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 'Il get from drinking eight 8-ounce
glasses of water — the 8 x 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.