Methodological Limitations in Hydration Research
Whilst there is a clear mechanistic basis for how hypohydration might impair endurance performance, methodological limitations perhaps make it difficult to ascertain the true effects at levels of hypohydration typically experienced by athletes. There are two main limitations of the evidence base in this area. First, until recently, no performance study had blinded subjects to their hydration status. Second, many of the methods used to induce hypohydration are both uncomfortable and unfamiliar to subjects. These limitations in the present literature make it difficult to draw robust conclusions.
Blinding Changes in Hydration
In sport and exercise nutrition research, studies examining the performance effects of a nutrition strategy (e.g. carbohydrate intake or dietary supplements) use blinded experimental designs to remove any potential associated placebo/nocebo effects, but this is not the case with hydration research. Previous research clearly demonstrates placebo effects are evident for carbohydrate [72], caffeine [73], sodium bicarbonate [74] or breakfast [75] consumption before exercise. Indeed, it seems unlikely that any reputable scientific journal would accept for publication a study examining the performance effects of a known ergogenic supplement if the subjects were not blinded to their treatments. However, this is exactly the case for studies examining differences in hydration, as the methods used to induce hypohydration (e.g. fluid restriction before/during exercise, heat exposure, diuretic administration) are overt, meaning subjects know which trials they are performing. Because athletes believe that hypohydration, at least significant hypohydration, impairs performance [76], the overtness of the methods used to elicit hypohydration might entirely explain the results observed. This is potentially a serious problem, requiring attention before we can establish the true performance effects of hypohydration. To date, studies have used two different methods to achieve this blinding: intravenous delivery of fluids [44, 67, 68], or delivery of fluid to the stomach through a gastric feeding tube [41,42,43].
In the first study of its type, Wall et al. [67] dehydrated trained cyclists by 3% body mass through exercise, followed by intravenous rehydration with approximately isotonic fluid in the 2-hour post-exercise, producing euhydration or hypohydration of ~ 2% and ~ 3% body mass at the start of a 25-km cycling time trial in 33 °C. Performance was similar between trials. Using a similar study design, Cheung et al. [68] produced euhydration or hypohydration (~ 2.1% body mass) via an intravenous infusion of isotonic saline during a 90-min standardised preload in trained cyclists. Subjects then completed a 20-km cycling time trial in 35 °C. Subjects performed two euhydrated and two dehydrated trials, with ad libitum mouth rinsing of water in one trial of each to remove sensations of thirst/dry mouth. Again, time trial performance was similar between trials, suggesting that neither hypohydration nor thirst influences performance. These interesting studies successfully blinded subjects to their hydration status and have potentially important implications. They suggest that when an individual’s knowledge of their hydration status is removed, hypohydration of 2–3% body mass does not impair exercise performance, meaning previous studies were potentially confounded by the lack of blinding. Interestingly, both these studies reported higher rectal temperature with hypohydration towards the end of their time trials, which in some exercise settings, might impair performance capabilities [77].
Our group has used a different approach to blind subjects, choosing to deliver water (or not, as the case may be) directly to the stomach via a gastric feeding tube inserted orally [41] or nasally [43]. In our first study [41], active but not specifically cycling-trained subjects completed eight blocks of 15 min cycling, separated by 5 min rest, in 34 °C. Water infused into the stomach was manipulated to either maintain euhydration or produce hypohydration (~ 2.4% body mass) by the end of the preload. Additionally, a small volume of water (~ 15 mL) was orally ingested every 10 min of the preload in both trials. Subjects then performed a performance test, where they had to complete as much work as possible in 15 min. To remove expectation effects related to hydration and assist with the blinding, subjects were told that drink composition was being manipulated. In contrast to our hypothesis and the results of previous studies [67, 68], 8% less work was completed in the hypohydrated trial (Fig. 2a). Subsequently, we used the same methods to explore the effects of blinded hypohydration in trained cyclists, except the 2-hour cycling preload was continuous rather than intermittent [43]. This time we recruited two pair-matched groups of trained cyclists, with both groups completing trials in 31 °C. Water intake was manipulated to either maintain euhydration or produce hypohydration (~ 3% body mass) at the end of the preload. Subjects then completed a time trial, where they performed a set amount of work (estimated to last ~ 15 min) as quickly as possible [78]. One group (blinded group) had water delivered through a nasogastric feeding tube with a similar cover story and oral water intake to our previous study [41], whilst in the other group (unblinded group) all water was provided orally in both trials. We hypothesised that hypohydration would impair performance in both groups, but that the impairment would be attenuated in the blinded group. In contrast to this hypothesis, we observed that the performance effects were remarkably similar between groups with performance decrements of 11% (blinded group) and 10% (unblinded group) caused by the hypohydration (Fig. 2b).
