Fluid Balance in Team Sport Athletes and the Effect of Hypohydration on Cognitive, Technical, and Physical Performance

To locate relevant articles for this review the literature search was conducted using PubMed and EBSCO databases. Multiple search phrases pertaining to “fluid balance”, “sweat losses”, “sweating rate”, “hypohydration”, “dehydration”, “team sport”, “performance”, “skill”, “cognition”, “attention”, “vigilance”, “decision making”, “memory”, “reaction time”, “intermittent”, “high-intensity”, “sprint”, “jump”, “power”, and “agility” were used. Other general inclusion criteria included English language and full-length articles published in peer-reviewed journals. Abstracts and unpublished observations were not included. The search period was through September 2016. A total of 75 original studies measuring sweating rate and/or fluid balance (involving ad libitum drinking, i.e., not controlled by study investigators) in athletes during training or competition were identified (see Table 1 for a general summary). The search located 20 original studies measuring the impact of hypohydration on performance during intermittent high-intensity protocols and involving team sport athletes as participants (see Tables 2 and 3 for details on individual studies).

Although racket sports (e.g., tennis) are typically considered individual sports, they are included in this review because they are team sports when played in “doubles” competition (a match between two pairs of players) and because of the intermittent high-intensity physical demands and technical skill requirements. Sports that require skill but do not rely heavily upon intermittent bouts of high-intensity running (e.g., golf) are not discussed here. Endurance, strength/power, combat, and esthetic sports are also outside the scope of this review.

Body fluid balance is primarily a function of an individual’s fluid intake (i.e., hydration practices) relative to his or her fluid losses (i.e., sweat) during exercise. The term “euhydration” refers to maintenance of “normal” baseline body water content, while the terms “hypohydration” and “hyperhydration” refer to body water deficits and excesses beyond euhydration, respectively. The term “dehydration” is defined as the process of the dynamic loss of body water or the transition from euhydration to hypohydration. The simplest method to assess an individual’s acute change in hydration status (or fluid balance) is to compare his/her body mass to baseline values [13]. For example, 3% hypohydration is defined as a water deficit equal to 3% BML. It is acknowledged that a small portion of body mass loss during exercise occurs due to substrate oxidation, that is, non-water mass loss through expiration of carbon dioxide. The reader is referred to other papers that discuss potential errors in hydration assessment methodologies in greater detail [13, 14]. Importantly, the BML method of hydration assessment and related terminology have been used in all relevant studies identified for the aims of this review and, therefore, will be used throughout the discussion that follows.

Because sweat is hypotonic compared with plasma [122], exercise-induced hypohydration is associated with an increase in plasma osmolality and a decrease in plasma volume (i.e., hyperosmotic hypovolemia). Hypovolemia results in a decrease in stroke volume and a compensatory increase in heart rate to maintain a given cardiac output [123‐125]. Hypovolemia and hyperosmolality delay the onset and decrease the sensitivity of the sweating and skin blood flow responses to hyperthermia [126‐128], thus increasing heat storage [125, 129]. Consequently, exercise performance that is dependent upon the cardiovascular and thermoregulatory systems, such as aerobic exercise in the heat, can be impaired by hypohydration [9, 12]. The physiological mechanisms underlying the effect of hypohydration on aerobic performance have been well studied (for reviews, see Sawka et al. [130, 131] and Cheuvront et al. [132]). By contrast, much less is known about the potential mechanisms for the detrimental effects of hypohydration on team sport performance. The next section summarizes the proposed mechanisms by which hypohydration could impair cognition, technical skill, and physical performance related to team sports.

The effect of hypohydration on cognition has been widely researched. While decrements in cognitive performance with hypohydration have been reported in some studies of athletes [97, 98, 100, 133], healthy young adults [134‐138], and military personnel [139, 140], other studies have found no effect of hypohydration [94, 95, 141‐146]. Furthermore, a clear mechanism by which hyperosmolality or hypovolemia per se would impair cognition is currently lacking (for a review, see Cheuvront & Kenefick [12]). In brief, hypohydration has been suggested to mediate decrements in brain function by decreasing cerebral blood flow, reducing brain volume, or increasing blood–brain barrier permeability. However, a consistent effect of hypohydration on these measures of brain function [147‐153] at the levels of BML typically reported in the cognition literature (e.g., 1–4% BML in team sport studies) has not been found.

