Managing Dehydration
NeuroNotes #3
A summary of this paper is available here: Understanding Dehydration
Purpose
To form a basic understanding of dehydration, how it affects the human body and any ways in which it can be prevented.
What is dehydration?
Dehydration is a drop of total body water below appropriate levels to allow optimal functioning of the body (Cheuvront & Kenefick, 2014). Dehydration is identified in different ways, with a distinction between medical terminology when treating dehydration and biological terminology when researching it. Medical terminology revolves around three identifications of dehydration: isotonic, hypotonic and hypertonic. There is, understandably, a focus on identifying symptoms related to these types of dehydration so that the fluid disorder can be treated effectively. This is necessary: in extreme cases, dehydration can cause shock, loss of consciousness, and lead to death (Subudhi et al., 2013).
However, until recently, a focus on the treatment of dehydration in its more severe forms has led to a lack of understanding in the general population of the importance of adequate hydration. 74% of the global population has access to clean drinking water (Ritchie & Roser, 2021) however many individuals in these countries are ‘low drinkers’, maintaining poor hydration levels so that their bodies continually work to conserve water. Although this will not result in immediate medical assistance being required (and may not be medically termed as dehydration), longer-term adverse effects, which are largely avoidable, will develop (Perrier, 2017). As such, knowledge and maintenance of adequate hydration levels have been shown to significantly improve short and long-term health outcomes (Liska et al., 2019; Perrier et al., 2020).
Biologically, there are two distinct types of dehydration: dehydration with salt loss and dehydration (largely) without salt loss (extracellular and intracellular dehydration, respectively). The body’s endocrine system controls the response to both these types of dehydration. Intracellular dehydration results in increased conservation of water by the kidneys and creates the feeling of thirst. Extracellular dehydration creates a narrowing of intracranial arteries (vasoconstriction) and reduced control of adequate blood volumes to internal organs. Extracellular dehydration is more likely to be due to causes such as diuretic medications, acute diarrhoea or vomiting. Intracellular dehydration is more common, less severe and usually due to poor fluid intake, particularly during sports activity and certain work roles. For a more detailed review of these two types of dehydration, see Cheuvront & Kenefick, 2014.
What are the effects of dehydration?
Both types of dehydration reduce muscle blood flow (González-Alonso et al., 1998), alter skeletal muscle metabolism (Febbraio, 2000), and there is an overall reduction in the amount of oxygen in the blood that the body can utilise (Craig & Cummings, 1966). These effects result in a variety of symptoms manifesting themselves. Aside from thirst (particularly for intracellular dehydration), sweating and thermoregulation ability is reduced (Sawka et al., 1985). Early studies found that a sub-optimal hydration level of just 2% created significant impairment to mental (Gopinathan et al., 1988) and physical function. However, results have been mixed, indicating that effects may vary depending on the situation and task, with a study that combined both dehydration and heat stress finding no significant difference in choice reaction time (Serwah & Marino, 2006). A 2014 review (Cheuvront & Kenefick) of the effects of dehydration in various sporting scenarios also settled on a 2% threshold but divided the expected deficits dependent on the type of activity being undertaken. Endurance was most affected and physical strength was minimally or not affected (see Judelson et al., 2007). It has been noted that the study of dehydration requires more consistency in the parameters used as different confounds (such as the way that dehydration is achieved) reduce the validity of findings (Lieberman, 2007).
Therefore, according to current research, dehydration primarily affects physical abilities. Endurance seems to be most affected, followed by strength and motor skills. Cognitive abilities seem least affected, with attention only minimally affected by differences in hydration (Krecar et al., 2014), however, they are likely to become increasingly affected due to distraction instilled by the realisation of physical exhaustion (Baker et al., 2007).
This variation in the type and severity of the effects of dehydration is at least partly due to the coping mechanisms that the body instigates to combat it. In a similar way that less urine is produced as the body recognises it needs to conserve more water, the brain employs resources in a different way so that potential cognitive impairment is overcome, at least in the early stages of dehydration. This is referred to as cognitive resiliency. Whereas over 2% of dehydration of body mass shows a reduction in physical abilities, cognitive abilities have shown little change even with a 4% reduction (Ely et al., 2012). As is (counterintuitively) observed during other physical states such as tiredness or negative moods, prefrontal cortex activation is increased when someone is required to carry out a task involving executive function while dehydrated (Kempton et al., 2011). Whether consciously or not, cognitive resiliency results in an enforced increase in concentration due to a recognition that this is required to achieve the same outcome.
