Despite it being well known that dehydration affects physical performance, less is known about the effects of dehydration on brain function. American researchers recently conducted a study to assess how much of an effect dehydration has on cognitive performance.
Dehydration occurs when more fluid, mostly water, is lost than ingested by the body. This results in the body not having enough water to carry out normal functions. Dehydration typically occurs due to exercise, high environmental temperatures, or disease.
Many studies have shown that optimal physical performance in activities such as exercise, muscular endurance, and occupational tasks is not possible with dehydration. There is less evidence, however, whether dehydration also affects cognitive performance.
Cognitive function is the process the brain undergoes to acquire knowledge and understanding. Brain activities included in cognitive function are memory, reasoning, attention, and language. Cognitive performance is the brain’s ability to utilize the knowledge or information acquired during this mental process.
At what point of dehydration is cognitive performance affected?
It is unclear how much water loss in the body (the percentage of body mass loss) results in impaired cognitive performance. Past research into the effects of dehydration on cognitive performance is inconsistent. However, different levels of dehydration clearly cause impaired cognitive function.
For example, soldiers who are in extreme conditions, such as the desert, are easily dehydrated. The dehydration can lead to confusion and delirium which leads to the soldiers having trouble navigating and completing operations. However, studies which have looked at the impact of dehydration on the cognitive performance of soldiers include other factors such as sleep deprivation, psychological stress, and low blood sugar levels.
It remains unclear at what point of the body becoming dehydrated is cognitive performance affected. There is evidence showing physical performance is affected when the body has lost 2% of water. Does cognitive performance also become impaired at this point? Or does the brain have protective mechanisms which kick in to protect the brain for longer?
Researchers in the US recently conducted a review to analyze the impact dehydration has on cognitive performance. They looked at factors such as how dehydration was caused and at what level of dehydration led to a decline in performance during cognitive tasks. It is easy to measure cognitive performance such as reaction time and accuracy during tasks that require either decision making, problem-solving, attention, memory judgment or eye-hand coordination. Their results were published in Medicine and Science in Sports and Exercise.
The review included 413 participants from thirty-three studies. The level of dehydration ranged from 1-6% of body mass loss. The method by which dehydration was induced and outcomes of cognitive performance including accuracy and reaction time varied. Dehydration was induced either by exercise, heat exposure, exercise with heat stress (meaning exercise was done at a higher temperature) or restricting the intake of fluids.
Dehydration did not affect all cognitive functions equally
The results showed that even though dehydration only slightly impaired cognitive performance, the effects were still significant. However, dehydration did not affect all cognitive functions equally. The impact it had on various cognitive tasks was quite different. Dehydration caused a greater impairment of cognitive tasks such as executive function (mental skills to get things done), motor coordination (body movements) and attention compared to tasks which required reaction time. It is not clear why different regions of the brain react differently to the loss of water, but it is possible that some areas of the brain are more susceptible.
The researchers found that studies reporting body mass loss greater than 2% in dehydrated individuals were linked to greater impairment of cognitive performance compared to individuals with body mass loss less than 2%. This suggests that the threshold at which dehydration impairs cognitive performance may be similar to that reported in the past for physical performance (i.e. exercise).
Limitations of this study include that it was a review of current literature and therefore it cannot provide explanations for the mechanism of action such as how dehydration is causing impairment of specific cognitive tasks. It can, however, provide guidance for future research.
Dehydration specifically impairs cognitive tasks requiring coordination and attention
In conclusion, this review showed that the effects of dehydration include impaired cognitive performance. Despite the differences in methods used by the various studies included and the range of results reported, the impact was clear. Cognitive tasks which required executive function, coordination, and attention were more likely to be impaired if an individual was dehydrated compared to tasks such as reaction time. Finally, the study also showed a positive association between dehydration and cognitive performance. The greater the level of dehydration, the greater the impairment of cognitive performance.
Written by Lacey Hizartzidis, PhD
Reference: Wittbrodt MT, Millard-Stafford M. Dehydration Impairs Cognitive Performance: AMeta-analysis. Med Sci Sports Exerc. 2018 Jul 10. doi:10.1249/MSS.0000000000001682.