An alternative explanation, previously described by Cheuvront and Kenefick [12], is that symptoms of hypohydration, such as thirst, headache, or negative mood states (e.g., fatigue), may distract subjects during cognitive tasks and subsequently impair performance. Moreover, individual variability in cognitive resiliency (ability to overcome the stressors of hypohydration) may explain, in part, the equivocal findings in the cognition literature [12]. For example, Szinnai et al. [145] found that 2.6% BML induced by water deprivation had no impact on cognitive-motor function, but significantly increased ratings of perceived effort and concentration necessary for test completion. Furthermore, Kempton et al. [147] showed that although mild hypohydration did not impair cognitive performance or cerebral perfusion, higher levels of neuronal activity (as indicated by a greater increase in the fronto-parietal blood oxygen-level-dependent response) were required to perform an executive function task. Thus, it may be that some individuals are better at increasing concentration sufficient to overcome symptomologic distracters of hypohydration and achieve the same level of performance as that of a euhydrated state [12]. In the team sport literature reviewed in Sect. 4.1, hypohydration consistently increased ratings of thirst, perceived exertion, and fatigue, but subsequent effects on cognitive performance were equivocal.

In team sports high-intensity efforts are performed within the context of intermittent exercise over a prolonged period of time (1–2 h). Thus, it is plausible that reductions in aerobic capacity [154‐157] or muscle endurance [117, 158] that have been shown to occur with hypohydration, could help explain the impaired physical performance (sprinting, lateral movements, and intermittent running capacity) reported in studies mimicking the demands of team sports training/play. In addition, hypohydration has been shown to result in decreased muscle blood flow [159, 160] and alterations in skeletal muscle metabolism (increased lactate, muscle glycogenolysis, and carbohydrate oxidation) [160‐163]. However, to date these findings have mostly been documented in prolonged cycling exercise, which is likely a result of the difficulty in obtaining invasive physiological measurements in team sport athletes on the field of play. To our knowledge, only one team sport performance study has measured the effect of hydration status on markers of muscle metabolism. Ali and colleagues [106] found that blood lactate concentration was significantly higher (7.2 vs. 3.7 mmol/L) when female soccer players drank no fluid (2.2% BML) compared with when they ingested fluid during the 90-min LIST protocol (1.0% BML). However, in this study, sprint and skill performance were not impacted by hydration status [106]. It is also interesting to note that, in the studies reviewed (Tables 2, 3), performance was no more likely to be impaired in sports with high aerobic demands (e.g., soccer) than sports that have more rest opportunities (e.g., basketball) or are lower intensity (e.g., baseball, cricket). This is somewhat surprising given the reported detrimental effects of hypohydration on endurance performance [9, 12], but it is likely that study limitations and various modifying factors play a role in the discrepancy in these findings (discussed in more detail in Sect. 5.5).

Another potential mechanism to consider is the hyperthermic effect of hypohydration, as some sport-specific studies have reported higher body core temperatures with fluid restriction versus fluid intake [94, 102, 103]. Drust et al. [164] reported that elevated core and muscle temperatures during a 40-min intermittent cycling protocol were associated with impaired repeated sprint performance. The authors concluded that the results may be related to the influence of hyperthermia on central nervous system function. Central fatigue, as indicated by an impaired ability to sustain maximal muscle activation during sustained contractions, has been implicated in exercise performance decrements associated with hyperthermia [165‐167]. It is thought that multiple factors (including core and skin temperature) likely provide afferent inputs for central nervous system integration and reduce motor drive to skeletal muscles. The reader is referred to a review by Nybo et al. [165] for a recent comprehensive discussion on physiological factors governing hyperthermia-induced fatigue. It is important to note that while hyperthermia (increased core temperature) can impair performance during prolonged exercise, increased muscle temperature could enhance certain aspects of physical performance [168]. In particular, improved sprinting performance has been reported in soccer [16, 169], perhaps as a result of improved muscle contractile properties and anaerobic power [170, 171], in hot versus temperate environments. However, these changes occur irrespective of hypohydration, as Mohr and colleagues [16] found faster peak sprinting speed in hot versus temperate conditions when players accrued similar levels of BML between trials with ad libitum fluid intake (1.9 and 1.8%, respectively). As such, optimal performance strategies may involve both maintenance of muscle temperature as well as the limitation of excessive hypohydration.

