The idea that we need to drink eight 8-ounce glasses of water a day to prevent dehydration is “thoroughly debunked nonsense,” according to a commentary published Tuesday in the journal BMJ.
There’s no good scientific evidence for recommending that level of daily hydration, yet that fact hasn’t deterred public health organizations and physicians from making “8 X 8” a cornerstone of a healthy lifestyle, writes Dr. Margaret McCartney, a general practitioner (and blogger) from Glasgow, Scotland.
Much of impetus behind the continuation of the “8 X 8” myth, McCartney adds, can be traced to aggressive marketing tactics by the bottled water industry, including the Hydration for Health initiative, which is sponsored by the French food giant Danone, maker of Volvic, Evian and Badoit bottled waters.
Hydration for Health cites studies that claim to back up the 8 X 8 recommendation, but any researcher taking a close look at that evidence would find “it to be weak and subject to selection bias,” says McCartney.
Recently, for example, Hydration for Health claimed that an Australian study showed that drinking a lot of fluids would prevent chronic kidney disease. But that study had significant methodological problems, McCartney points out. Furthermore, she adds, other, better-designed studies have shown no link between high hydration and a prevention of kidney diseases. In fact, some research has shown the opposite.
A stubborn myth
This isn’t the first time that a major medical journal has debunked the 8 X 8 myth. In fact, an earlier commentary (2007) in the BMJ included it in a list of “falsehoods we [physicians] unwittingly propagate as we practice medicine.”
In 2002, the American Journal of Physiology published a thorough debunking of the myth, and it was also addressed again in a 2008 editorial in the Journal of the American Society of Nephrology. “There is no clear evidence of benefit from drinking increased amounts of water,” concluded the authors of that editorial. “Although we wish we could demolish all of the urban myths found on the internet regarding the benefits of supplemental water ingestion, we concede there is also no clear evidence of lack of benefit. In fact, there is simply a lack of evidence in general.”
That lack of evidence extends to the “sub-myths” about supplemental water: the idea that children would improve their schoolwork if they drank more water, or that elderly people lose their sensitivity to thirst, or that drinking water decreases appetite and thus helps with weight reduction, or that even mild dehydration increases the risk of illnesses like kidney disease and urinary tract infections.
None of those have been proven. Yet we continue to believe them — and to lug water (mostly expensive, bottled water) around with us.
A murky source
Where did the idea that we needed at least 8 x 8 ounces of water daily come from? No one’s entirely sure, but the oldest source may be a 1945 recommendation by the Food and Nutrition Board of the National Research Council. It stated (without any supporting research) that “[a] suitable allowance of water for adults is 2.5 liters daily in most instances. An ordinary standard for diverse persons is 1 milliliter for each calorie of food. Most of this quantity is contained in prepared foods.”
Note the last statement in that recommendation: “Most of this quantity is contained in prepared foods.” That’s the part that seems to have been dropped over the years.
Of course, if the weather is hot and humid and/or you’re exercising and you’re feeling thirsty, or if you have a medical condition that can cause dehydration, you should make sure you drink plenty of fluids. And, yes, coffee and tea do count as fluid sources. As the 2002 editorial in the American Journal of Physiology pointed out, research has “cast serious doubt on the often asserted diuretic role of caffeinated drinks, except, possibly, in persons who have not ingested caffeine for nearly a week.”
But don’t overdo the fluids. Drinking too much water (overhydrating) during strenuous events can be deadly.