- Margaret McCartney, general practitioner, Glasgow
Water, water everywhere. Should doctors be telling people to drink more water as a public health issue? Hydration for Health, an initiative to promote drinking more water, held its annual scientific meeting in Evian, France, last week. The initiative has shown its fervour for water with recent adverts in the medical press, including the BMJ. The website states that its mission is “to establish healthy hydration as an integral part of public health nutritional guidelines and routine patient counselling so people can make informed choices.” It believes that “Healthcare professionals should be encouraged to talk with patients about the calorific content of SSBs [sugar sweetened beverages] when discussing lifestyle modification to manage overweight and/or obesity . . . Consumption of water in preference to other beverages should be highlighted as a simple step towards healthier hydration.” And healthier hydration is? “recommending 1.5 to 2 litres of water daily is the simplest and healthiest hydration advice you can give.”
Hydration for Health has a vested interest: it is sponsored and was created by French food giant Danone. This company produces Volvic, Evian, and Badoit bottled waters. The initiative’s website is bold and strident. Under a section entitled “We don’t drink enough water,” it states, “many people, including children, are not drinking enough . . . Children can be at greater risk than adults of feeling the effects of not drinking enough because of their smaller size . . . Elderly people often have a decreased sensation of thirst, which can lead to a higher risk of dehydration [and] evidence is increasing that even mild dehydration plays a role in the development of various diseases.”1
The “we don’t drink enough water” idea has endless advocates. Schools often insist that pupils bring a water bottle to school and tell pupils that they should drink eight glasses a day. Drinking eight glasses of water a day is recommended by all kinds of organisations, including the NHS, which says on the NHS Choices website: “Try to drink about six to eight glasses of water (or other fluids) a day to prevent dehydration.”2 This is not only nonsense, but is thoroughly debunked nonsense. In 2002, Heinz Valtin published a critique of the evidence in the American Journal of Physiology. He concluded that “Not only is there no scientific evidence that we need to drink that much, but the recommendation could be harmful, both in precipitating potentially dangerous hyponatremia and exposure to pollutants and also in making many people feel guilty for not drinking enough.”3 In 2008, an editorial in the Journal of the American Society of Nephrology reached much the same conclusion, stating that “There is no clear evidence of benefit from drinking increased amounts of water. 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.”4
Hydration for Health claims that drinking more will benefit us in all kinds of ways. It supports its claim that “many people still may not drink enough fluids” with two references. One is a 2006 discussion paper about mineral rich water from Clinical Nutrition.5 The authors make the statement that “the elderly consume insufficient liquid amounts, and they often drink when dehydration has already appeared,”5 which is supported by Italian government guidelines rather than primary research. The second paper, from the Journal of Gerontological Nursing, is a retrospective notes based study and does not distinguish between iatrogenic causes of dehydration such as diuretics and primary causes such as illness.6 In other words, Hydration for Health has presented no quality evidence supporting its initial assumption. The claims that mild hydration is linked to conditions such as stroke and gallstones are backed up mostly by level III evidence (descriptive or correlation studies rather than randomised controlled trials). Of the few instances where higher quality evidence was cited, for example, signs of mitral valve prolapse on echocardiogram relating to dehydration, this is of dubious clinical importance.7
The claim that children do not drink enough is referenced to an article in Nursing Times, written by the coordinator of the Water is Cool in School campaign.8 This feature article makes the important points that some children do not have free access to fluids at school (drinking “more” as Danone would have it, is different from drinking as needed or when thirsty) and that school toilets can be unpleasant. However, this article goes further and says that “mild dehydration of 1-2% loss of body weight (the level of dehydration when we first feel thirst) results in a measurable decline in mental performance.” The claim is, in turn, referenced to a review article by Rogers and colleagues. This study, however, found that “when thirst was low, the impact of water intake was detrimental rather than neutral or beneficial, showing that mental performance can be very sensitive to physiological perturbation.”9 In other words, there is still no evidence that we need to drink more than we naturally want, and there may be unintended harms from an enforcement to drink more water.
