Food and diet

Drink theory doesn’t hold water

“The belief that we need eight glasses of water a day to stay healthy is a myth,” reports the Daily Mail today. The Independent says that there is “no evidence that drinking eight glasses of water a day improves skin tone, aids dieting or prevents headaches”.

The newspaper reports are based on an editorial that concludes there is no proof of any benefit, except perhaps for people in hot climates. The authors of the editorial, however, note that the lack of clear evidence of any benefit is not the same as evidence that there is no benefit.

The editorial describes several studies that have looked at the issue, including a recent case study published in the same journal. This case study and the editorial have highlighted the potential dangers of too much fluid, which can dilute sodium in the blood.

Where did the story come from?

Dan Negoianu and Stanley Goldfarb from the University of Pennsylvania in Philadelphia wrote the editorial to provide some background to a related case study reported in the same journal. They also summarised the research and gave their opinions on the recommendation to drink eight glasses of liquid a day. The editorial was published in The Journal of the American Society of Nephrology , a peer-reviewed medical journal.

What kind of scientific study was this?

This was an editorial. It started by asking several questions, for example: Is the advice to drink plenty of water good advice? How is it that the intake of water affects kidney function and other physiological variables? Is there any evidence behind the current recommendations to drink eight glasses a day? If there is evidence that this amount of water is therapeutic, then what are the improved outcomes? The authors address these questions using their knowledge of the existing scientific literature about these issues.

What were the results of the study?

The authors highlight particular theories about the benefits of drinking an increased amount of water, for example, that it improves the kidneys’ ability to remove toxins, improves skin tone and organ function, and helps to control weight and prevent headaches. The authors then refer to a range of studies of different designs that have investigated these theoretical benefits. The studies include some internet sources, case studies and physiological laboratory experiments. The authors reference uncontrolled trials, case series and a randomised trial. They also quote a review from 2002 in support of their argument that “there is no clear evidence of benefit from drinking increased amounts of water”.

What interpretations did the researchers draw from these results?

The authors acknowledge that they “wish we could demolish all of the urban myths found on the Internet regarding the benefits of supplemental water ingestion”. They also concede that there is “no clear evidence of a lack of benefit… In fact, there is simply a lack of evidence in general”.

What does the NHS Knowledge Service make of this study?

Editorials are often used to alert readers to current issues of concern. The lack of evidence regarding the optimum amount of additional fluid, if any, that can be recommended for health is of concern. This was not a systematic review and therefore all the relevant scientific literature on this subject is unlikely to have been examined.

It is important to distinguish between two situations: where there is a lack of evidence for an effect and, alternatively, where there is concrete evidence of little or no effect. The authors make this point, but it is possible to misinterpret this by reading only the headlines of the newspaper reports. Better quality research is clearly required to investigate this issue.

Sir Muir Gray adds...

It may not do much good, but it also does no harm and therefore does more good than harm.

NHS Attribution