The recommendation to drink six to eight glasses of water a day to prevent dehydration "is not only nonsense, but is thoroughly debunked nonsense," argues GP, Margaret McCartney in this week's BMJ.
There is currently no clear evidence of benefit from drinking increased amounts of water, she says, yet the "we-don't-drink-enough-water" myth has endless advocates, including the NHS.
The NHS Choices website states: "Try to drink about six to eight glasses of water (or other fluids) a day to prevent dehydration," while many schools also feel it appropriate to insist that pupils are accompanied to school by a water bottle.
Other organisations, often with vested interests, reinforce this message, she says. For example, Hydration for Health (created by French food giant Danone - makers of bottled waters including Volvic and Evian) recommends 1.5 to 2 litres of water daily as "the simplest and healthiest hydration advice you can give." It also claims that "even mild dehydration plays a role in the development of various diseases."
But McCartney argues that there is no high quality published evidence to support these claims.
She points to several studies showing no clear evidence of benefit from drinking increased amounts of water and suggesting there may be unintended harms attached to an enforcement to drink more water.
"It would seem, therefore, that water is not a simple solution to multiple health problems," she writes.
For instance, reports that increased water intake in children can improve concentration and mental performance have not been confirmed by research studies, while data relating water drinking to a reduction in children being overweight are prone to bias.
While there are some conditions that do benefit from drinking increased water, such as in people with recurrent kidney stones, other evidence for preventing disease is conflicting, adds McCartney. In other words, this is a complex situation not easily remedied by telling everyone to drink more.
Untangling the evidence presented by Danone "results in weak and biased selection of evidence", she argues. Danone says we need "informed choices", but their own evidence does not support their call to action.
She concludes: "There are many organisations with vested interests who would like to tell doctors and patients what to do. We should just say no."