There is little disagreement regarding the critical need to maintain adequate hydration for human health and well-being. Similarly, optimal cognitive function is an essential aspect of human well-being. However, scientific understanding of the impact of hydration status on human cognition is woefully inadequate.1,2 Severe levels of dehydration not only impair cognitive function but lead to coma and death. However, we do not know the level of dehydration at which human cognitive state shows initial evidence of impairment and which behavioral functions are most affected by dehydration. It is possible that some otherwise healthy individuals fail to maintain optimal cognitive function because they do not maintain optimal hydration status. This is especially plausible because thirst has been shown to be an inadequate indicator of hydration status.3 It has not been established whether hydration status, which varies considerably day to day because of a wide variety of individual differences and environmental factors, influences cognitive performance, mood, and quality of life. Certain populations such as elderly people, individuals who engage in extensive exercise in the heat, children, and those with certain diseases, such as diabetes, may be particularly at risk of dehydration that has functional consequences.4-6
There are at least 3 key reasons why the effects of hydration on cognitive function have not been adequately determined. One reason is the general difficulty in assessing the effects of any independent variable on human behavior. Another is the difficulty generating consistent levels of dehydration in humans in a manner that does not confound dehydration effects with the effects of stressors that are used to induce dehydration. The third is the difficulty associated with accurately and reliably measuring human hydration status, as reviewed previously.7-9 The first 2 issues will be addressed in this article. In addition, the limited literature on the effects of dehydration on cognitive function will be briefly discussed.
Assessing the Effects of Hydration, Nutrition, and Environmental Factors on Human Cognitive Function: More Questions Than Answers
Psychologists have developed an extraordinary variety of techniques to assess human behavior. Virtually every behavior that can be identified can be assessed in some fashion, and most can be measured using several technologies. Cognitive function can be assessed directly with observation, cognitive tests, and questionnaires, as well as indirectly with sophisticated electrophysiological and brain scanning technologies.10 Thousands of tests of human cognitive function have been developed and are capable of assessing the most detailed aspect of functions such as memory, learning, reaction time, vigilance, and so on.11 Memory function is a particularly good example of the ability of psychologists to dissect a function and create a detailed taxonomy of its many aspects, as well as tests to assess each subfunction (for a detailed description of the many categories of memory, see Squire12). However, to a certain extent, assessing the microstructure of a specific function has impeded the development of capabilities for assessing the effects of independent variables (ie, hydration, nutritional status, or specific dietary components) on cognitive function. Because psychologists have so many different tests of nearly all recognized cognitive functions, each with substantially different characteristics, especially their sensitivity to the effects of independent variables, seemingly similar experiments can yield substantially divergent results.13 A good example is the literature regarding the effects of various nutritional interventions on human behavior. In many cases, different laboratories cannot reliably replicate study findings when seemingly small differences distinguish the tests used to assess the same aspect of cognitive function, such as vigilance. Caffeine provides a good example. There is widespread agreement across many laboratories that this food constituent and drug enhance vigilance and reaction time, but some laboratories are unable to document such effects, perhaps because they use insensitive versions of cognitive tests.14-18
Methodological Issues: Assessing the Effects of Hydration State on Human Cognitive Function
To accurately assess the effects of any independent variable, it is essential to be able to accurately, reliably, and repeatedly generate the condition of interest. Furthermore, it is crucial that techniques that generate the required condition not interact with the parameters to be assessed. Unfortunately, the techniques used to generate moderate levels of dehydration are not as dependable as might be desired and, in addition, impose additional stressors on subjects. For example, substantial individual variability typically exists in actual levels of hydration obtained when studies are designed to achieve specific levels of dehydration. The most common means of generating dehydration in the laboratory has been a combination of heat, exercise, and restricted fluid intake.3,19-21 However, because of individual differences in response to these stressors and the difficulty accurately measuring hydration status, hydration levels achieved vary substantially across subjects. In such studies, the control condition typically involves exposure to heat and exercise, while fully adequate levels of hydration level are maintained. It is not known if dehydration produced rapidly by this procedure is the same as dehydration occurring from inadequate fluid intake without the imposed stressors of heat and exercise. It is also not known if the combination of heat, exercise, and dehydration interact to produce greater deficits in cognitive function than dehydration alone.2