Finally, as with cognition, it is possible that psychological factors are also involved in hypohydration-induced decrements in physical performance. Negative mood states and other stressors associated with fluid restriction may distract athletes from giving their full effort toward performing the high-intensity exercise task. In support of this notion, most (10 of 11) team sport studies that measured subjects’ perceived exertion or fatigue found that ratings were significantly elevated in conditions of fluid restriction versus fluid intake (see Table 3). In addition, there may be some interplay between familiarization with hypohydration, perceived exertion, and the effect of hypohydration on performance. Although not specific to team sports, Flemming and James [172] reported some support for this concept in recreationally active men. In this study, 2.4% BML impaired 5-km treadmill running performance (by 5.8%) when subjects were unfamiliar with the hypohydration protocol. However, there was an attenuation of subjects’ ratings of perceived exertion and the performance decrement (1.2%) after completion of four familiarization sessions designed to habituate subjects with the hypohydration protocol. While these novel data are interesting, more research is needed before definitive conclusions can be made regarding the effect of familiarization with hypohydration on ratings of perceived exertion and performance.

The execution of sport-specific skill is a complex process, as it is dependent upon several aspects of physical and cognitive function. For example, a successful shot attempt during a basketball game requires a combination of fine motor (ball control) and gross motor (balance and coordination) skills, physical abilities (power, strength, and speed), concentration, and decision-making skills, among other factors. Thus, if hypohydration impairs cognition or physical performance, either directly through hyperosmotic/hypovolemia-induced changes in physiological function or as a byproduct of the distracting symptoms of hypohydration, then these mechanisms could also account for impaired execution of technical skills.

Other proposed mechanisms include changes in vestibular function, as some studies have reported impaired postural balance (increased body sway) with hypohydration after exercise [173‐176]. However, other studies have reported no impact of hypohydration on balance control [133, 141, 177]. Theoretically, balance is more likely to be impaired when hypohydration is combined with hyperthermia [175] or fatigue from previous exercise [173, 174]. However, Seay et al. [177] reported no relation between standing balance and levels of hypovolemia or hyperosmolality. As such, a clear physiological mechanism by which hypohydration could impair postural control has not yet been identified.

Future studies are needed to elucidate which physiological mechanisms or combination thereof may account for the detrimental impact of hypohydration on cognition, technical skill, and physical performance reported in some studies. For more details on potential mechanisms underlying the impact of hypohydration on various aspects of performance the reader is referred to previous reviews [12, 130, 132].

Many methodological differences among studies likely contribute to the inconsistent results reported across the literature. For example, the subject characteristics (e.g., sex, age, caliber of athlete), method of dehydration (e.g., passive heat, exercise, or exercise-heat stress), and/or BML differences between hypohydration and control trials varied considerably among studies. Thus, a relevant question that follows is: Do certain factors modify the impact of hypohydration on performance (i.e., are there interaction effects)? To date, no studies have addressed this question directly. However, when comparing studies across the literature (see Table 4) there seems to be no clear pattern regarding the impact of sex, age, or athlete caliber on the effects of hypohydration on team sport performance. This is due in part to the limited data available, as only six studies have included female subjects [63, 96, 98, 100, 106, 112] and only three studies have tested youth athletes [60, 101, 103]. A broad range of athlete calibers have been tested across studies with equivocal results within caliber (see Table 4), suggesting that a particular level of athlete is not more or less likely to be negatively affected by hypohydration based on the currently available data in team sports. However, studies directly comparing team sport athletes with different skill levels are needed.

