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Along with mood and energy deficits a dehydrated brain has to use a lot more energy to accomplish the same tasks, shows a study from King's College London.

Quote from Dehydration affects brain structure and function in healthy adolescents (2011):

It was recently observed that dehydration causes shrinkage of brain tissue and an associated increase in ventricular volume.

...

Dehydration following the thermal exercise protocol led to a significantly stronger increase in fronto-parietal blood-oxygen-level-dependent (BOLD) response during an executive function task (Tower of London) than the control condition, whereas cerebral perfusion during rest was not affected. The increase in BOLD response after dehydration was not paralleled by a change in cognitive performance, suggesting an inefficient use of brain metabolic activity following dehydration. This pattern indicates that participants exerted a higher level of neuronal activity in order to achieve the same performance level. Given the limited availability of brain metabolic resources, these findings suggest that prolonged states of reduced water intake may adversely impact executive functions such as planning and visuo-spatial processing.

Also according to an article on shape.com the energy usage of the brain increases, probably referring to "inefficient use of brain metabolic activity".

Why exactly does the brain shrink and increase energy usage after dehydration?

  • The shrinkage is easy to explain: less water = less blood volume = loss of water from tissues. But I don't see where you're getting an increased energy usage. Why do you say that? – Carey Gregory Dec 3 '19 at 5:16
  • @CareyGregory, I've read several articles that "summerize" this research like that, likely referring to the part "suggesting an inefficient use of brain metabolic activity following dehydration.". The metabolism of an organism defines which substances it will find nutritious. Metabolic rate also plays a strong factor in determining how much food an organism requires. Hedge, likely the " translation" to "energy usage". – Bob Ortiz Dec 3 '19 at 7:31
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    @CareyGregory I've added the URL to one article where it is "summarized" as such. – Bob Ortiz Dec 3 '19 at 8:20
  • How does one find out if his/her brain has shrank or not, or perhaps dehydrated or not? – jayarjo Dec 4 '19 at 8:16
  • @jayarjo. I don't know details about such measurements but apparently there is at least one method to do so. It sounds like a good question to post on this site. Maybe you can do some small research to those measurement methods and post a question? – Bob Ortiz Dec 4 '19 at 8:31

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