Drinking eight cups or two litres of water a day is longstanding advice. But is there any scientific basis for it, asks Dr Chris van Tulleken.

 

You know those ads that remind us that even a small drop in hydration（水合作用） levels can massively affect performance so you need to keep hydrated with whatever brand of isotonic（等压的） super drink they're selling? 

 

They seem pretty scientific don't they? Man in white coat, athlete with electrodes attached and so on. And it's not a hard sell because drinking feels right - you're hot and sweating so surely replacing that fluid must be beneficial. 

 

Well earlier this year sports scientists in Australia did an extraordinary experiment that had never been done before (British Journal of Sports Medicine, September 2013, Current hydration guidelines are erroneous: dehydration¹ does not impair² exercise performance in the heat, Wall BA). 

 

This group wanted to find out what happened to performance after dehydration. So they took a group of cyclists and exercised them until they lost 3% of their total body weight in sweat.

 

Then their performance was assessed after rehydration with either 1) nothing, 2) enough water to bring them back to 2% dehydration or 3) after full rehydration. 

 

So far nothing unusual, but the difference between this and almost every other study that's ever been done on hydration was that the cyclists were blind to how much water they got. The fluid was given intravenously without them knowing the volume. 

 

This is vital because we all, and especially athletes, have such an intimate psychological relationship with water consumption. 

 

Remarkably⁴, there was no performance difference between those that were fully⁵ rehydrated and those that got nothing. This study was part of a growing movement to "drink to thirst" which hopes to persuade athletes not to over hydrate with the potentially fatal consequence of diluting⁶ your sodium⁷ level, causing hyponatraemia.

 

Perhaps the result shouldn't be so surprising. Humans evolved doing intense exercise in extreme heat and dryness. We are able to tolerate losses in water relatively⁸ well whereas even slight over hydration can be far more dangerous. In simple terms, being too watery⁹ is as bad for you as being too concentrated. 

 

But what about the rest of us who aren't cycling around the desert in Western Australia? 

 

There is a very well accepted idea that we should drink about eight cups of water per day (two to three litres) in addition to our food and other drinks. 

 

We are awash with positive messages about the healing properties of water and how it will improve everything from our brains to our bowels¹⁰. And we know that without it we will die in days. 

 

It's a short leap of logic³ to think that if a lack of water is bad for for you then hydration must be good - purifying, cleansing¹¹ water washing though your organs must be beneficial, detoxifying. It surely improves your skin, helps you think, reduces your risk of kidney stones and turns your urine a lovely light, straw/champagne colour rather than the fetid orange syrup¹² you produce at the end of a long day where you haven't had time to drink.

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