Dehydration Study Shows How Summer Heat Can Lead to Bad Decision Making

The heat really might be frying your brain.


As summer temperatures soar to record highs, researchers are starting to realize that the heat can do weird stuff to our brains. Earlier in July, scientists demonstrated that heat waves can make college students dumber. Now, two studies out of the Georgia Institute of Technology add to the chaos by showing that dehydration can light up certain areas of the brain like fourth of July fireworks. While the fMRI scans in the study are lit, your dehydrated decisions, almost certainly, are not.

The papers show that even a little bit of heat-induced dehydration, which is stressful for the brain, can have measurable effects on cognitive function. But not all parts of the brain react equally. In one of the papers, published in early July in Medicine & Science in Sports & Exercise, scientists perform a meta-analysis of 33 previous studies, showing that dehydration leads to significant decreases in the performance of more complex tasks requiring focus and the “higher order functions required in decision.” However, it had little to no effect on simple tasks that measured reaction time.

Georgia Tech School of Biological Sciences professor Mindy Millard-Stafford, Ph.D., the lead author of that paper, says those executive-level functions apply to tasks like staying alert in a meeting or doing a math problem.

“In broad-brush terms, it seems that tasks that require more complex thinking, and complex processing appear to be impacted more,” Matthew Wittbrodt, Ph.D., a graduate research assistant turned postdoctoral researcher at Emory University, tells Inverse.

To pin down the mechanisms behind those declines in complex thinking, Wittbrodt is simultaneously working on another paper, soon to be released in the journal Physiological Reports. In his examination of fMRI brain scans of dehydrated individuals, he noticed that the brains showed more brain activity than the brains of non-dehydrated subjects. This was particularly pronounced in the thalamus, an area of the brain that is attuned to the body’s physiological state and is also implicated in mood, wakefulness, and consciousness.

“There are a handful of studies that show higher brain activation when you’re dehydrated. We’re not the only ones to see it,” Wittbrodt says. “We can’t exactly tease out why yet, but we did tend to see a lot of things happening in the thalamus, such as it shrinking with dehydration or greater activation. So it seems like the thalamus might be uniquely challenged.”

A figure from a previous study, examined in the meta-anaylsis showing higher levels of brain activity in the hours after exercise, when participants were dehydrated. 

Human Brain Mapping 

While it’s too early to pin down why researchers are seeing this greater activation during dehydration, Wittbrodt suggests that there might be a survival aspect at play. “From a survival aspect, you’re trying to figure out how to get water or how to manage that state that you’re in,” he says. This, potentially, could kick the brain into overdrive as it tries desperately to figure out a way to end the torturous state of dehydration. That extra activity, says Wittbrodt, is actually evidence that the brain becomes more inefficient as it becomes more dehydrated — it wouldn’t be working that if it were

“It’s just a matter of you needing more neural resources to complete the same task,” he Wittbrodt.

So, don’t let the brain scans deceive you. If you’re dehydrated — which you probably are, unless you’re actually drinking the Mayo clinic’s advised 11.5 to 15.5 cups per day — summer might not be the best time to think difficult thoughts. Which, lets admit, probably comes as a relief to most of us anyway.

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