The hydration status of tissues and cells is important in all organisms because water is involved in maintaining osmoregulation and correct pH, and is also needed for the effective removal of cellular waste. In mammals, water is tightly regulated and even small changes in the hydration of cells and tissues can affect their function. The brain tissue is particularly sensitive to even small fluctuations in hydration, and as dehydration occurs changes in neuronal physiology may become evident through mental and emotional perturbations. In humans, severe dehydration can lead to confusion and delirium, but even exercise induced dehydration of as little as 2 to 4 % can cause deteriorations in performance on mental tests. However, many such tests are administered in conditions of high heat and this may be a confounding variable. Longer-term less severe dehydration may cause milder changes in metal performance that are difficult to detect.
Light intermittent exercise can be used by researchers to induce dehydration and the effects on neuronal tissue measured using cognitive tests. Researchers1 used this approach on 25 healthy female subjects, in a placebo controlled study published in the Journal of Nutrition in 2012. Each subject underwent 3 treatments that involved taking a diuretic (forosemide 40mg), taking a diuretic and undertaking mild exercise or euhydration (as a control). Cognitive tests were used at rest and during the three treatments to asses perceived fatigue, headache, concentration, task difficulty and total mood disturbance. Hydration, as measured by serum osmolality, indicated that dehydration of around 1 % was achieved in the treatment groups, and this was significantly different to the control. This low level of dehydration resulted in higher heart rates and increased feelings of fatigue-inertia, anger-hostility and vigour-activity. Concentration and headache were also increased in the dehydrated subjects.
The dehydration in this study resulted in a loss of 1.36% of body mass and this caused disturbances in mood state. These results generally agree with previous reports showing decreased mental performance with increasing levels of dehydration. The wide variety of cognitive aspects measured by researchers suggests that dehydration can affect a wide variety of mental function. The exact reason for this disturbance is not known, but may relate to changes in the hypothalamus, where dehydration centres may detect hydration status and signal higher centres that regulate mood. They may also relate to a general deterioration in neuronal activity as dehydration increases. It is unclear if these results would be seen in longer-term studies of subjects exposed to sub-clinical dehydration, as may occur in daily living. Alcohol use, physically demanding jobs and behavioural aspects can all affect hydration and may therefore influence mental performance.