Dehydration and fatigue
Fatigue is described by muscle physiologists as a decline in muscle performance, whether this is from reduced muscle cell force or reduced motor drive from the central nervous system. Researchers in exercise describe it more loosely as a decline in overall performance (Knicker et al., 2011). The interplay between dehydration and fatigue has long been studied, particularly with regard to sports (e.g. Maughan & Leiper, 1994) and exercise in general (e.g. Montain & Coyle, 1992), though as fatigue results from the interaction between many physiological systems and the brain (Lambert, 2005), links between dehydration and fatigue across multiple experimental paradigms are inconsistent.
Furthermore, dehydration has been shown to increase antidiuretic hormone levels (Walsh et al., 1994). This hormone (vasopressin) is associated with migraines (Hampton et al., 1991) and other adverse effects. This leads to increased perceived exertion, which in turn can result in sub-optimal effort - a sign of fatigue (Edwards & Noakes, 2009). The significance of this perception of effort is evident when fatigued subjects make a conscious decision to stop exercising. While there may be a feeling of not being able to carry on, this is not confirmed by a defined level of metabolic activity, such as oxygen uptake levels or cardiac output, where activity must cease (Kayser, 2003). As dehydration makes feelings of fatigue more likely, this in turn means that the effort felt during exertion will reach a maximal level earlier than when normally hydrated. These effects on performance are exacerbated in warmer environments (Sawka et al., 2012).
It should be noted that fatigue is often viewed as a gradual build to a maximum that results in a severe reduction in performance or a halt to activities altogether. However, workers have been observed to self-regulate their fatigue levels with breaks in activities (whether consciously or not). In a study of 138 miners, variations in downtime due to fatigue were significant (Vernon, 1928). The average rest per hour was 7.3 minutes however in very poor conditions this was sometimes observed to increase to 28 minutes. Furthermore, production during those conditions was 20% poorer than in optimum conditions. This study was conducted almost a century ago and in general working conditions have improved significantly since that time however similar declines in worker performance due to a lack of fluid intake have been observed more recently (Wästerlund et al., 2004). The principle of finding areas and times of the day where worker fatigue is more prevalent and changing these conditions, or investigating potential causes such as dehydration, will not only improve worker well-being but also productivity.
Furthermore, fatigue (and any dehydration that may cause it) has been seen to be subconsciously controlled by athletes where possible, as is the case during football games. Periods of extreme physical activity are followed by periods of lower activity. Players who are able to conduct this process effectively contribute more in the second half of games when fatigue increasingly becomes a factor (Bangsbo & Mohr, 2005; Mohr et al., 2005). This role of the subconscious mitigating fatigue has been found in other areas, for instance when runners were informed that exercise intensity would not increase but it did. Both those runners and a control group reported similar levels of intensity, indicating that conscious preconceptions of the activity were not influential. However, under different conditions, where the higher intensity was warned, the control group reported greater increases in the level of perceived exertion. This indicates that preconceptions, when confirmed during the exercise itself, did influence the outcome; the body prepared for greater exertion so was better able to cope with it when it came (Hampson et al., 2001). This has practical implications when planning any physical activity. Improved knowledge of the upcoming task and an expectation that it will be physically demanding will allow the body to perform more successfully.
Combating dehydration and fatigue
Recognising dehydration
The most effective way to deal with dehydration is to avoid it. The human body consists of between 45 and 75% water, depending on age, sex and amounts of fat or muscle (Chumlea et al., 1999). Unfortunately, for all of its water content, it is not adept at storing excess water adequately. Therefore, forming habits that maintain appropriate levels of hydration is critical to the long-term health of the individual.
Humans have been found to vary their body mass naturally by 1% or less, across various climates. Therefore, adding a buffer of another 1%, if someone loses more than 2% of their average daily body mass, it is highly likely that this is due to dehydration (Cheuvront, 2010). In a medical examination to determine dehydration, blood, urine, saliva and tear tests can be used alongside monitoring vital signs and even ultrasound for extracellular dehydration. Assessing blood, primarily looking at blood concentration which increases during dehydration, is thought to be the most reliable (Cheuvront & Kenefick, 2014). This method of assessing dehydration is impractical for the general population on a daily basis. When urine is sampled medically, its volume and concentration is assessed. Although crude, this method is generally the easiest for individuals to mimic (length of urination and colour of urine). Diet affects urine, so the most reliable observation is made first thing in the morning when there has been a long break from food intake (Armstrong et al., 1994). If urine is dark in colour and low in volume, average fluid intake should be increased.
