“Dehydration elicits a small, but significant impairment on cognitive function.”

A recent study suggests that dehydration can impair cognitive function. In this post I outline the results of the study, what it means to the individual athlete or operative, and practical recommendations.

The maintenance of water and salt balance is essential for survival within the human body and all living organisms. Physiological and behavioural changes act to control our intake of fluids and electrolytes to compensate for any losses, typically through sweating. Fluid and electrolyte loss are common during exercise, particularly when performed in the heat or due to wearing encapsulating clothing. Also consider the fireman, soldier or motorsport athlete who are often performing physical activity, whilst wearing encapsulating and protective clothing in a hot environment. In these occupational duties and during exercise, dehydration, where water losses exceed water intake leading to a deficit in body water, can occur. So, if you consider your weight before and after an exercise session (omitting and fluids consumed) the difference in weight comes primarily from water loss.

Whilst the effects of dehydration on physical performance usually receive the greatest focus, a recent study (1)highlighted that dehydration can also result in a decline in cognitive function, for example decision making, problem-solving or judgement. Again, in the case of the soldier operating in hot environments, dehydration has shown to contribute to a decline in ability to navigate, perform military operations, reaction time, attention, memory and reasoning (2). I say contribute because other factors such as sleep deprivation could also have played a part in the results.

A reduction in body mass by 2% from dehydration has been suggested to impair physical performance (3). Interestingly, the meta-analysis found that studies in which participants were dehydrated more than 2% body mass showed greater impairment in cognitive function than those were body mass was equal to or less than 2 %. Adding to this finding the study reports that the decline in cognitive performance was related to the magnitude of water deficit i.e. the more dehydrated you become, the greater the decline in cognitive performance.

As the title of this article suggests, the main finding of the study was that dehydration can cause a small but significant decline in cognitive performance. An interesting finding was, however, that some cognitive tasks such as attention, executive function and motor-coordination were more severely affected than other tasks like memory or reaction time; meaning that fine skills such as basketball dribbling, for example, are more likely to become impaired. Regarding the reduction in motor coordination capabilities, these tasks mostly consisted of fine motor movements, such as those of the finger or hand, in response to a visual stimulus. Similarly, accuracy was more severely impaired than reaction time.

What does this mean for the individual?

Hydration is obviously an important component in an athlete’s training or competition nutrition strategy, but if we consider the risks at stake for the combat operative or emergency services, any decline in physical and, or, cognitive function is obviously concerning. The potential risks to motorsport athletes must also be appreciated as attention and fine motor skills are important to performance; a brief loss of concentration or a slip on the controls could have severe consequences beyond losing a position on the track. The study suggests that fine skills are at risk of being impaired by dehydration, such as dribbling a football or basketball. But these findings could also apply to high-skill sports such as mountain bike, motocross, endure or speedway for example.

Practical Recommendations

The greatest effect on cognitive function was seen when body mass (weight) loss was greater than 2%, so your hydration strategy should aim to avoid this. Beginning exercise adequately hydrated is important and whilst more complex methods of monitoring hydration status are available, monitoring pee colour is a good starting point. Whilst it may not be practical to monitor weight during a race or occupational situation, gaining insight into the possible magnitude of fluid loss could be conducted during training by pre- and post-weighing. Following this, allowing for environmental conditions which would suggest higher rates of sweat loss on hot days, you could implement a hydration strategy to maintain euhydration or at least avoiding body mass losses greater than 2 %.


  1. Wittbrodt, M.T. Millard-Stafford, M. (2018). Dehydration Impairs Cognitive Performance: A Meta-Analysis. Med Sci Sports Exerc.. doi: 10.1249/MSS.0000000000001682  [Epub ahead of print]
  2. Lieberman, H.R, Bathalon, G.P. Falco, C.M. Kramer, F.M. Morgan, C.A 3rd, Niro, P.(2005). Severe decrements in cognition function and mood induced by sleep loss, heat, dehydration, and undernutrition during simulated combat. Biol Psychiatry.2005 Feb 15;57(4):422-9.
  3. Sawka, M.N. Burke, L.M. Eichner, E.R. Maughan, R.J. Montain, S.J. Stachenfeld, N.S. (2007). American College of Sports Medicine position stand. Exercise and fluid replacement. Med Sci Sports Exerc. ;39(2):377-90


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