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Are Habitual Hydration Strategies of Female Rugby League Players Sufficient to Maintain Fluid Balance and Blood Sodium Concentration During Training and Match-Play? A Research Note From the Field

Jones, Ben; Till, Kevin; King, Roderick; Gray, Michael; O'Hara, John

The Journal of Strength & Conditioning Research: March 2016 - Volume 30 - Issue 3 - p 875–880
doi: 10.1519/JSC.0000000000001158
Research Note
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Jones, B, Till, K, King, R, Gray, M, and O'Hara, J. Are habitual hydration strategies of female rugby league players sufficient to maintain fluid balance and blood sodium concentration during training and match-play? A research note from the field. J Strength Cond Res 30(3): 875–880, 2016—Limited data exist on the hydration status of female athletes, with no data available on female rugby players. The objective of this study was to investigate the habitual hydration status on arrival, sweat loss, fluid intake, sweat Na+ loss, and blood [Na+] during field training and match-play in 10 international female rugby league players. Urine osmolality on arrival to match-play (382 ± 302 mOsmol·kg−1) and training (667 ± 260 mOsmol·kg−1) was indicative of euhydration. Players experienced a body mass loss of 0.50 ± 0.45 and 0.56 ± 0.53% during match-play and training, respectively. During match-play, players consumed 1.21 ± 0.43 kg of fluid and had a sweat loss of 1.54 ± 0.48 kg. During training, players consumed 1.07 ± 0.90 kg of fluid, in comparison with 1.25 ± 0.83 kg of sweat loss. Blood [Na+] was well regulated (Δ−0.7 ± 3.4 and Δ−0.4 ± 2.6 mmol·L−1), despite sweat [Na+] of 47.8 ± 5.7 and 47.2 ± 6.3 mmol·L−1 during match-play and training. The findings of this study show mean blood [Na+] that seems to be well regulated despite losses of Na+ in sweat and electrolyte-free fluid consumption. For the duration of the study, players did not experience a body mass loss (dehydration >2%) indicative of a reduction in exercise performance, thus habitual hydration strategies seem adequate. Practitioners should evaluate the habitual hydration status of athletes to determine whether interventions above habitual strategies are warranted.

Research Institute Sport, Physical Activity and Leisure, Leeds Beckett University, Leeds, West Yorkshire, United Kingdom

Address correspondence to Dr. Ben Jones, B.Jones@LeedsBeckett.ac.uk.

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Introduction

Understanding the habitual hydration strategies of athletes, encompassing sweat loss and fluid consumption is important for strength and conditioning coaches to either optimize hydration strategies or acknowledge habitual behaviors that are adequate (6). To date within rugby, hydration strategies (i.e., fluid intake and sweat loss) have only been reported for male rugby league (17) and rugby union (11,15) players, and male rugby league referees (10). To date, no data are available on the hydration strategies of female rugby league players; therefore, strength and conditioning coaches do not have an evidence base for which to either intervene or allow habitual practices to continue. There may be differences in fluid homeostasis, due to a biological sex effect (i.e., differences in thermoregulation (9) and metabolic heat production (1)), or behavioral differences (i.e., females are more compliant with fluid intake advice during endurance exercise (5)) between sexes.

Rugby league players are regularly involved in match-play and field-based training sessions, thus understanding any differences among the respective session types may allow developments in the preparatory practice (i.e., how players arrive) and habits during training and match-play. In addition to hydration assessments (i.e., body mass change, fluid intake, and sweat loss), investigating blood [Na+] changes in rugby players is also insightful for strength and conditioning coaches. For example, a recent study showed that on some occasions, rugby union players consumed excessive fluid and became hyponatremic (blood [Na+] <135 mmol·L−1) (11). Hyponatremia can lead to suboptimal muscle function (16), and previous observations of a female tennis player have shown severe implications for health (21). Female athletes have been shown to have a greater prevalence of hyponatremia in comparison with males (24). To the author's knowledge, no study has investigated the hydration habits and change in blood [Na+] in female rugby players, thus practitioners are unaware of what level of intervention or prescription is required for this population. Therefore, the purpose of this study was to establish the habitual hydration status on arrival, determine sweat loss, fluid intake, sweat [Na+], and Δblood [Na+] during field training and international match-play in female rugby league players.

