Food and diet

Why you should drink (water) before you drive

"Not drinking enough water has same effect as drink driving," The Daily Telegraph reports. A small study found participants made more mistakes on a driving simulator task when they were mildly dehydrated than when they had plenty of fluids.

This was a small trial of 12 men, studying the effect of mild dehydration on performance during a driving task. The men had a day of being hydrated or fluid-restricted prior to spending two hours in a driving simulator showing a view of a monotonous dual carriageway.

This was a crossover trial, meaning that all men acted as their own control, undertaking both hydrated and dehydrated conditions one week apart.

The researchers found men in the dehydrated state made around double the number of driving errors during the two-hour drive compared with the hydrated group.

Overall, the detrimental effects of dehydration on wellbeing and physical and mental performance are well-publicised, so the results are entirely plausible. But the study has many limitations, so it cannot provide solid proof.

These include the very small sample size and the fact that spending two hours in a driving simulator in an enforced state of dehydration or hydration may not be the same as driving in real life. The participants could have driven less carefully because they knew it was only a simulation.

Still, when you are in charge of several tonnes of metal moving at high speed, anything that could impair your concentration is a concern. We recommend topping up with food and water if you are going on a long drive, as well as taking regular breaks. 

Where did the story come from?

The study was carried out by researchers from Loughborough University and was funded by the European Hydration Institute.

It was published in the peer-reviewed journal, Physiology and Behaviour.

The UK media reliably reports the main theme of this research, but does not point out that, though based on an entirely plausible hypothesis, this small study actually provides very little conclusive proof. 

What kind of research was this?

This was a small randomised crossover trial looking at the effect of mild dehydration on driving performance during a long, monotonous driving simulation.

As the researchers explain, mild dehydration can cause symptoms such as headache, weakness, dizziness, fatigue, lethargy, and reduced alertness and ability to concentrate. This could affect both physical and mental performance in a variety of tasks, including driving.

The study was particularly interested in any possible link between dehydration and vigilance or response times during a driving simulation. The crossover design meant participants acted as their own controls, performing the task in both hydrated and dehydrated conditions.  

What did the research involve?

The study included 12 healthy men with an average age of 22, who were all tested in a driving simulator. After an initial visit to familiarise themselves with the set-up, the participants attended the lab on two separate occasions seven days apart. The hydrated and dehydrated conditions were given in a random order.

Each man filled in a food and drinks diary the day before each visit. They went to the test laboratory after a 10-hour overnight fast, where urine and blood samples were taken.

Subjective feelings of thirst, hunger, concentration and alertness were assessed on a visual analogue scale, where you plot yourself on a 100mm line from good to bad, such as "not thirsty" to "dire thirst".

The men went away for a day with the instruction to repeat their food intake of the previous day, with differences in fluid intake.

The hydrated group drank at least 2.5 litres of fluid throughout the day, while the dehydration group only had 25% of this fluid intake (expected to cause a 1% reduction in body weight over 24 hours).

The following morning, they returned to the test lab after another overnight fast and the blood, urine and visual scales were repeated. They were then given breakfast, along with water to drink – 500ml in the hydrated group and 50ml in the dehydrated group.

They were fitted with electrodes to measure their brain activity (an electroencephalogram, or EEG) and then completed a two-hour driving task in the driving simulator.

The car gave a computer-generated road projection of a monotonous dual carriageway with long straight sections and gradual bends.

Slow-moving vehicles were met occasionally and had to be overtaken. Otherwise, the driver was instructed to stay in their lane. After one hour of the task, 200ml of fluid was given to the hydrated group and 25ml to the dehydrated group.

After the driving trial, blood samples were taken and an assessment was again made of subjective feelings of thirst, throat dryness, hunger, concentration and alertness. 

What were the basic results?

Data is only reported for 11 of the 12 participants. One was excluded from the results for "displaying a high propensity to fall asleep during the driving task (perhaps caused by sleep deprivation)".

The day of fluid restriction caused a 1.1% reduction in body mass, compared with a 0.1% reduction in the people who drank normally on that day. Examination of their blood and urine samples also confirmed that they were less hydrated.

The two-hour driving test was split into four 30-minute sections. Both groups made more and more driving errors as the test progressed. However, the number of errors was consistently higher in the dehydrated group than in the hydrated group – significantly so after the first 30 minutes.

These were minor errors, and included drifting, car wheels crossing the rumble strip or lane line, and late braking. There were four major incidents (such as hitting the barrier or another car), but these were evenly distributed between the two groups.

Overall, there were 101 major or minor errors in the dehydrated group, compared with 47 in the hydrated group – a statistically significant difference.

There was no significant difference in brain activity between the groups throughout the trial, as measured by the EEG.

At the end of the trial, people in the dehydrated trial rated worse for feelings of thirst, throat dryness, hunger, concentration and alertness. 

How did the researchers interpret the results?

The researchers concluded that, "The results of the present study suggest that mild [dehydration] produced a significant increase in minor driving errors during a prolonged, monotonous drive, compared to that observed while performing the same task in a hydrated condition."

They say the magnitude of decrement was similar to that observed when driving after drinking alcohol (to a blood alcohol concentration of approximately 0.08%, which is the current UK legal driving limit), or while sleep-deprived. 


This small randomised crossover study suggests that men make more minor driving errors when dehydrated, similar to the effect of being over the alcohol limit or sleep-deprived.

The idea that dehydration worsens driving ability is plausible. However, despite the plausibility of these results, there are several important limitations, meaning that this study does not actually provide firm evidence.

Representation of the sample

The study included only 12 young healthy males, and one of them was excluded as it was thought his performance wasn't reliable enough during the trial. The performance of these 11 remaining men cannot be extrapolated to the general population, as there are too many potential variables, such as age, gender, and varying general driving abilities, alertness and concentration levels.

Sample size

With only 11 men analysed, it is possible that the results could have been completely different if a larger sample had been studied. As the researchers acknowledge, the small sample size means their study did not have the statistical power to examine how the number of driving errors was related to the degree of hydration.

The artificial scenario

Spending two continuous hours in a driving simulator viewing a monotonous computer-generated screen while in an enforced state of dehydration or hydration may not be the same as driving in real life. For example, in real life:

  • you know you are in a serious situation where errors can mean life or death
  • there are variations in scenery and other distractions, which could have either beneficial or detrimental effects (such as fresh air or loud noise) 
  • if you know you are feeling unwell, you can actually stop, have a break, have something to eat or drink, for example 

Unproven comparisons

Though the study – and hence the media – has made a comparison between dehydration, alcohol and sleep deprivation, these are indirect comparisons.

Overall, despite the study's limitations, the detrimental effects of dehydration on wellbeing and physical and mental performance are recognised. That this applies to driving is entirely plausible, but was not proven by this study.

But if you are driving and feel thirsty, it is highly recommended that you take a break and rehydrate. Anything that can impair your concentration while driving is a potential risk to health.

As this study points out, worldwide, an estimated 1.2 million people die and a further 50 million people are injured each year in road traffic accidents. Driver error is the leading cause of accidents.

Read more about road traffic safety.

NHS Attribution