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Dehydration and mental performance in adults

Dehydration adversely affects mental performance. In adults, symptoms of mild dehydration include light-headedness, dizziness, tiredness, irritability, and headache, as well as reduced alertness and ability to concentrate.(1,2,3) At greater levels of dehydration, the impact on mental functioning is more severe and can lead to clumsiness, dim vision, exhaustion and may even result in delirium.(4)

Types of mental performance affected

Different degrees of dehydration do not affect various mental functions to the same extent. Routine mental work such as symbol classification can deteriorate slightly as a result of dehydration. Motor coordination is sensitive to dehydration and can show a decline even at very low levels of dehydration (less than 1 per cent loss of body weight). As dehydration increases to 2 per cent, functions such as short-term and long-term memory, motor coordination, reaction times, and perceptive discrimination decrease significantly.(5,6)

Short-term memory, arithmetic ability and motor speed progressively deteriorate as the degree of dehydration increases. This seems to occur regardless of whether the dehydration is caused by exposure to heat and induced by exercise.(7) There appears to be a critical level of 2 per cent dehydration where the deterioration in mental performance becomes highly statistically significant. At even higher levels of dehydration, (3 per cent) a further significant decrease in performance is observed. Arithmetic and motor speed show a further significant decrease at 4 per cent dehydration.(8) In addition, at levels of dehydration above 2 per cent individuals report feeling more tired.(6)

Drinking enough water to compensate for the fluid loss, improves cognitive performance(6) and subjective mood ratings. For example, in one study, individuals reported an immediate, although not sustained, “alerting” and “revitalising” effect following water consumption regardless of whether they were initially thirsty or not.(2) However, the deterioration in mental function improved after 3.5 hours even in individuals who did not drink to replace lost fluid, although they felt increasingly tired.(7)

In studies of mental performance, dehydration is often achieved by means of exposure to heat, exercise or both.(5,8,7) These conditions may make the cause of any cognitive impairment difficult to interpret because physical exercise may actually enhance cognitive performance. Other studies have failed to show any affect on attention, memory, cognitive-motor function or neurophysiological function (either positive or negative) as a result of hydration status. This was despite participants reporting that they felt more "alert" following successive drinks of water, and that they experienced increased tiredness, reduced alertness and higher levels of perceived effort and concentration during dehydration.(9,10) Further studies are needed to better understand the relationship between dehydration and mental performance.

Mental performance and thirst

It is well known that eating a meal can cause a deterioration in mental (and physical) performance - this is called the post-prandial dip.(11,12) This is due to the body diverting energy and other resources to digest and absorb the meal and to store the nutrients. It has been found that the consumption of water alone can either improve or impair mental performance, depending on the level of thirst. When individuals felt very thirsty, their performance, in a rapid visual information processing task, improved by up to 10% following consumption of 330ml water. However, when individuals were not initially thirsty, their performance deteriorated after water consumption.(2)

This suggests that when people are not thirsty, the impact of drinking water can be detrimental to mental performance. This is because, like the post-prandial dip, the intake of water in excess of the body's requirements leads to the diversion of energy and physical resources to rebalance the body's water and salt levels. This impact may also depend on drink temperature. If the water is served cold , additional energy may have to be diverted to warm it, once ingested.

Dehydration and mental performance in children

There have only been a small number of studies looking specifically at the affect of dehydration in children, but these show that poor hydration can adversely affect children’s mental performance.(13) The extent to which this may impair learning ability is still unknown, but there are a number of initiatives in the UK to encourage water intake in school-age children in order to promote good health and support learning.(14,15)

In schools, where water is provided throughout the day and where fizzy drinks are banned, there have been reported improvements in concentration levels, academic performance and pupil behaviour.(16) Teachers in schools taking part in the Food in Schools water provision pilot project, reported that “the enhanced water provision contributed to a more settled and productive learning environment, as well as helping instil good habits.” Water reduces tiredness, irritability and distraction from thirst and can have a positive effect on pupils’ concentration throughout the day.(15) An experiment in a Scottish school also found that drinking water contributed to improvements in pupils' test results.(17) Schools who took part in the Sandwell Children’s Fund project to increase hydration amongst school children, through the provision of water filter systems and water bottles, reported that since the introduction of project, children showed improved concentration levels and were less tired and lethargic. They also reported that children complained less of having headaches and were more aware of the importance of keeping hydrated.(18)

Studies of young adults as discussed above, have shown that once thirst is felt, mental performance including memory, attention and concentration can decrease by about 10 per cent. Thirst is usually felt when dehydration reaches 0.8-2 per cent loss of body weight due to water loss.(1) For a 10-year-old child weighing 30kg this is the equivalent to two cans of drink (300ml each).