It is important to note that careful consideration needs to be given to the finer points of the methods used in these gastric infusion studies to ensure adequacy of blinding. First, the gastric tube must be fixed in position, by application of tape on the cheek, behind the ear, and onto the centre of the upper back, and dummy infusions must occur in dehydrated trials. Fluid infused must be maintained at body temperature (~ 37 °C), as too much deviation from this value will mean fluid is detected when it travels down the tube into the stomach. Fluid should be infused in small volumes every few minutes, as infusions of large volumes will increase gastric distension and may be detected by subjects. Finally, we recommend a clear, coherent and plausible cover story is used to remove any pre-conceived ideas about the effects of hypohydration and, if thirst might respond differently between trials, prevent subjects determining which trial they are completing.
The inconsistent findings between these blinded hydration studies [41, 43, 67, 68] suggest that further research is required before a firm conclusion can be made. However, perhaps differences in the methods used might also explain the results, as studies using intravenous rehydration report hypohydration does not impair performance [67, 68], whilst studies using intragastric rehydration do [41, 43]. These apparent differences might be caused by the divergent effects on physiology and perception produced by the different techniques.
Manipulating hydration status via intragastric water delivery produces differences in serum osmolality between euhydrated and hypohydrated trials [41, 43], caused by hypovolemia with hypohydration. Because serum osmolality is key for coordinating physiological fluid balance responses to hypohydration induced by exercise [1], it is likely that replicating typical responses is important in blinded hydration studies. The use of approximately isotonic saline in studies manipulating hydration status through intravenous rehydration [67, 68] means the hypertonicity produced is present (and similar) in both hypohydrated and euhydrated trials. As such, internal homeostatic signals indicative of hypohydration (i.e. an increased AVP concentration and renal water conservation, as well as intracellular dehydration) were likely activated in both trials. Thus, in these studies [67, 68], the internal physiological milieu would be consistent with, and likely sensed as, hypohydration, irrespective of body water. In contrast, euhydration/hypohydration induced by intragastric rehydration produces physiological responses consistent with those typically reported in unblinded studies (i.e. decreased plasma volume, increased serum osmolality, increased AVP concentration). Therefore, whilst body water might be manipulated by intravenous rehydration, if internal physiological signals still indicate hypohydration, this might feed into the control of self-regulated exercise intensity and performance [63, 64].
Additionally, perceptual responses related to fluid balance (e.g. thirst) might play a key role in how hypohydration impairs endurance performance [6, 8, 46], particularly as oral fluid ingestion may be important for fluid balance regulation [79]. In a hypohydrated state (~ 2.8% body mass loss) induced by exercise, ingestion of water followed by extraction from the stomach suppresses thirst and AVP (at least partially) within minutes, despite no recovery of plasma volume or osmolality either immediately or in the subsequent 80 min [79]. Additionally, Arnaoutis et al. [80] reported that exercise capacity in 31 °C, when hypohydrated (~ 1.9% body mass loss induced by exercise), was increased with ingestion of, but not mouth rinsing with (25 mL in both situations) water, when compared with a no water trial. The exercise capacity test lasted ~ 20 min, thus the ~ 100 mL ingested in that trial was not meaningful for fluid balance. This and other work [58] demonstrate that mouth rinsing water alone confers no performance benefit, suggesting fluid must be consumed to influence performance. Casa et al. [81] reported a strong trend (p = 0.07) for oral rehydration to increase exercise capacity compared with intravenous rehydration in recovery from hypohydration (4% body mass). Interestingly, oral rehydration reduced rectal and skin temperatures during exercise, as well as reducing other relevant variables (blood lactate/glucose concentrations and respiration rate) compared with intravenous rehydration. This suggests that oral fluid intake might also be important for some of the physiological effects associated with euhydration.
Taken together, the limited evidence available suggests that the swallowing of fluid might be an important factor involved in both regulatory and performance responses to hypohydration during exercise. Some have speculated that this response is possibly related to activation of oropharyngeal receptors [79, 80]. However, it is difficult to separate effects evoked by oropharyngeal responses from those evoked by gastric responses because fluid ingested is received by the stomach. Thus, it is difficult to discern if the swallowing of fluid per se, or the delivery of fluid to the stomach/gastrointestinal tract, controls these effects. Either way, as no fluid was swallowed in the studies of Wall et al. [67] and Cheung et al. [68], this might explain why hypohydration did not affect performance. In contrast, our studies, using intragastric rehydration combined with some oral rehydration (~ 15 mL every 5 min in euhydrated and hypohydrated trials), would have activated receptors present in oropharyngeal and gastric regions, possibly explaining the performance responses observed. Adams et al. [42] also used intragastric rehydration to produce blinded euhydration or hypohydration (~ 2.2% body mass) at the end of 2 h of exercise in 35 °C. Additionally, 25 mL water was ingested every 5 min of the 2 h to suppress thirst. Thirst was similar between trials, whilst the mean work rate was ~ 6% lower in the hypohydration trial, suggesting hypohydration can impair performance independent of thirst. It is important to note that these results [42] should not be interpreted as thirst not playing a role in hypohydration-induced impairments of performance, but rather that the effects are not fully mediated by thirst.
