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Medical myths: If you're thirsty you're already dehydrated

BY Max Pemberton

1st Jan 2015 Ask Dr Max

Medical myths: If you're thirsty you're already dehydrated

Someone said this to me recently, as I was gulping down a glass of water...

Where did the myth come from?

You hear it everywhere, usually from someone scolding you for allowing yourself to go so long without water that you’re actually thirsty—as though we can no longer trust our body to tell us when it needs something.

Yet the idea that if we’re thirsty then we’re already dehydrated is not true. It’s just as much a myth as the idea that you should drink eight glasses of water a day


What's the truth?

Your body is actually incredibly good at regulating water percentage to ensure that it’s in balance.

The more concentrated the blood is, the less fluid there is. This concentration is known as the “osmolality”. When it’s very high, then the blood is more concentrated and the person is dehydrated.

Research suggests that the sensation of thirst sets in when the concentration of fluid in the blood rises by two per cent. Clinical dehydration is the rise in the concentration of the blood by five per cent. So thirst kicks in well before actual dehydration.


So nothing to worry about?

The vast majority of people can trust their body. If they’re thirsty, then they should drink—and if they’re not, then they shouldn’t worry. Those who’ve had changes in blood flow to their brain (caused by strokes, for example) can be an exception, and may need to have their fluid intake monitored.



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