Here’s another reason to fill up your water bottle this summer: even mild dehydration can make you less smart, according to a new survey of research from the Georgia Institute of Technology.
For his doctoral dissertation, Matthew Mittbrodt, now of Emery University, reviewed 33 studies on dehydration and cognitive performance. His key finding: some mental tasks are significantly impaired when dehydration levels reach two percent.
And it’s pretty easy to reach that level. If you weigh 150 pounds (10 stone 7), for instance, a two percent loss of body mass due to dehydration equals three pounds—an amount you can easily lose during a hot, humid run.
Not all aspects of cognitive performance decline due to this mild amount of dehydration, though. The ones that are most affected are more complex—motor skills like hand-eye coordination, attention, accuracy, and executive function, which you use to keep several projects with varying deadlines moving forward simultaneously. (Executive function is what you use to prepare Christmas dinner so everything is ready to go at the same time.)
Simpler mental talks, like information processing (such as handling overdue invoices) and reaction time, aren’t as impaired.
Mild dehydration can easily pose a problem during the work day, because many office jobs require the exact functions most harmed by inadequate hydration.
“Office tasks requiring planning, arithmetic, logic, and/or other critical thinking are likely utilising the executive functions,” Melinda Millard-Stafford, Ph.D., a Georgia Institute of Technology professor who supervised Mittbrodt’s research review, wrote in an email.
A workplace example of the type of attention that can be impaired by dehydration is “being able to sit in a long afternoon meeting and maintain focus without being distracted,” Millard-Stafford said.
Driving and operating machinery are examples of manual labor that could be affected by dehydration harming motor skills.
If the two percent dehydration threshold sounds familiar, that’s probably because conventional wisdom used to hold that running performance starts to suffer once you’re dehydrated to that level. In the past few years, a more nuanced understanding of on-the-run dehydration has developed—it’s now believed that most runners can maintain performance even if their dehydration is a little greater than that two percent dehydration level, as long as they drink when they’re thirsty (rather than drinking to immediately replace every ounce sweated out).
There’s no disagreement, however, about the wisdom of rehydrating soon after you’re done running. Doing so will speed your recovery and leave you fresher for the next day’s run. This new research reviews adds another reason to do so: you’ll be smarter.
A version of this article appeared on Runnersworld.com