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Dehydration and Cognitive Performance

The Center for Human Nutrition, Omaha, Nebraska (A.C.G.)
Pate Rehabilitation, Dallas, Texas (N.R.G.)

Address reprint requests to: Ann C. Grandjean, Executive Director, The Center for Human Nutrition, Room 1024 505 Durham Research Plaza, Omaha, Nebraska 68105. E-mail: agrandje{at}unmc.edu

	ABSTRACT

TOP FOOTNOTES ABSTRACT INTRODUCTION NEUROPSYCHOLOGY THE EFFECT OF DEHYDRATION... FUTURE RESEARCH AND CHALLENGES REFERENCES

 
Human neuropsychology investigates brain-behavior relationships, using objective tools (neurological tests) to tie the biological and behavior aspects together. The use of neuropsychological assessment tools in assessing potential effects of dehydration is a natural progression of the scientific pursuit to understand the physical and mental ramifications of dehydration. It has long been known that dehydration negatively affects physical performance. Examining the effects of hydration status on cognitive function is a relatively new area of research, resulting in part from our increased understanding of hydration's impact on physical performance and advances in the discipline of cognitive neuropsychology. The available research in this area, albeit sparse, indicates that decrements in physical, visuomotor, psychomotor, and cognitive performance can occur when 2% or more of body weight is lost due to water restriction, heat, and/or physical exertion. Additional research is needed, especially studies designed to reduce, if not remove, the limitations of studies conducted to date.

Key words: dehydration, cognitive function, cognitive performance, fluid restriction, exercise, heat

Key teaching points:

• Research assessing the effects of hydration status on cognitive function grew from our understanding of hydration's impact on physical performance and advances in the discipline of cognitive neuropsychology.

• Decrements in physical, visuomotor, psychomotor, and cognitive performance can occur when 2% or more of body weight is lost due to water restriction, heat, and/or physical exertion.

• A major limitation of most studies conducted to date is the inability to determine the effects of dehydration independent of the effects of thermal stress, physical stress, and/or fatigue.

• Increasing our understanding of the effects of hydration status on cognitive performance could be applied to health care, education, and other areas where cognitive performance is assessed and/or treatment is rendered.

	INTRODUCTION

TOP FOOTNOTES ABSTRACT INTRODUCTION NEUROPSYCHOLOGY THE EFFECT OF DEHYDRATION... FUTURE RESEARCH AND CHALLENGES REFERENCES

 
The Panel on Dietary Reference Intakes for Electrolytes and Water (the Panel), under the oversight and assistance of the Standing Committee on the Scientific Evaluation of Dietary Reference Intakes (the DRI Committee), established reference values (DRIs) for dietary electrolytes and water intakes for healthy Americans and Canadians [1]. The DRIs expand and replace previous recommendations, the Recommended Dietary Allowances (RDAs), last published in 1989 [2]. The water section in the 1989 RDAs consisted of four pages of text and five references. The water section in the Dietary Reference Intakes for Water, Potassium, Sodium, Chloride, and Sulfate (water DRIs) consists of 81 pages of text and 24 pages of references, indicating our increased understanding of, and appreciation for, the essential nutrient water. One of the Panel's research recommendations was for additional studies on the effects of water deficits on cognitive performance.

The intent of this paper is to provide a concise overview of the evolution of research in the area of dehydration and cognitive performance.

	NEUROPSYCHOLOGY

TOP FOOTNOTES ABSTRACT INTRODUCTION NEUROPSYCHOLOGY THE EFFECT OF DEHYDRATION... FUTURE RESEARCH AND CHALLENGES REFERENCES

 
Brain-Behavior Connection
History is replete with individuals who attempted to establish the source of human behavior. In early times, competing hypotheses included beliefs that behavior was controlled by the soul, spirit, or some part of the body (e.g., heart, brain). Around 500 BC, Alcmaeon of Croton located mental processes in the brain, thus establishing the brain hypothesis. Philosophers and physicians debated the merit of the brain hypothesis for the next 2,000 years. Modern thinking started with Descartes (1596–1650) who established that reasoning came from the "mind." Behavioral observations made by experimenters (after they created lesions in animal brains) and physicians (who had patients with lesions in certain parts of the brain) further established the brain as the control center for both thoughts and behaviors.

The discipline of neuropsychology further established the brain-behavior relationship by drawing on information from many disciplines (e.g., anatomy, biology, pharmacology, physiology) [3]. Phineas Gage and Patient HM were two such events [4, 5]. In 1848, a construction accident resulted in a 4-foot iron rod entering Phineas Gage's right cheek and exiting the top of his head. Although Gage survived, he suffered extensive damage to his frontal lobe. This event provided some of the earliest evidence that specific areas of the frontal lobe may be involved in emotion and personality. In 1953, surgeons removed portions of Patient HM's medial temporal lobes to treat his epilepsy. While his seizures improved, the surgery caused profound difficulty with memory (i.e., selective amnesia). Because his impairment was surgically induced, and thus the exact location of the damage known, observations could be made regarding the location of injury relative to subsequent changes in cognitive abilities.

