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What can we conclude?

So what can we conclude about the hydration and driving study?

The simple answer is not a lot.

  • The study population was made up of 12 healthy males with an average age of 22 and they only used the results from 11 of them because the 12th wasn’t reliable enough – he kept falling asleep. It’s possible that testing another 12 subjects might have produced different results? Twelve is a small number and the results may be due to chance. We’ll be looking at this again next week, but for now the main thing to bear in mind is that studies with small numbers of participants are not considered reliable. Also, how representative are these 12 young men of the population of drivers as a whole? Not very representative at all. Drivers can be male and female adults of all ages. Also, if we look at risk analyses conducted by insurance companies, two things are immediately clear. First, younger people aged below 25 years are much more likely to be involved in accidents. This may be because older people have more experience in driving. Second, women drivers are statistically less likely to be involved in accidents than men of a similar age. So participants of this study are all likely to be amongst the most risky drivers in the population.

All this means it’s difficult to apply the evidence in the study to the population as a whole.

  • This was a simulation, so would the same results be seen when people are driving a car? The test is similar to sitting at a computer screen and drivers are not making ‘life and death’ decisions. In real life they may behave differently. For example, drivers may stop and take a break if they’re feeling tired or losing concentration. You can see the simulator in this video clip where one of the researchers discusses the study.

  • The researchers make a link between this research and some previous research looking at the effect of reduced sleep and/or low levels of alcohol and uses this link to say that the effects of mild dehydration and low doses of alcohol are similar. There is no direct comparison, there are additional measures of driving errors in the hydration study (late braking) so the evidence does not support this claim.

You can read a more detailed analysis produced by the NHS Choices ‘Behind the Headlines’ (BtH) service

Dehydration increasing the burden on family doctors

Apart from the misleading statement that “up to one in five patient consultations could be prevented if people simply hydrated properly” (presumably a misreading of the numbers), the content of Mirror news story appears to come almost word for word from the press release.

Indeed all the information about how the survey was conducted comes from the press release. This research has not been published – unlike the study on dehydrated drivers.

As we have so little information it leaves us with more questions than answers.

  1. This was a small percentage (0.45%) of the 66,757 doctors on the UK GP Register and we don’t know whether they are representative of this group as a whole.

  2. We don’t know how the doctors who took part in the survey were selected. Did they self-select – something that is likely to happen with online surveys? If so they may have done so because they are more concerned about dehydrated patients than those who did not.

  3. How the questions were developed and phrased is not clear. This is important because the way in which questions are phrased is known to have an impact on how we answer them.

  4. The press release suggests there is a problem because “60 per cent of the UK drink just one glass of water or less a day” referencing the Zenith International UK Bottled Water Drinks Report 2014 as supporting evidence. Unfortunately, a copy of the report costs nearly £3000, so additional data from this is not available to us to judge the reliability of the evidence

As noted, the top level results of the survey reported in the press release were that one fifth (21%) of GP visits were due to tiredness and fatigue. Of these, “dehydration was thought to be the cause of one in ten consultations” (actually 12%). Presenting the information this way suggests that doctors are being overrun with patients who just need to drink a bit more. However, this is 12% of 21% which is slightly more than 2.5%. If true, that is still a very significant figure as UK GPs have millions of consultations every year.

The press release also quotes UK National Health Service guidance as saying adults should drink 8-19 200ml glasses of fluid a day and children 6-8 glasses. However, the web page given to support this doesn’t actually state a recommended quantity of fluid. Elsewhere, NHS Choices states that “In climates such as the UK’s, we should drink about 1.2 litres (six to eight glasses) of fluid every day to stop us getting dehydrated. “ Click here for the article

Interestingly, the comment is about drinking fluid rather than water. NHS advice states “Water is a healthy and cheap choice for quenching your thirst at any time. It has no calories and contains no sugars that can damage teeth” but goes on to say that “plain tea, fruit tea and coffee (without added sugar) can also be healthy”. Click here for the article

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Making Sense of Evidence: The Informed Health Consumer

Cardiff University