What you've heard
Drink at least eight glasses of water a day
Verdict
False as a rule
False as a rule. Drink to thirst, more in heat and hard exercise, and never past thirst in long events.
Do you need eight glasses of water a day?
False as a rule. Drink to thirst, more in heat and hard exercise, and never past thirst in long events.
The rule seems to have grown from a 1945 US allowance of about two and a half liters a day that came with two caveats: most of it was already in food, and "sensations of thirst usually serve as adequate guides to intake except for infants and sick persons." The allowance traveled; the caveats did not. A physiologist who went looking for the evidence summed up: "No scientific studies were found in support of 8 x 8." The US National Academies later concluded that most healthy people "adequately meet their daily hydration needs by letting thirst be their guide," and found no convincing evidence that caffeine dries you out. The rule can even do harm. At one Boston Marathon, about one runner in eight tested at the finish had too little sodium in their blood, most often those who drank so much that they gained weight during the race.27
Where the book tests it
What the book recommends instead
Cut the ultra-processed share first, beginning with sugary drinks and processed meat
ProbableEvidence grade: Probable. Good observational evidence with a plausible mechanism, or small or few trials.How: swap sugary drinks, including energy drinks, for water, tea, coffee or sparkling water; keep juice to a small glass; make bacon, ham and hot dogs occasional; change what is in the cupboard, so the easy option at home is food that was recently an ingredient. Start with the meals you eat most often. Why: in controlled feeding, people ate about five hundred calories a day more on ultra-processed menus, and drinks and processed meat carry the clearest long-term signals.
Build meals on plants, legumes, and minimally processed protein and fats
ProbableEvidence grade: Probable. Good observational evidence with a plausible mechanism, or small or few trials.How: fill half the plate with vegetables and fruit, a quarter with whole grains or potatoes, a quarter with protein, often beans, lentils or fish; cook with olive or another liquid plant oil; add a handful of nuts most days. Work up to at least 25 grams of fiber a day, slowly, to limit gas; in practice that means beans or lentils most days, whole grains in place of white bread and rice, and fruit or vegetables at every meal. Include a fermented food with live cultures most days, such as plain yogurt, kefir or kimchi (EmergingEvidence grade: Emerging. Early, small, animal-only, or actively contested.; one small trial, see Chapter 6). Why: this is the pattern with trial support for heart events, and it lowered blood pressure within weeks (StrongEvidence grade: Strong. Multiple randomized controlled trials, or consistent large cohort studies with a well-understood mechanism. for blood pressure).
Drink to thirst
ProbableEvidence grade: Probable. Good observational evidence with a plausible mechanism, or small or few trials.How: water with meals and when thirsty; coffee, tea, milk and water-rich food all count. Drink more in heat and hard exercise, and in long events never drink past thirst. Why: thirst is the one intake signal the modern world has left mostly intact.
Sources
- 27.Food and Nutrition Board, National Research Council (1945). Recommended Dietary Allowances, Revised 1945. Reprint and Circular Series No. 122 (August 1945). Washington, DC. https://dspace.gipe.ac.in/xmlui/bitstream/handle/10973/36074/GIPE-031998.pdf; 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, 283(5), R993-R1004. https://doi.org/10.1152/ajpregu.00365.2002; 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; 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 (488 runners gave blood at the finish; weight gain during the race went with about four times the odds of low sodium) (the 1945 text: "Most of this quantity is contained in prepared foods"; low sodium was also more common in slower runners)
Note numbers are those of Chapter 5, Eat.
This verdict is one of the "What you've heard" boxes in Chapter 5, Eat, where the book tests the claims people repeat against the studies behind them.
Educational content, not medical advice. Disclaimer.