Health myths: Drink eight glasses of water per day
26 August 2013 by Caroline Williams
It's the myth that just won't go away. Almost everyone thinks they
don't drink enough water, but the idea that we all should drink lots
of it – eight glasses per day – is based on no scientific data
whatsoever.
No one really knows where the eight-glasses idea comes from. Some
blame the bottled water industry but plenty of doctors and health
organisations have also promoted it over the decades. The source
might be a 1945 recommendation by the US National Research Council
(NRC) that adults should consume 1 millilitre of water for each
calorie of food, which adds up to about 2.5 litres per day for men
and 2 litres for women.
According to Barbara Rolls, a nutrition researcher at Penn State
University and author of the 1984 book Thirst, this amount is about
right for people in a temperate climate who aren't exercising
vigorously. And 1.9 litres is what you'll get from drinking eight
8-ounce glasses of water – the 8 ×8 rule – as per the US version of
the myth.
What most people don't realise, though, is that we get a lot of that
water from our food, as the NRC pointed out at the time. Foods
contain water and are broken down chemically into carbon dioxide and
more water. So if you are not sweating buckets you need only about a
litre a day – and 1.2 litres is what you will get from the eight
150-millilitre glasses recommended by the UK's health service.
But any talk of glasses is misleading because there is no need to
drink pure water. The fluids that people drink anyway, including tea
and coffee, can provide all the water we need, says Heinz Valtin, a
kidney specialist at Dartmouth Medical School in Lebanon, New
Hampshire, who has reviewed the evidence (Regulatory Integrative and
Comparative Physiology, vol 283, p R993).
According to the myth, however, caffeinated drinks don't count
because they are diuretic, stimulating the body to lose more water
than it gets from the drink. Not true. A comparison of healthy
adults in 2000 found no difference in hydration whether they got
their water from caffeinated drinks or not (Journal of the American
College of Nutrition, vol 19, p 591). Even one or two mildly
alcoholic drinks will hydrate you rather than dehydrating you.
Hydrophilics respond by saying that pure water is better than other
drinks. Even this claim is arguable, but the crucial point is that
if you are a healthy individual already drinking enough tea, milk,
juice or whatever, there is no evidence that swigging down water as
well will achieve anything other than making you go to the bathroom
all the time.
The final aspect of this myth is that we need to force ourselves to
drink because by the time we are thirsty we are already seriously
dehydrated. Not so. Rolls showed nearly 30 years ago that we get
thirsty long before there is any significant loss of bodily fluids.
It takes less than a 2 per cent rise in the concentration of the
blood to make us want to drink, while the body isn't officially
regarded as dehydrated until a rise of 5 per cent or more.
So relax and trust your body. Don't force yourself to gulp down
gallons of water if you don't want to – that can be dangerous – just
drink the beverage of your choice whenever you're thirsty.
This article appeared in print under the headline "1 Drink eight
glasses of water per day"
Caroline Williams is a freelancer based in Surrey, UK