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Health myths: Drink eight
glasses of water per day
• 26 August 2013 by Caroline Williams
• Magazine issue 2931. Subscribe and save
• For similar stories, visit the Food and Drink Topic Guide
Read more: "Don't swallow it: Six health myths you should ignore"
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