Question: Should we drink eight litres of water daily?
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There are few things as refreshing as a drink of water on a hot day, or a hot shower when we're grubby. Water is essential to all life, and it's the universal solvent that washes away grime.
Human psychology gives us useful shortcuts, but the eight-litres-a-day myth falls into the trap of "if a bit of something is good, then a lot must be even better". Aristotle would recognise this when he classified the form of false logic "reductio ad absurdum".
The myth plays into the idea that the world is awash with man-made chemicals, so we need to detox. Some say that you can sweat it out (untrue), and that drinking excessive quantities of water will also help.
Wendy Doyle from the British Dietetic Association says kidneys are efficient at removing toxins, and drinking more than you actually need won't remove them, it just means more trips to the loo.
The myth appears to have originated from a 1945 US Food and Nutrition Board recommendation that adults should drink 2.5 litres daily. That's about eight glasses, which has morphed in some retellings to a crazy "eight litres".
You excrete about 1600 millilitres of water per day, another 900 millilitres through sweating and breathing. That means you need to replace about 2520 millilitres, but you get some of that just by eating. That means you need to drink about 1-1.5 litres per day, but it doesn't have to be straight water. It can be tea, milk, coffee or any kind of drink.
Drinking too much water can even be dangerous. A 28-year-old woman died after drinking six litres of water in three hours for a radio station's "Hold Your Wee for a Wii" competition.
One man had a heart attack and died after drinking 10 litres on successive days. He was constantly drinking and vomiting, then refilling his glass. A post-mortem found the level of sodium in his blood was only 115, instead of the normal 140.
Excessive water cannot be cleared by the kidneys, leading to a condition known as hyponatremia. That water leaves the blood and enters the cells, which makes them swell. It can affect athletes after heavy exercise, and even sports drinks can cause hyponatremia when consumed to excess. The rule of thumb when exercising is to drink as much you sweat, but it's not easy to measure.
The best advice is, drink when you're thirsty.
Response by: Rod taylor, Fuzzy Logic
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