Introduction
Hydration has been implicated in having both health and performance benefits during exercise. Evidence has shown that a change in body mass, resulting in as small as 2% dehydration, causes decreases in exercise performance (19,24,30 cardiovascular strain (18,21,31 5,30,38 cardiovascular strain that results from hypohydration is of particular importance for this review, especially when exercise is being performed in the heat.
Cardiovascular drift is the term used to describe the effects that prolonged exercise has on cardiovascular function; to maintain cardiac output, there is an increase in heart rate (HR) and decrease in stroke volume (SV). The cause of cardiovascular drift is controversial in that some experts attribute cardiovascular drift to increased skin blood flow and reduced central venous return whereas others assert that cardiovascular drift is caused by a reduction in ventricular filling time directly associated with the increase in HR (44 cardiovascular strain (10,12,45
Overall, dehydration impairs exercise performance and endangers overall health. During the process of dehydration, blood volume in the body decreases as plasma volume is reduced (30 42 −1 are common during intense exercise, and if fluid replacement does not minimize the sweat losses during exercise, an athlete increases their risk of dehydration (11 performance by promoting hyperthermia and impairing cardiovascular function through reduced sweat rate and skin blood flow, respectively (11
Research examining the influence of dehydration on physiological measures such as core body temperature and HR are well established. Recently, results have shown that when exercising in the heat, core body temperature increases by 0.22° C for every additional 1% body mass loss (BML) (22 21
The purpose of this systematic review was to evaluate and analyze the data regarding the changes in HR for every 1% change in body mass loss (ΔBML) during exercise in the heat. We further analyzed the data to determine whether preexercise hydration state (euhydrated or hypohydrated) influenced the magnitude of HR change during exercise in the heat and whether intensity and mode (running or cycling) of exercise affected the change in HR at a given percentage of change in BML.
Methods
Experimental Approach to the Problem
Although it has been established that during prolonged exercise in the heat HR increases because of cardiovascular drift and is further exacerbated with increasing levels of dehydration, various ranges of the magnitude increase in HR have been reported. To evaluate a more accurate value for the magnitude in which HR increases during intense exercise in the heat at increasing levels of dehydration, a systematic review was conducted. We searched the following electronic databases without limits on language or dates: PubMed, SPORTDiscus, ERIC, CINAHL, and Scopus. The search was performed in February of 2013 using the following search terms in various combinations: dehydration, euhydration, hypohydration, water consumption, heart rate, athletic performance , running, bicycling, triathlon, football, time trial, exercise, exercise and performance , endurance, and strength. Previously identified articles, review articles, and reference lists of available studies were cross-referenced for additional studies that met the inclusion criteria. A total of 245 articles were identified from this search.
Study Selection
The specific inclusion criteria used to identify eligible articles before data analysis included (a) valid measure of HR, (b) exercise in the heat (>26.5° C), (c) the level of dehydration must reach at least 2%, which is the level at which performance is affected, (d) there must be a comparison group (a euhydrated group or a graded dehydration protocol), (e) if experimental design involved a rehydration phase, only oral rehydration was considered for inclusion, and (f) subjects must be healthy active human beings. Studies were excluded based on the criteria outlined in Table 1 . A euhydrated group or graded dehydration protocol was used to find the differences in HR between the more dehydrated group and less dehydrated group.
Table 1: Exclusionary criteria used to identify appropriate articles for the systematic review.
Titles and abstracts of 176 articles were excluded because they did not meet the inclusion criteria specified above. Of the remaining 69 studies, 46 were excluded after a full review of the text. In the remaining 23 articles, 20 (1,2,4,5,7,14,16,17,19–21,23,26–31,39,43,45 Figure 1 ). When the articles lacked the necessary data for extraction (e.g., failure to report mean values and SDs for %BML or HR), data were obtained through personal communication with the respective authors.
Figure 1: Selection process for articles included for the systematic review. PEDro = Physiotherapy Evidence Database.
