The Impact of ‘Voluntary Dehydration’

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The Impact of ‘Voluntary Dehydration’

In the ground-breaking 2015 Loughborough study, driver dehydration led to a sharp increase in driver errors, an increase in thirst perception and significantly reduced levels of concentration and awareness. While the adverse effects of driver dehydration has now reached global exposure, and while much has been written about studies that induced dehydration in one way or another via exercise, heat stress or diuretics; very little research has been done on ‘voluntary dehydration’ that easily achieved state where, all else being equal, we simply do not drink enough.

Scientists from Massey University ran such a study entitled Hypohydration per se affects mood states and executive cognitive processing: results from a face-valid model for studying some consequences of ‘voluntary dehydration’. Published 14 September 2015 and available online at Extreme Physiology & Medicine, the study can be split into hypothesis, methodology and results.


Without inducing dehydration (as per methods discussed above), and while controlling confounding factors such as sleep, diet and caffeine; the aim of the study was to measure the cognitive effects of ‘not drinking enough’ i.e. ‘voluntary dehydration’.


The test group comprised of 24 males with a mean age of 26 years ± 6 y. The laboratory environment was kept at 20°C and 600 lux (slightly brighter than the standard office environment). Test subjects were given at least three sessions in which to familiarise themselves with the environment before completing the two experimental sessions. In the 24 hours prior to the experimental sessions, the men followed similar diet, sleep and caffeine intake with the only difference being how much fluids they consumed – usual ad libitum (euhydration – the absence of absolute or relative hydration or dehydration) versus complete restriction (hypohydration – dehydration). During the experimental sessions, their hydration levels, mood states (using psychological rating scale POMS) and aspects of cognition (logical reasoning, working memory, executive processing) were assessed.


For all subjects sleep duration was a mean of 8.5 hours; calorie intake a mean of 9391 kilojoules; and caffeine intake a mean of 54mg; while water intake in the euhydration phase was a mean of 1436 ml ± 1311 mL compared to the hypohydration phase in which water was restricted to a mean of 285 ± 446 mL.

In all cases of fluid restriction where mild dehydration (hypohydration) was achieved, mood states (which covered tension-anxiety, depression-dejection, anger-hostility, fatigue-inertia, vigour-activity, and confusion-bewilderment) were detrimentally affected, along with performance of working memory and executive processing.

In their post-study discussion, the researchers stated

‘In populations (e.g. illness/disease, ageing) or occupations (e.g. aviation, military) where voluntary dehydration may be common [as is very often the case for HGV drivers where lack of access to regular hydration leads to driver dehydration] these findings indicate that even mild hypohydration, typical of a working day, may have detrimental consequences for mood and some executive cognitive functions.’

Proof yet again that even mild dehydration adversely affects performance across the board, and while this might not mean the difference between life and death in many industries, it might be the tipping point in others. The Driving Hydration package provides practical ways to combat voluntary dehydration, thereby ensuring a healthier and safer journey for all concerned.

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