake in Premenopausal Women With Recurrent Urinary Tract Infections: A Randomized Clinical Trial." JAMA Internal Medicine, 178(11), 1509-1515. https://doi.org/10.1001/jamainternmed.2018.4204 . Four co-authors were Danone Research employees.
  6. 17.Stookey, J.D., and Kavouras, S.A. (2020). "Water Researchers Do Not Have a Strategic Plan for Gathering Evidence to Inform Water Intake Recommendations to Prevent Chronic Disease." Nutrients, 12(11), 3359. https://doi.org/10.3390/nu12113359 . Both authors disclose occasional consulting for Danone Research.
  7. 27.Kenefick, R.W. (2018). "Drinking Strategies: Planned Drinking Versus Drinking to Thirst." Sports Medicine, 48(Suppl 1), 31-37. https://doi.org/10.1007/s40279-017-0844-6 . Published in a supplement supported by the Gatorade Sports Science Institute, a division of PepsiCo.
  8. 28.Hakam, N., Guzman Fuentes, J.L., Nabavizadeh, B., Sudhakar, A., et al. (2024). "Outcomes in Randomized Clinical Trials Testing Changes in Daily Water Intake: A Systematic Review." JAMA Network Open, 7(11), e2447621. https://doi.org/10.1001/jamanetworkopen.2024.47621
  9. 29.Kenney, E.L., Long, M.W., Cradock, A.L., and Gortmaker, S.L. (2015). "Prevalence of Inadequate Hydration Among US Children and Disparities by Gender and Race/Ethnicity: National Health and Nutrition Examination Survey, 2009-2012." American Journal of Public Health, 105(8), e113-e118. https://doi.org/10.2105/AJPH.2015.302572 (54.5% in 2009-2012.)
  10. 15.Bethancourt, H.J., Swanson, Z.S., Nzunza, R., Huanca, T., et al. (2021). "Hydration in relation to water insecurity, heat index, and lactation status in two small-scale populations in hot-humid and hot-arid environments." American Journal of Human Biology, 33(1), e23447. https://doi.org/10.1002/ajhb.23447
  11. 30.Kaplan, H., Thompson, R.C., Trumble, B.C., Wann, L.S., et al. (2017). "Coronary atherosclerosis in indigenous South American Tsimane: a cross-sectional cohort study." The Lancet, 389(10080), 1730-1739. https://doi.org/10.1016/S0140-6736(17)30752-3
  12. 31.Maughan, R.J., Watson, P., Cordery, P.A., Walsh, N.P., et al. (2016). "A randomized trial to assess the potential of different beverages to affect hydration status: development of a beverage hydration index." American Journal of Clinical Nutrition, 103(3), 717-723. https://doi.org/10.3945/ajcn.115.114769 (72 men; hydration compared over four hours.)
  13. 32.LMNT (2026). Product page for an electrolyte drink mix listing sodium 1,000 mg per serving. https://drinklmnt.com/products/lmnt-recharge-electrolyte-drink (accessed 19 September 2026).
  14. 33.US Food and Drug Administration (2024). Sodium in Your Diet. https://www.fda.gov/food/nutrition-education-resources-materials/sodium-your-diet (Americans average about 3,400 mg of sodium a day against a recommended limit of 2,300 mg.)
  15. 34.World Health Organization (2011). Hardness in Drinking-water: Background document for development of WHO Guidelines for Drinking-water Quality. WHO/HSE/WSH/10.01/10/Rev/1. https://cdn.who.int/media/docs/default-source/wash-documents/wash-chemicals/hardness-bd.pdf (Food supplies over 80% of calcium and magnesium intake, drinking water typically 5% to 20%.)
  16. 35.Azoulay, A., Garzon, P., and Eisenberg, M.J. (2001). "Comparison of the mineral content of tap water and bottled waters." Journal of General Internal Medicine, 16(3), 168-175. https://doi.org/10.1111/j.1525-1497.2001.04189.x
  17. 36.Rosanoff, A., Weaver, C.M., and Rude, R.K. (2012). "Suboptimal magnesium status in the United States: are the health consequences underestimated?" Nutrition Reviews, 70(3), 153-164. https://doi.org/10.1111/j.1753-4887.2011.00465.x (About 48% of Americans in 2005-2006 took in less magnesium from food than the estimated average requirement.)
