Introduction: Hypohydration exceeding 2% body mass is known to impair endurance performance (Sawka et al., 2001; Cheuvront et al., 2010). It is hypothesized that the central nervous system, specifically the brain, is negatively affected by hypohydration, leading to an impairment in endurance capacity. To date, few brain imaging studies have investigated the functional impact of exercise-induced hypohydration and this precipitates the main objective of this study. Methods: Ten endurance-trained sportsmen (mean±sd: age 23.3±1.1 years; body fat 10.5±2.4%; VO2peak 65.6±5.0 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 separate randomized, counterbalanced trials. Participants underwent MRI scans at baseline and post-fluid replacement for assessment of brain volume and fMRI imaging during a plantar flexion task. Endurance capacity was assessed by running to exhaustion at 75% VO2peak. Core temperature, heart rate and endurance timings were assessed using paired sample T-test with p<0.05 considered significant. Results: Results from the endurance run showed similar heart rates between the euhydrated (EU) and hypohydrated (HH) trials at time of exhaustion (EU: 171±10 bpm, HH: 172±9 bpm; p=0.184). Core temperature before the start of endurance run were similar between conditions (EU: 36.7±0.2°C, HH: 36.7±0.3°C; p=0.836). The absolute change in core temperature in the endurance run were similar between trials (EU: 2.7±0.2°C, HH: 2.7±0.3°C; p=0.939). However, the rate of increase in core temperature was higher in hypohydrated subjects (EU: 0.06°C/min, HH: 0.07°C/min; p=0.006). Endurance capacity was reduced with hypohydration (EU: 45.2±9.3 min, HH: 38.4±10.7 min; p=0.033). Fold changes in total brain volume were lower in HH trials than EU trials (EU: 1.007, HH: 0.993; p=0.003). BOLD (blood-oxygen-level dependent) activation in the precentral gyrus (motor), postcentral gyrus (somatosensory) and supplementary motor area (SMA) during the plantar flexion task were similar between conditions (PreCG: p= 0.314, PostCG: p=0.332, SMA: p=0.677). Conclusion: Our findings demonstrate that the cardiovascular system is remarkably defended in endurance-trained athletes despite loss of body fluids. Total brain volume is reduced with hypohydration but notably the functional activity of the brain is well-preserved in these endurance athletes. The decline in endurance capacity could possibly be due to other factors such as the greater heat strain accumulated in hypohydrated individuals.
© Copyright 2016 21st Annual Congress of the European College of Sport Science (ECSS), Vienna, 6. -9. July 2016. Published by University of Vienna. All rights reserved.