You may have heard a recommendation that we
drink half our body weight in ounces of water every day. Is this necessary ...
The answer is: it depends —
on the climate, the season, and one's activity level and diet. An active
person in southern California in summer will drink a lot more water than a
sedentary person in Montreal in winter. Also, a vegetarian will typically need
less water than a meat eater, because meat has a lot of salt and toxins in it
that have to be excreted, and it also makes the body run warmer. But the
kidneys have to filter all this water, so an unnecessarily high level of water
consumption doesn't come without a cost. As Michio Kushi says in Macrobiotic
Dietary Recommendations, "It
is best to drink only when thirsty."
The whole concept of drinking so much water is
based on a misunderstanding. As the British
Medical Journal and numerous other sites point out, "The advice
to drink at least eight glasses of water a day can be found throughout the
popular press. One origin may be a 1945 recommendation [by the U.S. National
Research Council's Food and Nutrition Board] that stated, 'A suitable
allowance of water for adults is 2.5 litres daily in most instances. An
ordinary standard for diverse persons is 1 millilitre for each calorie of
food. Most of this quantity is contained in prepared foods.' If the last,
crucial sentence is ignored, the statement could be interpreted as instruction
to drink eight glasses of water a day." (Footnotes omitted. For other
sites, do a search for "2.5 liters water day 1945".)
Another site, by Dov Michaeli MD, PhD, observes:
The controversial 1992 bestseller Your
Body’s Many Cries for Water, which calls for a minimum of eight to 10
glasses of pure water a day (not coffee, not soda), probably played a role
in spreading the myth, as has the bottled-water industry, which has exploded
since the 1980s.
And the scientific evidence?
In a word: none.
Exhortations to drink more water invariably
include a reference to Dr. Batmanghelidj's book. Although I haven't read his
book, I understand that his conclusions were based on a study of prisoners in
Iran. Much of Iran has a hot climate, and one might reasonably suppose that
these prisoners were often not in good health and didn't get enough to eat.
More water certainly might have helped them. But to generalize to the rest of
Moreover, Dr. Batman's book apparently says
that if we're even a little thirsty, we're in the initial stages of
dehydration, and we should immediately drink. What's the justification for
this? If we're a little hungry, does that mean we're in the initial stages of
starvation, and we should immediately sit down and eat? If we're a little
tired, does that mean we're in the initial stages of exhaustion, and we should
immediately lie down and sleep? The body is capable of operating in a range of
It's been said that thirst is not always a
reliable indicator, because in some people, the hypothalamus has lost its
ability to gauge our need for water, and it can take six weeks to reset. Okay,
maybe people coming off a meat-eating diet need to drink a little more water
for six weeks. This might be a reasonable recommendation.
Finally, a study
in the June 2008 issue of the Journal of the American Society of Nephrology
reviews the classic medical advice to drink eight glasses of water a day and
also concludes it's a myth, as a news article about the study points
Drs. Dan Negoianu and Stanley Goldfarb of the
University of Pennsylvania in Philadelphia reviewed the scientific
literature published on the health effects of drinking lots of water to get
to the roots of widely accepted urban legends about water's health benefits.
"Our bottom line was that there was no
real good science — or
much science at all —
behind these claims, that they represent probably folklore," Goldfarb
... "When you
take in a lot of water, all you do is put out more urine but not more toxins
in the urine," Goldfarb said."