Generally speaking, studies investigating the behavioral effects of dehydration are not conducted using double-blind procedures.2 This is a problem because the expectations of volunteers regarding the effects of dehydration could influence test results, as most individuals believe that being dehydrated will have adverse effects. Of course, when investigators are not blind to treatment conditions, bias also may exist. Although it is not easy to overcome such methodological problems, procedures can be used to mitigate biases that are inadvertently created. For example, volunteers can be given some fluid by mouth in all conditions to confuse them about the test condition, or they can be given varying amounts of fluid via intravenous lines so they are not directly aware of their fluid intake. In addition, testing procedures can be instituted to ensure that the investigators responsible for conducting behavioral tests are blind to the treatment condition.2,10,13
There are at least 2 other methods available to dehydrate humans: allowing individuals to gradually dehydrate by not providing fluid or using the drug furosemide (Lasix), a potent diuretic. It is difficult to achieve substantial levels of dehydration using the fluid abstinence paradigm. In 28 hours, without any fluid intake and consumption of dry foods, only about 2.6% dehydration occurs.22 In addition, such a long period without fluid consumption is itself inherently stressful. Administration of furosemide causes a rapid and substantial loss of water. However, administration of this drug produces isotonic dehydration, whereas water restriction and fluid deprivation typically produce hypertonic dehydration. Therefore, dehydration induced by furosemide (Lasix) may not be representative of real-world dehydration.
Studies Examining the Effects of Dehydration on Human Behavior
It is surprising how little information is available regarding the effects of dehydration on human cognitive function. Reviews of the literature indicate that definitive conclusions regarding the effects of dehydration on cognitive function are not possible, given the small number of published studies, methodological problems associated with many studies, and inconsistency of the results across similar studies.1,2
Two important studies of dehydration were conducted using a combination of heat and exercise to induce a range of dehydration in healthy young males. These studies were both conducted over 20 years ago at the Indian Defence Institute of Physiology and Allied Sciences.19,20 Both studies suggest that dehydration levels of 2% or more impair various aspects of cognitive function, in particular, short-term memory, reasoning, and hand-eye coordination. However, examination of the data presented by these articles suggests that dehydration of only 1% may be sufficient to disrupt certain aspects of cognitive function2 (Figure). There are many positive elements to these studies, particularly the careful attempts to achieve specific levels of dehydration and inclusion of control euhydrated testing conditions. However, these studies had some significant shortcomings, especially the limited nature of the behavioral testing and the severe environmental conditions imposed to achieve rapid dehydration. In addition, like most other studies of dehydration, only a relatively small number of subjects were tested. If dehydration has only subtle effects on cognitive function until substantial fluid loss has occurred, as appears to be the case, this is a particularly critical issue. Other studies that achieved similar levels of dehydration as those of Gopinathan et al19 and Sharma et al20 under well-controlled conditions have failed to find any effects on cognitive function.23
One study that provides useful information on hydration and cognitive state was conducted using the water deprivation technique.22 The investigators deprived young male and female volunteers of all fluids for 28 hours and achieved a 2.6% mean body mass loss. The volunteers also participated in a control session but became somewhat dehydrated on average (0.75%) during that arm of the study. Several tests of cognitive performance and mood were administered from 24 to 28 hours after the start of each session. Few changes in cognitive performance were observed, but there were effects of dehydration on self-reported levels of tiredness, alertness, effort, and concentration (P < 0.05) and some evidence of cognitive performance decrements in females. The results of this study suggest that mood is more likely to be affected by modest levels of dehydration than cognitive performance and that women are more sensitive to dehydration than men. The failure to maintain euhydration during the control session makes it difficult to draw any definitive conclusions from this study. A similar study24 was conducted using the fluid deprivation technique, but it used a shorter dehydration period (approximately 12 hours), and the investigators do not report any measure of hydration status. Not surprisingly, the results are inconclusive, but there was some evidence of adverse effects of dehydration on mood state, in particular, alertness.
It is clear that understanding the effects of dehydration on cognitive function will require considerable additional carefully controlled research. Such studies should use state-of-the-art methods to assess accurate hydration status and adequate numbers of test subjects. Because the cognitive functions that are sensitive to moderate levels of dehydration have not been identified, a variety of behavioral tasks should be used. In addition, it is essential that blinding procedures be implemented, to reduce the effects of any preconceived notions of subjects or investigators regarding the effects of dehydration on cognitive function.
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