Autonomic Adaptations mediate the effect of Hydration on Brain Functioning, Cognition and Mood: A Randomised Controlled Trial.

Dehydration (water loss: 2.0% of body weight) has significant negative effects on physical and mental performance. In the current study, the effects of minor (water loss <1.0% of body weight) hypo-hydration on CNS function, mood and cardiovascular function were measured. On two mornings twelve male participants were exposed to a temperature of 30oC for four hours and either did or did not drink two 150ml glasses of water during that time. Subjects not given water lost 0.6% body weight. Subjects participated in a fMRI protocol during which they completed a modified version of the Paced Auditory Serial Addition Test (PASAT): a challenging arithmetic task designed to elicit autonomic arousal. Throughout the morning the subjects had their heart rate variability (HRV) monitored. At periodic intervals, subjects rated their thirst and mood by visual analogue scales and at the end of the PASAT task, they rated its difficulty. Changes in body temperature and fluid loss (perspiration and urine) and urine osmolality were monitored. Decreases in functional magnetic resonance imaging (fMRI) blood oxygen dependent level (BOLD) activity in the orbitofrontal cortex, ventral cingulate gyrus, dorsal cingulate cortex, hypothalamus, amygdala, right striatum, post-central gyrus and superior parietal cortex were seen when participants were hypo-hydrated. These deactivations were associated with reduced HRV, greater perceived effort and more anxiety. Drinking water also prevented a decline in memory and mood. These data are discussed in relation to a model that describes how autonomic regulatory and interoceptive processes may contribute to the cognitive and affective consequences of hypo-hydration. This trial was registered at clinicaltrials.gov NCT03525470.