No matter how mild, dehydration is not a desirable condition because there is an imbalance in the homeostatic function of the internal environment. This can adversely affect cognitive performance, not only in groups more vulnerable to dehydration, such as children and the elderly, but also in young adults. However, few studies have examined the impact of mild or moderate dehydration on cognitive performance. This paper reviews the principal findings from studies published to date examining cognitive skills. Being dehydrated by just 2% impairs performance in tasks that require attention, psychomotor, and immediate memory skills, as well as assessment of the subjective state. In contrast, the performance of long-term and working memory tasks and executive functions is more preserved, especially if the cause of dehydration is moderate physical exercise. The lack of consistency in the evidence published to date is largely due to the different methodology applied, and an attempt should be made to standardize methods for future studies. These differences relate to the assessment of cognitive performance, the method used to cause dehydration, and the characteristics of the participants.

Key teaching points:

• This paper reviews the existing findings about the impact of dehydration in the main cognitive skills explored so far.

• Children and the elderly are the populations most vulnerable to dehydration.

• Healthy young adults are also at risk of a decrease in their cognitive performance when hydration is not adequate.

• Attention, psychomotor, and immediate memory skills, as well as assessment of the subjective state, are the brain capabilities most vulnerable to mild or moderate dehydration.

• The relationship between hydration and cognitive performance is an emerging area of study of undoubted practical interest in which much research still need to be carried out.