Jenna M. Burchfield, Matthew S. Ganio, Brendon P. McDermott, Nicole E. Moyen, Cory L. Butts, Keeley Treece & Matthew A. Tucker

Human Performance Laboratory, Department of Health, Human Performance, and Recreation, University of Arkansas, Fayetteville, Arkansas

INTRODUCTION: Information regarding effects of dehydration (DY) and heat stress on mood and perceptual measures is controversial since the two conditions could confound one another. PURPOSE: To assess the independent and combined effects of mild DY through 24-h fluid restriction and heat stress via passive heating on mood, thirst, and thermal sensation in females. METHODS: Eight healthy females (24.3 ± 4.6 y, 162.4 ± 4.2 cm, 77.3 ± 12.9 kg) volunteered in two randomized, repeated-measures trials, both involving passive heating to a 1.0°C core temperature (TC) increase from baseline. 24-h prior to heat stress, females either remained euhydrated (EU) or became DY via fluid restriction. Upon arrival, percent body mass (BM) change was calculated by comparing their nude BM to a 3-day baseline BM record. EU individuals received fluid during heating to maintain euhydration. Percent BM change and blood (osmolality) and urine (osmolality and specific gravity) biomarkers were measured prior to and throughout testing. Thirst, thermal sensation, and mood (Brunel Mood Scale; BRUMS) were assessed via questionnaires before heat stress, at 0.5ºC, and 1.0ºC increases in core temperature. RESULTS: EU started the heating euhydrated according to BM change from baseline (-0.4±0.9%), urine specific gravity (USG; 1.009 ±0.007), urine osmolality (UOSM; 331±241 mOsm/kgH2O), and plasma osmolality (POSM; 286±4 mOsm/kgH2O). DY participants started heat stress with a BM change from baseline of -1.2±0.4%, USG of 1.027±0.004, UOSM of 1058±130 mOsm/kgH2O, and POSM of 290±4 mOsm/kgH2O. Despite physiological confirmation of dehydration, there were no differences in mood state, thirst, or thermal sensation between EU and DY at baseline (p > 0.05). Further the effect of heat stress on these variables was similar between groups. Independent of group, thermal sensation significantly increased with heat stress from baseline to 1.0°C TC increase (4.6 ± 0.8 to 6.7 ± 0.3, p < 0.05). However, independent of heat stress, DY subjects had significantly decreased Vigor (41.9 ± 3.6 v 37.3 ± 2.3 arbitrary units p < 0.05) and increased thirst (3.8 ± 0.5 v 6.6 ± 0.5, all p < 0.05). CONCLUSION: These data indicate that DY and heat stress do not additively affect mood or perceptual measures. Although heat stress does not affect mood, emphasis should still be placed on keeping participants hydrated when testing effects of heat stress, as hydration status alone affected Vigor and thirst independently of heating. Heat stress and dehydration were not additive in this setting. Heat stress failed to have an independent affect on mood or thirst measures, but dehydration alone decreased Vigor.

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