Reports made by the BBC in 2000 that increased water intake in children led to an improvement in test results created widespread media excitement.10 However the research has not been published in a peer reviewed journal, and the water intervention seems to have been part of a raft of changes in school. The Nursing Times article mentioned a three year research project in schools by the School of Health Sciences at Leeds Metropolitan University looking at the “effects of access to water for pupils and the links between water and learning, ” after anecdotal reports from schools that attention spans, concentration, and behaviour are “noticeably improved by frequent intakes of small amounts of water.” This study has also not been published. One randomised cluster controlled trial has related water drinking to a reduction in children being overweight.11 However, the authors note that they may have been at risk of selection bias, due to some schools and some children choosing not to take part, and the study was done as part of a larger educational programme about weight and diet. It seems, therefore, that water is not a simple solution to multiple health problems.
Professor Stanley Goldfarb, physician and nephrologist at the University of Pennsylvania, was coauthor of the 2008 editorial in the Journal of the American Society of Nephrology, mentioned above, that concluded there is no clear evidence that drinking more water is beneficial. After he wrote his article, he was contacted by Danone, and taken out to dinner by two of its representatives. They didn’t try to dissuade him from his views, but they did show him a graph intimating that sales fell after the editorial was published.
Professor Goldfarb told me: “The current evidence is that there really is no evidence [for drinking extra water]. I agree, if children drank more water rather than getting extra calories from soda, that’s good for weight loss, and self evident. As far as reducing the amount of calories in adults, that’s been looked at many times and there is no evidence that drinking water before meals reduces appetite during a meal. However if you flavour it, you can reduce calorific intake,” possibly by altering appetite perception.12
What about adults? The other claim being made by Hydration for Health in a press release for its conference is of a “recently published study from Dr Strippoli of the University of Sydney identifying a positive relationship between low fluid intake and the occurrence of chronic kidney disease.”13 This cross sectional study, published in Nephrology in 2011 found an association between high fluid intake and lower risk of chronic kidney disease (CKD).14 The authors conclude that “higher intakes of fluid appear to protect against CKD.” However, the finding is only an association and is prone to confounding, and the study did not investigate whether more fluid intake can prevent chronic kidney disease.
Additionally, there is more research in this area than that cited by Hydration for Heath. Professor Goldfarb is aware of other studies that do not show a link, or even show the opposite. A study from Canada, part of the Modification of Diet in Renal Disease study, found that high urine volumes and low osmolarity were related to faster decline in kidney function.15 Professor Goldfarb does not tell his patients with other types of kidney disease to drink extra water, saying that many people are relieved about this.
“It’s a complicated story” he says. “One definitely benefits from it if there are recurring kidney stones. Will it reduce kidney stones in people who have never had them? It’s really just speculation. The other disease where increased fluids can be recommended is in polycystic kidney disease. This suppresses anti-diuretic hormone, which decreases the risk of development of cysts in people with a genetic risk. The downside of that with drinking all day, you are likely to have to get up in the middle of the night; so you have to factor in sleep deprivation.”
What about the medical old favourites, such as drinking plenty to ward off urine infections?
“Well, if young women have repeated urine infections, drinking lots probably reduces lower urine infections. But it may have an adverse effect on upper urine infections—excess water may reduce the concentration of inhibitory factors which would otherwise help with infections.”
In other words, this is a complex situation not easily remedied by telling everyone to drink more. What does he think of the bottled water industry telling us to drink up? “If anyone is selling anything . . . they don’t have a right to make claims that are not substantiated but have a right to sell their products. Here in the US, gallon for gallon, bottled water is more expensive than oil. And the other concern is all the debris that goes along with the bottles—so from a societal point of view it’s kind of silly—municipal water supplies are so carefully monitored.” Indeed, in the UK, the British bottled water industry website describes a year on year rise in bottled water consumption which now exceeds 33 litres annually per person.16
Examination of the evidence presented by Danone shows it to be weak and subject to selection bias. Danone says we need “informed choices,” but its evidence does not support its call to action. There are many organisations with vested interests who would like to tell doctors and patients what to do. We should just say no.
Cite this as: BMJ 2011;343:d4280
Competing interests: The author has completed the ICJME unified disclosure form at www.icmje.org/coi_disclosure.pdf (available on request from the corresponding author) and declares no support from any organisation for the submitted work; no financial relationships with any organisation that might have an interest in the submitted work in the previous three years; and no other relationships or activities that could appear to have influenced the submitted work.
Provenance and peer review: Commissioned; not externally peer reviewed.