Dehydration Lowers Cognitive Abilities And Changes The Shape Of Your Brain

While it remains unclear how the changes in brain morphology observed in the study relate to the cognitive deficits, one thing is clear: true dehydration has potent effects. Maridav/Shutterstock

Recent research has pushed back on widely perpetuated hydration myths that would lead you to believe that guzzling two liters a day is absolutely required, finding instead that simply replacing the water that is displaced in urine and sweat will keep the body functioning properly.

“With an hour of moderately intense activity, with a temperature in the mid-80s, and moderate humidity, it’s not uncommon to lose a little over 2 pounds of water,” lead investigator Mindy Millard-Stanford said in a statement from July.


“If you do 12-hour fluid restriction, nothing by mouth, for medical tests, you’ll go down about 1.5 percent,” she said. “Twenty-four hours fluid restriction takes most people about 3 percent down. If you drop 4 or 5 percent, you’re going to feel really crummy."

Hoping to add upon the current literature, Millard-Stanford’s team at the Georgia Institute of Technology initiated a new study exploring whether exercise and heat stress alone can impact cognition, and how dehydration would exacerbate the matter.

The series of experiments, documented in Physiological Reports, involved 13 volunteers asked to perform a simple attention and motor function test over a 20-minute period while inside an fMRI scanner. Before the test, they engaged in one of three activity sessions: in the control, subjects lounged in a temperate room for about 90 minutes, and for the other two, they completed a 150-minute-long session of walk-rest cycles on a treadmill in a hot, 45°C (113°F) room, either with access to enough water to replace lost fluid or with no water (achieving ~3 percent loss).

A research scientist in Mindy Millard-Stafford's exercise physiology lab withdraws sweat from a pouch on the back of a volunteer. Sweat is analyzed for electrolyte concentration among other biochemicals. Credit: Georgia Tech / Christopher Moore

All three sessions were conducted on separate days, and the fMRI tests were administered in air-conditioned rooms after a 45-minute cool-down.


Overall, subjects got worse at the dot task over the course of the 20 minutes, regardless of which activity they did before. This may be due to boredom, however subjects’ average performance following the exercise with water session was significantly worse than their control session score (about 8 percent), and their performance after exercise without water was twice as poor (16 percent worse).

The fMRI images showed that the cerebrospinal fluid-filled ventricles in the brain contracted when participants exercised, sweated, and drank water; yet when dehydration is added to the mix, the ventricles expanded. As intriguing as this finding may be, it may not have anything to do with the cognition test results.

fMRI image of a test subject's brain reveals expanded ventricles and a more compact thalamus. Credit: Georgia Tech / Christopher Moore

"The structural changes were remarkably consistent across individuals," Millard-Stafford said in a more recent statement. "But performance differences in the tasks could not be explained by changes in the size of those brain areas."

Brain scans also revealed that neural firing patterns are different after dehydration kicks in.


"The areas in the brain required for doing the task appeared to activate more intensely than before, and also, areas lit up that were not necessarily involved in completing the task," first author Matt Wittbrodt said. "We think the latter may be in response to the physiological state: the body signaling, 'I'm dehydrated'."

Millard-Stafford’s next hydration research goal is to determine if electrolyte beverages can prevent exercise-related cognitive declines better than plain water. Considering that the media has been flooded with research conducted by industry-sponsored groups or institutions, it would be nice to have some objective data on this subject.