Food · 4 min read ·

Hydration: how much water you really need

Two litres a day? Eight glasses? The recommendations are simpler than they seem — and more than just water counts towards them.

In a nutshell

1How much to drink

EFSA recommendation 2.0 / 2.5 L EFSA considers a total water intake of 2.0 L a day for women and 2.5 L for men adequate. Water from all drinks and from food counts. This applies to moderate temperatures and normal activity; in heat and with heavy exertion, losses can reach about 8 L a day.1
Moderate evidence — measurements in 5,604 people 23 countries Measuring body water turnover in 5,604 people from 23 countries showed that water needs vary with age, body size, physical activity, pregnancy and climate — temperature, humidity and altitude.3

A single number such as “eight glasses a day” therefore does not fit everyone.3

2Coffee and tea

Weak evidence — one study 4 cups In a study of 50 men who regularly drank coffee, 4 cups of coffee a day (4 mg of caffeine per kilo) hydrated them just as well as the same amount of water; only sodium excretion differed.4

Caffeine and its safe doses are covered in the article Coffee.

3During sport

ACSM recommendation under 2% The American College of Sports Medicine: the goal of drinking during exercise is to prevent a water loss greater than 2% of body weight, which impairs performance. You can find out how much you sweat by weighing yourself before and after training.2
Moderate evidence — observational study 13% At the Boston Marathon, 13% of the 488 runners tested had low blood sodium (hyponatraemia) at the finish and 0.6% critically low. It was linked with weight gain during the race — that is, drinking more than the body sweated out — and with finishing times over 4 hours.5

So during long events, do not drink “in advance”: you should not weigh more after the race than before it.5

4Water and weight loss

Weak evidence — a small study ~2 kg In a study of 48 people aged 55–75, participants on a diet drank 500 mL of water before each main meal and lost about 2 kg more over 12 weeks than with the diet alone.7

A review of studies found a similar effect only in people who were also dieting; in others the results were inconsistent.6

What to take from this

Sources

Written on , based on studies available at that date. Newer research may change the conclusions.

  1. EFSA (2017). Dietary Reference Values for nutrients: Summary report. EFSA Supporting Publications 14(12):e15121. doi.org/10.2903/sp.efsa.2017.e15121
  2. Sawka M. N. et al. (2007). American College of Sports Medicine position stand. Exercise and fluid replacement. Med Sci Sports Exerc 39:377–390. doi.org/10.1249/mss.0b013e31802ca597
  3. Yamada Y. et al. (2022). Variation in human water turnover associated with environmental and lifestyle factors. Science 378:909–915. doi.org/10.1126/science.abm8668
  4. Killer S. C., Blannin A. K., 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:e84154. doi.org/10.1371/journal.pone.0084154
  5. Almond C. S. D. et al. (2005). Hyponatremia among runners in the Boston Marathon. N Engl J Med 352:1550–1556. doi.org/10.1056/NEJMoa043901
  6. Muckelbauer R. et al. (2013). Association between water consumption and body weight outcomes: a systematic review. Am J Clin Nutr 98:282–299. doi.org/10.3945/ajcn.112.055061
  7. Dennis E. A. et al. (2010). Water consumption increases weight loss during a hypocaloric diet intervention in middle-aged and older adults. Obesity 18:300–307. doi.org/10.1038/oby.2009.235

This is general information and does not replace a doctor. If you have heart or kidney disease, your doctor may limit your fluid intake — follow their advice.