Heart rate has been shown to vary significantly when someone is dehydrated and moves from sitting to standing positions. Given the growing prevalence of wearables, in theory, this measure could be used as a warning of dehydration. However, studies have been in very controlled environments without additional stresses that may vary heart rate (e.g. Witting & Gallagher, 2003; Cheuvront et al., 2012). In the field, this type of control (and the ability to accurately infer when a subject is sitting or standing) is improbable. Nonetheless, as sensors and algorithms become more complex, it is a technique worth investigating. If average heart rates for different activities can be assessed and allocated to sitting or standing conditions, the variation in these averages could indicate dehydration.
Physical symptoms such as headaches, apathy and fatigue have long been identified as early indicators of dehydration however these symptoms are present among many other maladies. Thirst is an obvious symptom, although, in extracellular dehydration, the symptom may not present until over 6% of body mass has been lost, at which point severe dehydration has already occurred (Nadal et al., 1941).
Remaining hydrated
There is still debate as to whether rehydrating only when ‘feeling thirsty’ is adequate (e.g. Saker et al., 2016) or not (e.g. de Castro, 1988). Given the short and long-term effects of dehydration and the lack of accuracy when an individual gauges their hydration level, a habit of consuming adequate water volume is the most reliable way to avoid dehydration. Furthermore, when the body loses a significant amount of fluid quickly (through profuse sweating, vomiting or diarrhoea), a person who maintains adequate hydration will recover quicker and be in a healthier position overall compared to someone who is frequently dehydrated.
There is now an understanding that the ability to remain hydrated varies between individuals. This may be due to personal circumstances (e.g. amount and quality of sleep in the past 24 hours) and intrinsic factors such as efficiency at regulating metabolic pathways (Edwards & Noakes, 2009). Awareness of changes in climate will also aid hydration. Studies of military personnel adapting to new operational environments show that those involved in moderate exercise early in deployment acclimatise more quickly, maintaining lower internal body temperature during further exercise and therefore reducing fluid intake requirements to stay hydrated (Parsons et al., 2019).
Recognising when involuntary dehydration may be a risk due to social norms will help mitigate it. In environments that may induce heat stress, 250ml of water every 20 minutes is suggested (Kenefick & Sawka, 2007). While experimenting with the optimum amount of fluid intake prior to and during exertion (Maughan & Leiper, 1994) is helpful for athletes who are already optimising all areas of their health, it falls short in motivating the general population to maintain adequate fluid intake. Nevertheless, fluid intake prior to strenuous exercise (Burke & Hawley, 1997) and also after exercise (Von Duvillard, 2000) is similarly important. Adequate availability of water in places of expected exertion, alongside guidelines and prompts, will continue to be essential until there are improved ways to monitor hydration.
Hyponatremia
It is not enough to encourage all members of the population to drink more; consuming too much water can be as detrimental as consuming too little. Further research is required in this area to establish more robust water drinking guidelines, and this is most evident in the variation of guidelines from different governments. Currently, the UK government recommends around 1.6 litres of fluid intake per person per day, while that of Europe is 2.5 litres and in the US averages 3.2 litres (see table in the companion post here).
In the early 20th century, Lawrence of Arabia and his fighters consumed large amounts of water before desert fights, believing that this would keep them hydrated (Montain & Ely, 2012). Studies attempting Lawrence of Arabia’s technique have shown that increased water consumption in one bout does little to help ongoing hydration, and may indeed be harmful, with abnormally low sodium concentrations (hyponatremia) observed in athletes who have consumed too much water over too short a period of time (Hew-Butler et al., 2017).
Older people are also at risk of both dehydration (Suhayda & Walton, 2002) and hyponatremia, with physiological control systems significantly changed in people over 65 (and gradually prior to that), leading to an increasing lack of sensitivity to hydration requirements (Kenney & Chiu, 2001). Side effects of medication also lead to hyponatremia, and this is more prevalent in older people (Palmer et al., 2003).
Conclusion
Dehydration affects endurance and by contributing to fatigue also affects the motivation to make cognitive decisions. The strains that dehydration puts on the body, particularly the kidneys, can lead to long-term health problems.
However, as dehydration involves a loss of only 2% of body mass and hydration requirements are different for each individual, it is hard to ensure consistent adequate hydration. Furthermore, while dehydration creates health issues, over-hydration does as well, and pressure on the general population to increase water intake may be counterproductive in many cases.
Continued improvement in the understanding of the negative effects of dehydration and over-hydration and appropriate dissemination of such information to the general public is necessary. Maintaining balanced hydration levels throughout the day would be the most appropriate message to convey. As research has shown drinking requirements vary due to several factors, over time improved guidelines should be made available taking into account sex, age, exertion levels and local temperatures, much as guidelines for body-mass index do.
Forging positive hydration habits early in life will result in a greater chance that adequate hydration will be maintained during variations in temperature and exertion, and later in life, when a person is more at risk of adverse effects.
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