Mild Dehydration Impairs Cognitive Performance and Mood

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Mild Dehydration Impairs Cognitive Performance and Mood

The American College of Sports Medicine quantifies mild dehydration as a 1% loss in body mass. Previous blogs dealt with the ground-breaking Loughborough study that documented how driver hydration impacts both mental and physical performance. The startling results showcased how mild dehydration resulted in double the driver errors, sharply increased thirst perception, and reduced concentration and levels of awareness. Equally disturbing was that insufficient driver hydration was seen to be as dangerous as driving while under the influence of alcohol or sleep deprived.

This blog, while not dealing specifically with driver hydration, deals with another study that assessed the effects of mild dehydration on mental ability and mood – all of which corroborate the findings of the Loughborough study and underpin the Driving Hydration solution. The study entitled ‘Mild dehydration impairs cognitive performance and mood of men’ appeared online at PubMed and ResearchGate; and was published in the British Journal of Nutrition in 2011.

Analysis of the study can be split into hypothesis, methodology and results.


Past studies evaluating the effects of dehydration on mental and physical performance along with mood, achieved dehydration in their subjects by having them exercise in hot conditions. The effects of dehydration and hyperthermia (the condition of having a body temperature greatly above normal) were therefore intertwined, and the detrimental effects of dehydration alone could not be isolated. More recent studies forwent hyperthermia, but the aimed for levels of dehydration were more severe (between 2.6 -2.8%).

The goal of this study was to examine the effects of mild dehydration (as stated previously, the American College of Sports Medicine quantifies this as a 1% drop in body mass) on performance and mood, without incurring hyperthermia.


The test group comprised 26 young men, with an average age of 20 years. They participated in three randomised, single-blind (only experimenters know the makeup of the test – thereby eliminating subjective bias), repeated-measures (each condition includes the same subjects) trials:

  1. Exercise-induced dehydration plus a diuretic
  2. Exercise-induced dehydration plus a placebo containing no diuretic
  3. Exercise while maintaining euhydration (normal state of body water content, the absence of absolute or relative hydration or dehydration) plus placebo.

Each trial had the men complete three 40 minute treadmill walks at 5.6 km/hour, on a 5% grade in a 27.7 °C environment. During each trial an extensive computerised cognitive test battery was administered – tasks examined vigilance, reaction time, short-term spatial memory and pattern recognition, learning, and logical reasoning, amongst others. Also administered was the Profile of Mood States questionnaire – a standard validated psychological test that assesses transient, fluctuating feelings and longer-lasting mood states; and the symptom questionnaire – assessing headaches, concentration and task difficulty.


The level of dehydration achieved in the study, without incurring hyperthermia, is similar to the level of mild dehydration encountered routinely by adults during a normal work day. The results showed that even mild dehydration led to changes in ability – “Dehydration degraded specific aspects of cognitive performance: errors increased on visual vigilance and visual working memory response latency slowed. Fatigue and tension/anxiety increased due to dehydration at rest and fatigue during exercise.”

In conclusion, the study found that mild dehydration in men (sans hyperthermia) resulted in adverse changes in vigilance and working memory, and levels of tension/anxiety and fatigue increased.

Proof yet again that even mild dehydration affects performance and that driver hydration is a crucial factor in helping to minimise driver errors and reduce vehicle incidents – the Driving Hydration solution has everything transport managers and directors need to educate their drivers about the importance of driver hydration and provides practical steps to ensure they remain adequately hydrated throughout the day.

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