Cognitive Assessment
Information processing (e.g. mental processes such as perception and memory) became the dominant model for understanding cognitive function during the 1960s. Information processing psychologists viewed cognitive abilities as task-specific and were concerned with ways that specific skills were utilized for particular tasks. This provided an important theoretical basis for neuropsychology—the study of how the brain affects cognitive, behavioral, and emotional functioning [6].

Advances in neuropsychology have been accompanied by a proliferation of tests or assessment tools (commonly referred to as neuropsychological tests or neuropsychological measures) originally designed to identify individuals with brain injuries. With the establishment of neuropsychological research groups in the 1950s and 1960s, tests to measure specific cognitive functions were developed. Clinically, neuropsychological measures are often administered in particular groups (called batteries) to allow for analysis between patterns of test performance and behavior. In research, neuropsychological measures allow for cognitive functioning to be assessed for comparison to other variables. The use of neuropsychological assessment tools in assessing potential effects of dehydration, coupled with advancements in assessing the effects of hydration status on physical performance, led to scientific discovery.

	THE EFFECT OF DEHYDRATION ON PHYSICAL AND COGNITIVE FUNCTION

TOP FOOTNOTES ABSTRACT INTRODUCTION NEUROPSYCHOLOGY THE EFFECT OF DEHYDRATION... FUTURE RESEARCH AND CHALLENGES REFERENCES

 
Researchers demonstrated in the 1940s that persons under thermal and physiologic stress need to pay special attention to fluid and salt intake [7–10]. Since hydration status has the potential of being critical to performance, military personnel have served as subjects for numerous studies. The incidence of heat illness in American troops during World War II was, in part, the impetus for early research. A landmark study demonstrating the impact of various levels of water intake on marching in the heat was conducted by Pitts and colleagues [10]. Six experiments were conducted on a single subject marching in a heated room (100° F, 35–45% RH). During two of the experiments, the subject drank no water. During two experiments, the subject consumed water to satisfy thirst; and for two he was forced to drink water every 15 min in amounts equal to sweat loss (Table 1). Discussing their results, the authors stated, "It should be emphasized that during work men never voluntarily drink as much water as they sweat, even though this is advantageous for maintaining heat balance, but usually drink at a rate approximating about two-thirds of the water loss in sweat." Subsequent studies have confirmed and expanded our collective knowledge regarding the importance of adequate hydration on physical performance. See Murray [11], Maughan [12], and Kenefick [13] in this publication.

Leibowitz and colleagues, recognizing the work of Bursill and others who had investigated the impact of psychological stress on reaction time, conducted a study to determine if changes in physiological stress will show the same effect under conditions of constant central task load [15]. Subjects lost either 2.5% or 5% of their body weight (BW) by exercising intermittently (20-min work periods) on a treadmill for 6 h in a heated chamber. Reaction time to central and peripheral visual stimuli was tested while subjects walked on the treadmill. Following the 6 h, subjects spent an additional 2 h in the chamber participating in a series of physiological tests. No effect was seen on reaction time to central visual stimuli, but a faster response time to peripheral visual stimuli occurred (Table 2). However, the effects of dehydration could not be determined independent of the heat and exercise related stress. The authors concluded that physiological stress resulting from high heat and hypohydration does not adversely affect either peripheral or central reaction times. In fact, response time to peripheral visual stimuli improved with practice. Leibowitz and colleagues concluded that for short periods, effects of heat stress could be overcome in highly motivated and experienced subjects. They also acknowledged the probable effect of practice effect stating:

"Considering both phases of the present experiment, it is clear that practice was the dominant variable with respect to detection of peripheral stimuli. The large number of other variables, including a stress level rarely used in psychological experiments, was of insignificant or minor importance."

View this table: [in this window] [in a new window]   Table 2. Cognitive and Motor Control Functions Reported to Be Affected by Dehydration

 

Yoram Epstein of the Israel Defense Forces Physiological Research Unit and his colleagues assessed the combined effect of varying heat loads and mission intensities on psychomotor performance [16]. Using a TV computerized game, nine healthy young men performed a rifle marksmanship task while in a climatic chamber under three different effective temperatures (21°, 30°, and 35° C). To complicate the tasks, the targets appeared in three different sizes and configurations and the subjects had to follow directions from the instructor, who was located in an adjoining control room. Results showed little effect of heat load on the subjects’ abilities to perform easy tasks, but the number of target hits were reduced 17.5% on complicated tasks in the heat (Table 2). The authors concluded that the effects of exercise intensity and heat load on deteriorating performance are synergistic; psychomotor performance deteriorates even before physiological responses are impaired; and even highly motivated subjects are affected by heat load, especially when assigned to complex tasks that require a high state of vigilance, cooperation, and coordination.