Clinical signs of mild dehydration (below 3 percent loss of body weight) include restlessness and increased alertness. Children with moderate dehydration may be lethargic, but then show irritability when touched. For moderate to severe dehydration, children may feel dizzy, agitated, irritable, restless and confused.(13) A study of the relationship between voluntary dehydration and cognitive performance in elementary school children aged 10-12 years in southern Israel found that at the beginning of the day there were no significant differences in cognitive performance between the hydrated and dehydrated groups. At mid-day, however, the hydrated group performed better in four of the five cognitive tests compared to the dehydrated group, especially on a short-term memory task.(19) It should, however, be noted that heat exposure may have an independent role in the impairment of mental performance.(6) Performance on complex mental tasks, such as continuous rehearsal of digits for an immediate recall, tended to deteriorate as voluntary dehydration increased during the school day. Mental tasks based on automated skills, perceptual or procedural, long-term memory retrieval, and controlled thinking with lower cognitive demands were less affected by chronic dehydration.

Even moderate heat stress can adversely affect young people’s mental performance. When a group of 17 year olds were exposed to increasing temperatures within a range likely to be found in typical classrooms, moderate heat stress of only a few degrees centigrade above the optimum, had a marked effect on mental performance. Sentence comprehension and reading speed were worst at 26-27oC, and boys performed the multiplication task least well at 28oC. Tasks involving concentration and clear thinking were most adversely affected. The students were least alert when heat stress reached the point where their body could no longer maintain its optimum temperature by heat loss through the skin alone, ie just before sweating began.(20) Dehydration is known to impair thermoregulation and reduce the efficiency of the body’s temperature control mechanisms.(21,22)

Why does dehydration impair mental function?

The brain plays a vital role in the body’s response to dehydration. Osmoreceptors in the brain monitor plasma osmolality. This, and other physiological information, enables the brain to control water intake by altering thirst, and varying water excretion from the kidneys by adjusting the amount of antidiuretic hormone (ADH, or vasopressin) released.(23) Hyperosmolality of body fluids causes cells to shrink as water leaves them by osmosis, in order to compensate for the fluid deficit in extracellular compartments. Brain cell volume may decrease by up to 10-15 per cent (24), which leads to neurological features including increased excitability, irritability, muscle twitches, brisk reflexes, and spasticity. Ultimately seizures and coma can occur.(25) Computerized brain volumetry has revealed that dehydration (resulting from lack of fluid intake for 16 hrs) results in overall brain shrinkage of 0.55%.(26)

The adverse effects of dehydration on cognitive function highlight the importance of adequate fluid intake. Whilst there is evidence to show that cognitive function is impaired in mild dehydration, however, the precise pathophysiological mechanisms responsible for this impairment remain unknown. In the case of severe dehydration, such effects may be due to hypovolaemia (reduced water volume) and subsequent cerebral hypoperfusion (inadequate blood flow and oxygenation). One explanation for the effect of dehydration on cognitive function has been proposed by Cohen.(27) He suggests that, acute stressors such as dehydration compete for increased attention and awareness with other, parallel cognitive processes thereby compromising overall cognitive performance. Dehydration is also known to increase circulating levels of stress hormones such as cortisol which, in humans, is associated with a decrease in cognitive function, particularly a worsening of active learning and short-term memory.(13,28)

Wilson and Morley(4) recommend the integration of both cellular and hormonal theories to explain cognitive dysfunction in dehydrated states because of the complexity of the neurobiological mechanisms involved in cognition. Water homeostasis is normally well maintained by the body, with rapid restoration of function following a disturbance, such as water deprivation. In severe cases, or where there is more than one simultaneous disturbance, an excessive homeostatic response may occur that triggers damaging physiological processes. For example, cellular responses to dehydration may act at a molecular level, via the increased accumulation of intracellular calcium, to trigger neuronal death. Such damage to brain cells suggests that simple fluid replacement may not fully reverse the cognitive dysfunction arising from severe dehydration and further research is needed.

Dehydration and mental decline in older people

Older people are particularly susceptible to the adverse effects of dehydration, which include slowed pschomotor processing speed and poorer attention and memory performance.(29) Dehydration is a reliable predictor of increasing frailty, progressive deterioration in cognitive function and an overall reduction in the quality of life.(4)

Cognitive impairment often comes before the onset of functional decline, particularly in older dehydrated patients. Figure 1 is a model showing how this arises, based on the areas of the brain that are most vulnerable to the effects of dehydration. These areas are the reticular activating system, which subserves attention and wakefulness; the autonomic structures that regulate psychomotor and regulatory functions; and the cortical and mid-brain structures, which are responsible for thought, memory and perception. As various areas of the brain become progressively impaired by increasing levels of dehydration, they impact sequentially on cognitive function, task processing, functional decline and ultimately quality of life.(4)

Figure 1: Theoretical model of the clinical trajectories of cognitive dysfunction resulting from variable degrees of dehydration(4)

 
 
 
 
 
 
 
 
 
In older people, urge incontinence has been found to be worst in individuals who also have a decrease in cognitive performance and under perfusion of the frontal lobes of the cerebral cortex. This suggests that changes in brain hydration levels may be partially responsible for this condition.(13) (see also Urinary Tract and Older People)

Last updated: December 2006

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