  18. 20.National Academies (2004, 11 February). "Report Sets Dietary Intake Levels for Water, Salt, and Potassium To Maintain Health and Reduce Chronic Disease Risk." https://www.nationalacademies.org/news/report-sets-dietary-intake-levels-for-water-salt-and-potassium-to-maintain-health-and-reduce-chronic-disease-risk
  19. 37.Killer, S.C., Blannin, A.K., and Jeukendrup, A.E. (2014). "No evidence of dehydration with moderate daily coffee intake: a counterbalanced cross-over study in a free-living population." PLoS ONE, 9(1), e84154. https://doi.org/10.1371/journal.pone.0084154
  20. 9.Popkin, B.M., D'Anci, K.E., and Rosenberg, I.H. (2010). "Water, hydration, and health." Nutrition Reviews, 68(8), 439-458. https://doi.org/10.1111/j.1753-4887.2010.00304.x (Americans: an estimated 22% of water from food; watermelon, lettuce and spinach are 90% to 99% water; crackers, cereals and pretzels, 1% to 9%.)
  21. 38.Chen, Q.Y., Khil, J., and Keum, N. (2024). "Water Intake and Adiposity Outcomes among Overweight and Obese Individuals: A Systematic Review and Meta-Analysis of Randomized Controlled Trials." Nutrients, 16(7), 963. https://doi.org/10.3390/nu16070963
  22. 22.American College of Sports Medicine; Sawka, M.N., Burke, L.M., Eichner, E.R., Maughan, R.J., et al. (2007). "American College of Sports Medicine position stand. Exercise and fluid replacement." Medicine & Science in Sports & Exercise, 39(2), 377-390. https://doi.org/10.1249/mss.0b013e31802ca597
  23. 39.Centers for Disease Control and Prevention, National Institute for Occupational Safety and Health. Heat Stress: Workplace Recommendations. https://www.cdc.gov/niosh/heat-stress/recommendations/index.html (accessed 19 September 2026).
  24. 24.Hew-Butler, T., Rosner, M.H., Fowkes-Godek, S., Dugas, J.P., et al. (2015). "Statement of the Third International Exercise-Associated Hyponatremia Consensus Development Conference, Carlsbad, California, 2015." Clinical Journal of Sport Medicine, 25(4), 303-320. https://doi.org/10.1097/JSM.0000000000000221 . The recommendation, the two 2014 football deaths and the list of symptoms are quoted from the accompanying release: Wolters Kluwer Health (2015, 29 June). "'Drink when thirsty' to avoid fatal drops in blood sodium levels during exercise." ScienceDaily. https://www.sciencedaily.com/releases/2015/06/150629100926.htm
  25. 13.Phillips, P.A., Rolls, B.J., Ledingham, J.G., Forsling, M.L., et al. (1984). "Reduced thirst after water deprivation in healthy elderly men." New England Journal of Medicine, 311(12), 753-759. https://doi.org/10.1056/NEJM198409203111202 (24 hours without water; older men aged 67 to 75, young men 20 to 31.)
  26. 40.Hooper, L., Bunn, D.K., Abdelhamid, A., Gillings, R., et al. (2016). "Water-loss (intracellular) dehydration assessed using urinary tests: how well do they work? Diagnostic accuracy in older people." American Journal of Clinical Nutrition, 104(1), 121-131. https://doi.org/10.3945/ajcn.115.119925
  27. 41.Borghi, L., Meschi, T., Amato, F., Briganti, A., et al. (1996). "Urinary volume, water and recurrences in idiopathic calcium nephrolithiasis: a 5-year randomized prospective study." Journal of Urology, 155(3), 839-843. https://pubmed.ncbi.nlm.nih.gov/8583588/
  28. 42.World Health Organization (2024). Diarrhoeal disease (fact sheet, 7 March 2024). https://www.who.int/news-room/fact-sheets/detail/diarrhoeal-disease

This topic carries 42 notes in the research manuscript; the 28 above are those behind the material on this page. Full source register.