Quality Assessment
The methodological quality of the 23 studies was examined using the Physiotherapy Evidence Database (PEDro scale) (46
An a priori inclusion score of 5/10 on the PEDro scale was selected for the study to be included in the data analysis. A PEDro score of 5/10 was chosen because blinding of the participants and researchers was impossible when assessing hydration status in which fluid was restricted or limited before or during exercise. Three studies (32,35,40 Table 2 . One study (21 1,5,14,16,17,28,45 2,7,18,20,23,25–27,29–31,39 4 43
Table 2-a: Characteristics of the 20 studies included in the influence of BML on changes in HR during exercise in the heat.*
Table 2-b: Characteristics of the 20 studies included in the influence of BML on changes in HR during exercise in the heat.*
With the exception of 3 studies (4,23,43 43 4 23 1,5,14,16,17,28,43,45
Three studies (5,19,21 26,27 43 1 16
Data Extraction and Management
The following information was extracted from the included studies: study design, number of participants, environmental temperature (ambient temperature and relative humidity), types of exercise, duration of exercise, HR after exercise, %BML after exercise, and whether the participants began the experimental trial in a euhydrated or hypohydrated state. Only immediate postexercise HR and %BML was synthesized for this review because these 2 variables were the specific focus of the original research question. The number of subjects, mean, and SD values of HR and %BML from each trial were inputted into RevMan Software Version 5.2 (Copenhagen: The Nordic Cochrane Centre, The Cochrane Collaboration, 2012.) to calculate weighted mean difference and 95% confidence intervals (CI) using a random-effects model. The weighted mean difference values were further used to calculate the change in HR for every 1% ΔBML. If any of the studies included multiple trials that examined levels of dehydration compared with a control (graded dehydration protocol) or different protocols (varied intensities or exercise protocols), each separate trial was included for analysis.
As part of the inclusionary criteria, the level of BML because of dehydration must reach at least 2% in addition to a comparison group that is euhydrated or part of a graded dehydration protocol. We termed the groups exceeding 2% BML as “greater dehydrated” and the comparison group (group <2% BML) as “less dehydrated.” For example, for studies in which groups obtained postexercise % BMLs of 1 and 3%, the “less dehydrated” group was the group that had BMLs equally 1%, whereas the group that had BMLs equally 3% were labeled as the “greater dehydrated” group.
Results
Data Synthesis
For each comparison, the change in HR for every 1% ΔBML was calculated by dividing the weighted mean HR difference by the weighted mean % ΔBML. Combined, all included studies (1,2,4,7,14,16,17,19–21,23,26–31,43,45 5,25,39
A variety of exercise types, intensities, durations, and environments were used in the selected studies. Seventeen studies (1,2,4,14,16,17,19–21,23,26,27,29–31,45 7,28,43 2,4,14,16,17,19–21,23,29,30,45 1,26,27,31 7,28 43 1,2,7,26–28,31 4,14,16,17,19–21,23,29,30,43,45 2,4,7,14,17,43,45 2,14 4,7,43 17,45 Table 2 ).
Postexercise Measurements
The weighted mean difference in HR between groups (subjects who were more hypohydrated and subjects who were less hypohydrated) in all trials was 8 b·min−1 (95% CI, 6–10) (Figure 2 ), with the greater dehydrated group exhibiting a higher HR after exercise. The weighted mean difference in %BML was greater in the greater dehydrated group (2.53% [95% CI, 2.04–3.02]). Weighted mean difference in HR was greater in the greater dehydrated groups for those arriving euhydrated than hypohydrated (9 b·min−1 [95% CI, 7–11] and 8 b·min−1 [95% CI, 4–12], respectively). In subjects arriving euhydrated and hypohydrated, the greater dehydrated group exhibited a greater %BML; weighted mean differences were 2.59% (95% CI, 2.07–3.11) and 2.41% (95% CI, 1.57–3.26), respectively. The weighted mean differences in HR in variable intensity and fixed intensity trials were 3 b·min−1 (95% CI, −2 to 8) (Figure 3 ) and 9 b·min−1 (95% CI, 7–11), respectively (Figure 4 ). The mean difference in HR in cycling only trials and running only trials was 8 b·min−1 (95% CI, 6–10) and 9 b·min−1 (95% CI, 5–14), respectively. Change in HR with increasing levels BML was positively correlated (R = 0.66, R 2 = 0.43) and is shown in Figure 5 .
Figure 2: Weighted mean difference in postexercise HR between groups that were more dehydrated (>2% BML) and groups less dehydrated (<2% BML), all trials. BML = body mass loss; HR = heart rate.
Figure 3: Weighted mean difference in postexercise HR for variable intensity trials between groups that were more dehydrated (>2% BML) and groups less dehydrated (<2% BML). BML = body mass loss; HR = heart rate.
Figure 4: Weighted mean difference in postexercise HR for fixed intensity trials between groups that were more dehydrated (>2% BML) and groups less dehydrated (<2% BML). BML = body mass loss; HR = heart rate.
