No, you don’t need to be drinking two litres of water every day

It’s a myth you need to drink two litres of water—that’s around eight glasses—every day in order to maintain good health.  You probably need far more, but the good news is that you’re already consuming more than you realise.

In fact, on average we need to consume around six litres a day, but that doesn’t have to come just from drinking water.  Any beverage we drink contributes to the total, as does the food we eat—especially fruits and vegetables—and our environment, which is another source of liquid.

The optimum intake depends on our age, weight, location, level of activity and gender.  And it varies enormously around the globe, with some people consuming just one litre a day, say researchers from the University of Wisconsin-Madison.  

But the idea we need to be consuming two litres of liquids a day is a myth that isn’t supported by science, the researchers say.  For one thing, most people don’t take into account the water that’s in their food, and that counts towards total intake.

To find out our true liquid consumption, the researchers used trackers on 5604 people aged from eight days to 96 years from 23 countries to assess their levels of ‘water turnover’, the amount of water used by the body every day.

The amounts consumed varied enormously, and depended on age, sex, outside temperature and physical activity.  

And the same is true for the amounts we need to be consuming.  For instance, a 20-year-old man weighing 11 stone and living at sea level in a country such as the UK where the average air temperature is 10C (50F), and who did average amounts of physical activity, needs to consume around 3.2 litres of liquid a day.  A nine-stone woman of the same age, location and activity level needs just 2.7 litres.

Doubling your activity increases your liquid needs by an extra litre, while a 50 percent increase in humidity raises your liquid requirements by around one-third of a litre.

Weight gain also means you need more water.  An eight-stone person needs around 2.5 litres a day, while someone weighing 15 stones needs five litres.

It’s complicated, and just telling people to drink eight glasses of water a day is simplistic and could be interfering with the body’s sophisticated management of water.  It urinates out the water it doesn’t need and triggers thirst when we do need to drink.

Consciously drinking more just to follow the stipulation of a myth is a little like breathing more, simply because oxygen is good for us, the researchers say.  In fact, drinking too much water can be dangerous, and could trigger hyponatremia, a life-threatening condition where the kidneys start diluting the sodium content of blood. 

Science, 2022; 378: 909-15

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