This systematic review looked at whether being dehydrated or well hydrated changes executive function in adults. Executive function covers thinking skills such as memory, attention and self-control. The review reports no new data of its own. It summarized 33 earlier studies with 3,636 participants in total. All participants were adults older than 18. Several studies used special groups, such as doctors, nurses, soldiers, pilots and college athletes.
Most studies were small, short-term trials. Dehydration was usually caused by exercise, heat or limiting fluids, generally to about 2% body mass loss. Some trials gave set amounts of water, such as 300 mL during 4 hours of heat exposure.
The results were mixed. Of the 26 studies with a dehydration condition, 13 found clear harmful effects and 12 found no clear effect. One found that dehydration helped on a memory task. Of the 27 studies on rehydration, 16 found improvements. Where effects showed up, they were mainly in working or short-term memory and in attention.
The authors say strong conclusions are hard to draw. Only 8 of the 33 studies were rated positive for quality, and the rest were rated neutral. Most studies had fewer than 20 participants and were likely too small. Only one study looked at effects over a longer time. Heat and exercise made it hard to separate dehydration from those conditions. Practice on the tasks may also explain some positive results. The review did not cover children.
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Source: Katz B, Airaghi K, Davy B. Does Hydration Status Influence Executive Function? A Systematic Review. Journal of the Academy of Nutrition and Dietetics (2021). doi:10.1016/J.JAND.2020.12.021