As shown in Table 4, one of the factors that does seem to modify the impact of hypohydration on performance in team sport athletes is the method of dehydration. When dehydration was induced via exercise in the heat, subsequent performance was usually impaired with respect to cognition, skill, sprinting, lateral movements, jumping/power, and intermittent running capacity (the only exception was jumping performance in one study [103]). By contrast, when dehydration was induced via exercise alone, subsequent performance was impaired in ≤ 50% of the studies within each of the performance categories. This finding is perhaps not surprising given the well-established deleterious effect of environmental heat stress and subsequent heat strain on aerobic performance [130, 155, 165, 178] and muscle function [158, 164, 179], as well as mood states and perceived exertion [165, 180, 181], which may in turn impact aspects of team sport performance. It is important to note that the studies using heat and exercise to induce hypohydration also involved higher levels of BML. In addition, some of these studies included a rest period between the dehydrating exercise/heat protocol and the commencement of performance drills to allow body core temperature to return to baseline values [97, 102, 103]. Thus, it could be argued that hypohydration per se was responsible for the impaired performance. Nonetheless, in real life it can be difficult to separate out the effects of hypohydration and heat stress, as the two are closely linked when training/competing in warm–hot environments (i.e., exercise in the heat increases sweat rate thereby magnifying fluid losses) [9].

Another factor related to the method of dehydration is the timing of body water loss with respect to completion of the sport-specific protocol and performance tests. Most studies were designed to determine the effects of dehydration, accrued throughout sport-specific training/play, on performance. Mixed results were reported with this methodological approach. By contrast, some studies established hypohydration in the hours before [97, 102, 103] or in some cases the day before [98, 114] the sport-specific tests. A detrimental effect of hypohydration was reported in all five of these studies (albeit most of these studies also involved higher levels of hypohydration (>2% BML) and/or heat stress). This methodological approach allows for a more systematic investigation of the effects of hypohydration (e.g., a standard level of BML) and provides insight on performance effects when athletes begin training/competition in a hypohydrated state (which may be applicable for tournaments or back-to-back training sessions). However, it is not applicable to scenarios where athletes begin exercise in a euhydrated state, and, as such, impacts on the ecological validity of the study findings. It would be interesting for future research to directly compare the effects of previous dehydration versus in-game dehydration on performance.

The level of BML reached in hypohydration trials and the inclusion of a proper euhydration control trial are other important factors to consider. In the studies reviewed, most included a control (fluid intake) trial to compare performance against that of hypohydration (i.e., fluid restriction) trials. However, across studies, varying degrees of BML accrued during the control trials. That is, some studies aimed to replace fluid losses and maintain euhydration (< 1% BML [9]), while others involved ad libitum intake or prescribed a fixed volume of fluid intake that resulted in mild to moderate hypohydration in the control trials. For example, several studies in soccer dehydrated athletes to ~2–3% BML in the fluid restriction trials, but also accrued 1–2% BML in the fluid intake trials [95, 105, 106, 113]. By contrast, in most basketball [97, 102, 103], baseball [85, 114], and cricket [83, 109] studies, subjects maintained euhydration (< 1% BML) in the control trials. The differences in study design are likely due, in part, to attempts to implement ecologically valid fluid intake patterns, which reflect differences in drinking opportunities across sports. Nonetheless, the inconsistency makes it difficult to assimilate results across the literature.

For the purpose of this discussion (and Table 4), BML differences between control trials and hypohydration trials were calculated for each study. In this regard, most investigations involved a 1–2% BML difference between control trials and hypohydration trials. In these studies, there were mixed results in how hypohydration impacted cognition, sprinting, lateral movements, and intermittent running capacity, and no or little effect of hypohydration on jumping/power and skill. Only five studies involved a 3–4% BML difference between trials, but all found impaired performance with hypohydration [83, 85, 97, 102, 114]. It is important to note that these studies also involved exercise in the heat as the method of dehydration, so it is unclear whether the heat stress also contributed to the performance impairment. Nonetheless, two out of three studies involving graded levels of hypohydration (~ 1–4% BML) have indicated that ~3–4% BML was more likely to impair performance than ~1–2% BML [85, 97, 102].

Figure 2 shows a Venn diagram illustrating the likelihood of team sport performance impairments with hypohydration. Based on the studies reviewed, decrements in cognitive, technical, or physical performance seem more likely with higher levels of hypohydration and heat stress. However, as discussed in Sects. 5 and 6, other factors may play a role, but currently lack sufficient research in team sports. These include high aerobic demand, hypohydration at baseline, and individual differences in the response to hypohydration (e.g., low cognitive resiliency).