The notion that oropharyngeal/gastrointestinal stimulation following drinking might be important for performance is supported by the results of another recent blinded study [44]. Intravenous rehydration was used to maintain euhydration or induce hypohydration (~ 1.5% body mass) with water ingested in both trials (25 mL every 5 min). In contrast to previous studies using intravenous rehydration for blinding [67, 68], hypohydration impaired endurance performance. However, it must be noted that the plasma volume expansion produced by saline infusion in the euhydrated trial could also explain the results, as pre-exercise plasma volume expansion of the magnitude observed has previously been shown to enhance performance [82]. However, taken together, these studies might suggest that oral fluid intake is necessary to maximise performance responses to euhydration. Indeed, this is a theory that reconciles the discordant performance responses observed in blinded hydration studies to date.
The findings of Funnell et al. [43] are particularly important for interpreting previous work investigating the performance effects of hypohydration, as one might hypothesise (as we did) that knowledge of hypohydration might exaggerate any negative performance effects. Therefore, these results suggest that when hypohydration of ~ 3% body mass is present, impairments in endurance performance are not caused or exaggerated by a lack of study blinding. This suggests the conclusions of previous work, where hypohydration was ≥ 3% body mass, are unlikely to be confounded. However, it is possible the negative performance effects of previous unblinded studies, where hypohydration is < 3% body mass (including our own), may be inflated or explained by a placebo/nocebo effect. At lower levels of hypohydration (< 1–2%), confidence that the change in body water is outside typical euhydrated fluctuations is reduced [1]. Thus, it seems likely that the lower the level of hypohydration, the greater the chance that any associated negative performance effects are exaggerated or explained by placebo/nocebo effects.
It is important to note that the aforementioned blinded hydration studies do not necessarily provide evidence about the mechanisms by which hypohydration influences performance. What these studies do is to begin to build a strong foundation on which to understand if hypohydration, at a level commonly experienced in athletic settings, influences performance. On balance, our view is that the evidence to date strongly suggests that when sufficient hypohydration is present (possibly > 2% body mass), endurance cycling performance in the heat is compromised, at least when all typical physiological and perceptual symptoms are present.
Uncomfortable and Unfamiliar Dehydration Methods
As well as being overt, the methods used to induce hypohydration in the scientific literature are often atypical of subjects’ normal behaviour and produce uncomfortable symptoms/side effects. For example, common techniques used to induce hypohydration include: prolonged passive fluid restriction [32, 35], exercise-induced dehydration combined with fluid restriction during or after exercise [27, 40] or diuretic drug administration [26]. Fluid restriction causes thirst, whilst diuretic use can cause polyuria, both of which are uncomfortable. These dehydration methods increase feelings of headache [60] and can increase sensations of pain during exercise [61], likely explaining the negative influence of hypohydration on mood [60,61,62], as well as the impairment of performance of vigilance-related tasks [82]. Thus, some of the effects of hypohydration on performance reported in the literature might actually be associated with this discomfort, rather than hypohydration per se. A related methodological consideration here is drinking during exercise at a rate below what an athlete would do if provided fluid ad libitum, which is outside the scope of this review and interested readers are directed to recent review articles [3, 6, 7, 9].