Figure 5: The relationship between the change in HR and increasing levels of body mass loss. HR = heart rate.
Across all trials, the mean change in HR for every additional 1% ΔBML was 3 ± 2 b·min−1 . For trials where subjects arrived euhydrated, the mean change in HR for every additional 1% ΔBML was 3 ± 5 b·min−1 . For the trials in which subjects arrived hypohydrated, the mean change in HR for every additional 1% ΔBML was 3 ± 2 b·min−1 . Analyzing fixed and variable intensity trials (time trials or graded exercise tests), the change in HR for every 1% ΔBML was 4 and 1 b·min−1 , respectively. Change in HR for every 1% ΔBML for cycling and running only trials was 3 ± 2 and 3 ± 2 b·min−1 , respectively.
Discussion
The results of this review indicated that HR is increased with increasing levels of dehydration. Our analysis revealed that HR was increased by an average of 3 b·min−1 for every 1% ΔBML. The increases in HR as a result of the increased levels of dehydration coincide with the results of Montain and Coyle (30 cardiovascular strain ; HR is increased and SV is subsequently decreased. The increase in cardiovascular strain as a result of exercise-induced dehydration has been shown to decrease performance (3,37
Increased cardiovascular strain , occurring during prolonged physical activity in temperate and hot environments, has been identified as a factor impairing physical performance when exercising in a hypohydrated state (8,33 8 17,19–21,30,31,45
Results from our review showed that there were no differences in the change in HR for every 1% ΔBML for subjects arriving either euhydrated or hypohydrated before exercise. The effects of cardiovascular drift have been shown to be in proportion to the degree of dehydration and hyperthermia (17,19,30 performance from the studies included in our review, it is evident that arriving in a hypohydrated state has detrimental effects on performance , thus suggesting hydration status as a key determinant of subsequent exercise performance .
When examining the differences in HR with increasing levels of dehydration in cycling and running trials, our results showed that there were no differences between either exercise modes. This is in support of previous literature (34 15
Studies that required subjects to perform exercise using a time trial or graded exercise protocol saw negligible differences in HR with increasing levels of dehydration (4,7,17,25,45 9 performance . During a race, for example, a runner who is dehydrated will have an overall slower finish time over a runner who is hydrated, although both runners are performing at their maximum capacity. This is due to the physiological strain placed on their bodies, in that HR will be higher at a lower intensity in the more dehydrated runner; so although they may perceive themselves performing maximally, they will be running at a slower pace and be unable to pace themselves evening during the duration of the race (7,41
Increasing levels of dehydration during prolonged exercise, especially in the heat, have profound effects on exercise performance . The American College of Sports Medicine (36 6 performance . In addition, there is supporting evidence recommending that minimizing fluid losses before and during exercise will enhance performance during prolonged exercise (8,13,33 2,4,7,14,17,28
This review was not without limitations. For the purposes of this review, we focused on exercise in the heat. Although we found that HR was increased at increasing levels of BML, we cannot generalize this across all situations because we did not take into account the temperate and cold environmental conditions. Second, all but 3 of the studies were performed in a controlled-laboratory setting and all but 5 studies had subjects exercise at a fixed exercise intensity. To be able to generalize these results during real-world athletic competitions, additional research needs to be conducted analyzing the changes in HR at increasing levels of BML in the field setting. Nonetheless, the presented studies support the notion that HR is increased with increased BMLs during exercise.
Practical Applications
The evidence presented in this review may be useful to both athletes and coaches. The results indicated that there is an additional increase in HR for every 1% change in BML at given exercise intensities during exercise in the heat. This increase in HR with increasing levels of dehydration can cause a decline in athletic performance , not only during training but also during competition. For example, an athlete arriving for a practice session or competition will have an increased HR of about 6–10 b·min−1 during exercise if they are 2–3% dehydrated. This increase in HR exacerbates cardiovascular drift and the level of perceived fatigue, thus decreasing exercise performance , especially during endurance events and exercise in the heat.
Beginning exercise in a euhydrated state and minimizing fluid losses throughout exercise will attenuate the increases in cardiovascular strain and increase overall performance . Athletes who begin exercise in a euhydrated state and minimize fluid losses throughout exercise will be able to exercise at a higher intensity and have a lower perception of exertion and fatigue. As a result, athletes will be able to achieve greater gains during training to help maximize athletic performance during competition. In addition, maintaining an appropriate level of hydration during exercise in the heat is of particular importance because the physiological stresses of heat exposure are responsible for performance decrements.
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