Effects Of Exercise-induced Hypohydration On Brain Structure And Function, A MRI Study

Abstract

Hypohydration exceeding 2% body mass is known to impair endurance capacity. It is hypothesized that the central nervous system, specifically the brain, is negatively affected by hypohydration, leading to a decline in endurance capacity. PURPOSE: To investigate the effects of exercise-induced hypohydration on the brain. METHODS: Ten trained endurance males (mean±sd: age 23.3±1.1 years; body fat 10.5±2.4%; VO2peak 65±5 ml kg-1 min-1) were dehydrated to -3% body mass by running on a treadmill at 65% VO2peak in a 25°C environment, before drinking to replace 100% or 0% of fluid losses in two randomized, counterbalanced trials. Participants underwent MRI scans at baseline and post-fluid replacement to examine brain volume, functional activity and cerebral perfusion. Magnetic resonance spectroscopy was used to measure brain temperature (at primary motor cortex) before and during the dehydration run. Endurance capacity was assessed by running to exhaustion at 75% VO2peak. Results were assessed using paired sample T-test with p<0.05 considered significant. RESULTS: MRI results demonstrated a reduction in total brain volume in hypohydration (HH) as compared to euhydration (EU) trials (EU: 1.007, HH: 0.993; p=0.003). BOLD (blood-oxygen-level dependent) activation in the primary motor (M1) and somatosensory cortex (S1) during a plantar flexion task were similar between conditions (M1: p= 0.314, S1: p=0.332). Global and regional cerebral perfusion remained unchanged between conditions (Global: p= 0.055, M1: p= 0.447, S1: p=0.458). Brain temperature measured at baseline was higher than core temperature (Brain: 37.7±0.5°C, Core: 36.7±0.3°C; p<0.0001). However, both temperatures were similar during exercise (Brain: 38.2±0.4°C, Core: 38.6±0.3°C; p=0.110). Endurance capacity was reduced with hypohydration (EU: 45.2±9.3 min, HH: 38.4±10.7 min; p=0.033). CONCLUSION: Under hypohydration, the endurance capacity is impaired and total brain volume is reduced. Brain functional activity and cerebral perfusion are notably well-preserved. Brain temperature could be regulated within a narrower homeostatic range than the core temperature. Supported by DIRP Grant, PA No. 9015102335 and funding from ERGOTECH, South Africa.