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 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.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(密切的)psychological relationship with water consumption.
Remarkably, there was no performance difference between those that were fully 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 diluting your sodium 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 relatively well whereas even slight over hydration can be far more dangerous. In simple terms, being too watery is as bad for you as being too concentrated.
What’s the main idea of the passage?

A．There is no scientific basis for drinking eight cups of water a day

B．People used to drink eight cups of water a day

C．An experiment about whether people should drink eight cups of water.

D．People shouldn’t believe longstanding advice.

what does the word“dehydration ”in paragragh 4 mean？

A．doing exercise	B．losing water
C．drinking water	D．having a rest

What is NOT TRUE according to the passage?

A．Drinking too much water is bad for us

B．Sports scientists in Australia did an extraordinary experiment for the first time.

C．Those that were fully rehydrated and those that got nothing performed differently.

D．The cyclists didn’t know how much water they got.

What can we learn from the last paragragh?

A．Humans can bear losing water relatively well

B．It’s dangerous to drink water.

C．Athletes should drink more water than others.

D．This study was part of a growing movement to "drink to thirst".

what is Dr Chris van Tulleken’s attitude towards the. longstanding advice？

A．positive

B．doubtful

C．supportive

D．indifferent