Do you really need to
drink eight cups of water a day?
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 levels can massively affect performance so you need to
keep hydrated with whatever brand of isotonic 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: dehydration does not impair 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 re-hydration with either 1) nothing, 2) enough water to bring them back to 2%
dehydration or 3) after full re-hydration.
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.
Remarkably, there was
no performance difference between those that were fully re-hydrated 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.
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 bowels. And we know that without it we will die in days.
It's a short leap of logic to think that if a lack of water is
bad for for you then hydration must be good - purifying, cleansing water
washing through 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 syrup
you produce at the end of a long day where you haven't had time to drink.
So I've looked through the literature and I
found a review article saying all of this and more. It was written by a group
of respected physicians from American and French hospitals and it clearly
supported the widely held belief that you should drink two to three liters of
water a day.
It said that people with a high urine output
have a lower rate of kidney stone disease, that the flushing action of the
water may reduce the risk of a urinary tract infection (especially in women
after sex). Perhaps most importantly, they referenced a surprising study which
showed that paradoxically an increased intake of water increased the risk of
bladder cancer. But only tap water. And there's the clue.
A footnote at the end of the article explained
that what you thought was a scientific article in a scientific journal is in
fact a supplement, sponsored by a major mineral water manufacturer. All of the
authors received honoraria from this company, which also provided medical
writing assistance. So this isn't research, it's marketing.
And this is one of the reasons we're even
discussing this - because increasingly drinking water doesn't just come out of
our taps for free. It's sold to us by the same clever people that sell us
yoghurts with bacteria in them that probably don't do us much good, something I
look at separately in the television series I've been making. And these
companies pretty consistently recommend two to three litres of water per day.
So where did that number come from and is
there any reason to think it correct?
Well the grain of truth is this - people in
temperate climates who are not doing sustained physical exercise do need around
six to eight cups per day but that can be contained in food, alcohol or
caffeinated beverages.
Yes, beer and coffee do not dehydrate you to
any noticeable extent (there's a nice paper where some medical students got to
drink quite a lot of beer and had their urine studied - British Medical Journal
(Clin Res Ed), December 1982, Acute biochemical responses to moderate beer
drinking, Gill GV).
There is no evidence that adding the eight
cups of water to everything else you drink will do you any good and it could do
you harm (American Journal of Physiological - Regulatory, Integrative and
Comparative Physiology, November 2002, Drink at least eight glasses of water a
day. Really? Is there scientific evidence for "8x8"? Valtin H).
But the great thing is that just like a
top-level athlete you don't need to worry about exactly what that total daily
requirement is because your body will sort it all out for you.
If you drink too much you pee it out. If you drink
too little you get thirsty and pee less. It's all exquisitely well-controlled
in the same way that your intake of oxygen is well-controlled.
Saying that you should drink more water than
your body asks for is like saying that you should consciously breathe more
often than you feel like because if a little oxygen is good for you then more
must be better.
Like most things in life there's a Goldilocks
amount - not too little and not too much. With this in mind, next week I'll
deal with the health benefits of porridge and how to avoid being eaten by
bears.
The Magazine on water
For most people a
drink of water is a clear, tasteless liquid that quenches the thirst but Faustino Munoz can distinguish
between dozens of different sorts of water, a hobby that started as a child, while
helping his mother cook.
There are many things
that divide the north and the south of Britain - politics, the weather - but water is probably the
oddest, says Kathryn
Westcott.
We are regularly advised to drink more water: it clears skin,
reduces tiredness and aids concentration, but could drinking too much water
be dangerous ?
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