Altered brain structure with preserved cortical motor activity after exertional hypohydration: A MRI study

Hypohydration exceeding 2% body mass can impair endurance capacity. It is postulated that the brain could be perturbed by hypohydration, leading to impaired motor performance. We investigated the neural effects of hypohydration with magnetic resonance imaging (MRI). Ten men were dehydrated to approximately 3% body mass by running on a treadmill at 65% maximal oxygen consumption (V O2max) before drinking to replace either 100% [euhydration (EU)] or 0% [hypohydration (HH)] of fluid losses. MRI was performed before start of trial (baseline) and after rehydration phase (post) to evaluate brain structure, cerebral perfusion, and functional activity. Endurance capacity assessed with a time-to-exhaustion run at 75% V O2max was reduced with hypohydration (EU: 45.2 9.3 min, HH: 38.4 10.7 min; P 0.033). Mean heart rates were comparable between trials (EU: 162 5 beats/min, HH: 162 4 beats/min; P 0.605), but the rate of rise in rectal temperature was higher in HH trials (EU: 0.06 0.01°C/min, HH: 0.07 0.02°C/min; P 0.01). In HH trials, a reduction in total brain volume (EU: 0.7 0.6%, HH: 0.7 0.9%) with expansion of ventricles (EU: 2.7 1.6%, HH: 3.7 3.3%) was observed, and vice versa in EU trials. Global and regional cerebral perfusion remained unchanged between conditions. Functional activation in the primary motor cortex in left hemisphere during a plantar-flexion task was similar between conditions (EU: 0.10 1.30%, HH: 0.11 0.31%; P 0.637). Our findings demonstrate that with exertional hypohydration, brain volumes were altered but the motor-related functional activity was unperturbed.