There is considerable inter-individual variation with regard to tolerance of hypohydration [10]. Data from competitive endurance events report that greater body mass loss is weakly associated with better race performance [84,85,86,87,88]. This has, by some, been misinterpreted as evidence that hypohydration produced during prolonged endurance exercise might enhance performance. Clearly, association does not prove causation, but it seems likely the direction of the relationship would be the opposite, such that faster racing precipitates increased body mass loss (i.e. hypohydration). Faster racing means reduced time available to drink, increased metabolic heat production and sweat rate [89], and possibly decreased gastric emptying of ingested fluids with effects on gastrointestinal comfort [90]. This was nicely demonstrated by Dion et al. [91], who reported, in a controlled laboratory experiment with ad libitum drinking, that faster racing lead to a greater sweat rate and hypohydration at the end of a half marathon, but did not alter drink ingestion. However, the finding that endurance athletes can finish [80,81,82,83,84] and even win races in world-class times [92] with as much as 10% body mass loss is intriguing. Because of issues related to fluid availability (e.g. drink station placement or difficulties with transporting fluids) or gastrointestinal comfort at higher exercise intensities [90], well-trained endurance athletes are likely to regularly perform hypohydrated in their normal training and/or competition [16, 22,23,24,25]. This may increase resilience to hypohydration-induced discomfort and reduce the impact of hypohydration on performance, closing the gap between euhydrated and hypohydrated performance. Therefore, some researchers [6, 39, 93, 94], us included, have postulated that repeated exposure to hypohydration might mitigate some of the negative performance effects.
There is, however, limited empirical evidence to draw on at this time. Merry et al. [93, 94] reported that training status alters the fluid balance regulatory [93] and physiological [94] responses to hypohydrated exercise. Despite this, the performance impairment with hypohydration was similar in trained and untrained subjects [94]. However, one of the untrained subjects performed 16% better in the hypohydrated trial compared with the euhydrated trial, which seems unlikely. Removal of this subject changed the interpretation of the data, such that having a higher aerobic fitness attenuated the performance impairment caused by hypohydration. However, whilst greater aerobic fitness might alter physiological responses to hypohydrated exercise, we hypothesise that familiarity with a hypohydration stimulus, rather than fitness, might attenuate performance impairments. Therefore, the methods used to induce pre-exercise hypohydration by Merry et al. [94] were possibly unfamiliar to both groups.
To date, the study by Fleming and James [39] is the only study to directly assess the effect of repeated familiarisation with hypohydration. In this study, active, but not endurance-trained, subjects performed euhydrated and hypohydrated trials in a randomised crossover manner both before and after four exposures to the hypohydration stimulus. Euhydration or hypohydration (~ 2.4%) was produced by manipulating fluid intake in the 24 h before and during a 45-min steady-state run prior to a 5-km treadmill time trial. Hypohydrated performance before the four hypohydration exposures was significantly slower (–5.8%) than when euhydrated, but only 1.2% slower after familiarisation. Although there was no significant difference between hypohydrated and euhydrated trials after familiarisation, there was a strong trend (p = 0.064), with nine of the ten subjects running slower in the hypohydrated trial (compared to ten before familiarisation; Fig. 3). For all subjects, hypohydrated performance improved, while euhydrated performance did not change after familiarisation. Interestingly, responses for perception of effort during the 45-min steady-state run mirrored the performance effects observed.
Therefore, this study suggests that repeated familiarisation (on five occasions) with a hypohydration stimulus can attenuate, but not abolish, the negative performance effects of hypohydration, at least for running. This might go some way to explaining why well-trained endurance athletes (who will likely have years of exposure to hypohydration) might be able to finish and seemingly perform well in competitive events, despite sometimes substantial hypohydration at the end of the race [84,85,86,87,88, 92]. Clearly, further research is needed, but where maintenance of euhydration is not possible and athlete health and safety permits, perhaps strategic familiarisation with the anticipated hypohydration (method and magnitude) might be a prudent ergogenic strategy [95]. It is anticipated that for most athletes this would simply represent continuing normal weekly training, because, at least for endurance activities, training likely presents a similar or reduced opportunity to consume fluid (i.e. no drink stations, limited support/ability to carry fluids). However, in situations where fluid availability is low in competition and high in training, some ‘competition-specific fluid intake training’ might be beneficial.
It is also important to consider the results of the study of Fleming and James [39] in the context of the numerous other studies that have used uncomfortable hypohydration methods before testing performance capabilities. They suggest that the results of these previous studies, where uncomfortable and unfamiliar methods have been used to induce hypohydration, might exaggerate the negative performance consequences of hypohydration and, therefore, these studies should be carefully interpreted.
Furthermore, weight loss induced through dehydration has been theorised to possibly increase performance in activities where body mass must be carried [6]. However, Ebert et al. [31] reported that after a 2-hour cycling preload, time-to-exhaustion in uphill cycling (8% grade) was reduced with hypohydration of ~ 2.5% compared with euhydration, suggesting that hypohydration still impairs performance even when body mass must be carried. Moreover, a number of studies have reported that endurance running performance (i.e. where body mass is carried) is impaired by hypohydration [26, 32, 38], suggesting that, at least for unfamiliar/novel hypohydration, the associated mass loss is unlikely to be ergogenic. Whether this holds true for well-trained athletes used to experiencing the levels of hypohydration experienced during racing is unclear and clearly requires further work.