Gopinathan and colleagues used cognitive performance tests to assess the effect of various degrees of dehydration on mental performance [17]. Eleven soldiers between the ages of 20–25 years from tropical regions of India were dehydrated by 1%, 2%, 3%, or 4% BW by restricting water while exercising in a climatic chamber at 45° C and 30% RH. When subjects reached the target degree of dehydration, they recovered in a thermoneutral room. A subject was considered recovered if resting heart rate and oral temperature returned to initial resting levels or if consistent levels were attained during the last 30 minutes of the recovery phase. Tests to assess arithmetic ability, short-term memory, and visuomotor tracking were administered at the beginning of the study and when the subject recovered. Significant decreases in mental performance occurred at dehydration levels of 2% BW or more (Table 2). For a more extensive review of this study, see Lieberman [18].

Cian and colleagues conducted a study to compare the effects of euhydration, exercise-induced dehydration, heat-induced dehydration, and hyperhydration on cognitive function [19]. Hyperhydration was induced using a solution of water and glycerol. Eight healthy men, whose usual training was running four 1-h sessions per week, were unacclimated to heat. All subjects received four treatments in a cross-over design. After achieving the targeted degree of hydration, the subjects sat for 90 m in a thermoneutral environment (25° C, 40% RH). Upon completion of the recovery phase, the subjects exercised using an arm-crank ergometer until they could no longer maintain the power required to exercise (approximately 15–20 min). The cognitive test battery was administered prior to intervention (while subjects were rested and hydrated), 30 min after completion of the hydration manipulation phase, and again 15 min following an arm-crank test in which the subjects exercised to exhaustion. The average level of dehydration achieved during the two dehydration trials was –2.8% BW. Results showed that, compared to the euhydrated state, both dehydrated states resulted in increased fatigue, increased tracking errors, increased reaction time to making a decision, and decreased short-term memory (Table 3). The hyperhydrated state differed from euhydration only in short-term memory. The investigators postulated that the positive effect on memory might be attributed to hydric overload but that, based on the available data, the exact mechanism could not be identified, nor could other indirect effects of glycerol be dismissed.

	FUTURE RESEARCH AND CHALLENGES

TOP FOOTNOTES ABSTRACT INTRODUCTION NEUROPSYCHOLOGY THE EFFECT OF DEHYDRATION... FUTURE RESEARCH AND CHALLENGES REFERENCES

 
Collectively, the available research indicates that decrements in physical, visuomotor, psychomotor, and cognitive performance can occur when 2% or more of BW is lost due to water restriction, heat, and/or physical exertion. As is apparent in the brief review of studies herein, although similarities exist in study findings, the research designs have varied significantly. Regardless of design limitations, currently available data, and more importantly the implications of potential negative effects from hypohydration and dehydration, dictate that further investigation is needed [1]. Especially valuable will be studies with experimental designs allowing for discrimination of confounders such as thermal stress, physical stress, fatigue, and other factors.

In addition to the aforementioned physical confounders, control of other factors is also necessary to reduce ambiguity. Examples of potential confounders include:

• length of the dehydration phase
  
• time of day that neuropsychological assessment is conducted
  
• macronutrient and micronutrient composition of the diet, as well as non-nutritive compounds
  
• circadian rhythm
  
• quantity and quality of sleep
  
• individual differences (e.g., IQ, resourcefulness, motivation, competitiveness, psychopathology)
  

Practice effect (i.e. improvement in performance that can occur on neuropsychological measures simply because of repeat exposure to test procedures and stimuli) must also be considered. Practice effects can artificially inflate performance on many neuropsychological tests when given at test-retest intervals of weeks, months, or even years. Practice effect was noted by Leibowitz and colleagues [15] and continues to be a challenge in neuropsychology [22]. See Lieberman regarding recommendations for future research and discussion of study design and conduct [18].

Despite the limited data available in the area of hydration and cognitive performance, one cannot help but consider potential applications, especially as related to health care and general well-being. For example, hydration status may be a confounding variable for an individual undergoing neuropsychological evaluation in clinical or educational settings. Thus, research is needed to determine if hydration status should be considered during assessments for dementia (e.g., Alzheimer's, vascular), acquired brain injury (e.g., stroke, traumatic brain injury), learning disabilities, and other cognitive performance evaluations.

	FOOTNOTES

TOP FOOTNOTES ABSTRACT INTRODUCTION NEUROPSYCHOLOGY THE EFFECT OF DEHYDRATION... FUTURE RESEARCH AND CHALLENGES REFERENCES

 
Conflict of Interest Disclosure: ACG declares that she does not (nor does any member of her immediate family) have a present corporate or government relationship that presents a conflict of interest with regard to this manuscript. The author is a member of the Beverage Institute for Health and Wellness Advisory Council, serves as Scientific Advisor to the ILSI NA Technical Committee on Hydration, and has previously received unrestricted research grants from the Coca-Cola Company.

NRG has no conflicts of interest to declare in connection with this work.

Received July 16, 2007.

	REFERENCES

TOP FOOTNOTES ABSTRACT INTRODUCTION NEUROPSYCHOLOGY THE EFFECT OF DEHYDRATION... FUTURE RESEARCH AND CHALLENGES REFERENCES

 

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