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Methods

Experimental Approach to the Problem

The design of the study was observational in nature, to provide data on the players' habitual hydration status on arrival, sweat loss, fluid intake, sweat [Na+], and Δblood [Na+], and to establish postexercise hydration status during 1 international match and 4 field training sessions. Players were advised that observations were to establish sweat loss, in addition to blood and sweat [Na+], as to not encourage excessive or abnormal drinking habits. All training sessions lasted approximately 90 minutes and the match lasted 80 minutes. Training sessions started at 1,300 hours and kick-off time on the day of the match was 1,500 hours. Although no preobservation controls were employed, the data collected on arrival to match-play and training explored the habitual arrival status of international female rugby league players.

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Subjects

Ten international female rugby league players (age, 24.7 ± 5.8 years and body mass, 73.8 ± 16.0 kg) volunteered to participate in the study. The cohort consisted of 5 positional backs and 5 positional forwards. All players were observed during training and match-play. Despite the limited sample size, given the small population group of international female rugby league players in the United Kingdom (i.e., the squad; n = 25), the sample recruited was deemed appropriate to represent this specific population. All protocols received institutional ethics approval with written consent provided and signed by subjects.

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Procedures

On arrival, 60 minutes before kickoff for match-play and 30 minutes before training, a urine sample was provided and analyzed for osmolality. Players were required to towel dry, to remove any sweat before body mass assessment, wearing underwear, determined to the nearest 100 g using calibrated digital scales (Seca 700, 1321008; Hamburg, Germany). Fingertip blood samples were taken for the analysis of [Na+] while players assumed a semirecumbent position for 5 minutes. Absorbent sweat patches (Tagaderm + Pad, 3M; Loughborough, United Kingdom) were applied to the forearm, scapula, chest, and thigh (20).

Individually labeled, preweighed commercially available sports drinks (≈500 ml) and preweighed drinks bottles (≈1,000 ml) containing water were provided for ad libitum consumption during training and match-play (i.e., prematch and at halftime). The fluid available during actual match-play was water. The commercially available sports drink was Na+ free and the choice of the coaching staff not researchers. For match observations, all fluid containers were labeled with prematch (consumed between prematch body mass assessment and kickoff), halftime, and during match-play, which indicated when that specific fluid should be consumed ad libitum. This was not required for training as players used their individual fluid containers for the duration of training. A researcher acted as the water carrier during match-play to ensure compliance with individual bottles. Players were advised not to spit out any fluid or use their fluid for anything other than fluid intake, which was adhered to.

A preweighed individually labeled jerry can and a Shewee (Shewee Ltd., Tadworth, United Kingdom) was provided after body mass assessment, to collect any urine output at any time (aggregated for prematch and during halftime for match-play and for the duration of the training session) to allow calculations of sweat loss. No food was consumed between body mass assessments.

After match-play and training, before any food or fluid intake, sweat patches were removed and players were encouraged to provide a urine sample to ensure an empty bladder before postexercise body mass determination. After this, a fingertip blood sample was taken in the same manner as preexercise. Percentage body mass change was calculated from the arrival and postexercise body mass assessments.

All preweighed fluid containers were reweighed postexercise through triplicate analysis independently, using bench top scales (resolution 0.001 kg, CS-2000; Ohaus Corp, Parsippany, NJ, USA). Differences in mass from pre–match-play to post–match-play or training determined fluid intake and urine output. Sweat loss was calculated (body mass before − body mass after + fluid intake − urine output), although it is acknowledged, other mass determinants exist (13).

Urine samples were analyzed for osmolality using a calibrated freezing point osmometer (Gonotec Osmomat 030-D, 040906, Berlin, Germany), which had a coefficient of variation (CV) of 1.4, 1.3, 0.6%, determined against stock solutions of 100, 300, and 850 mOsmol·kg−1, respectively. Urine osmolality was interpreted as euhydration (≤700 mOsmol·kg−1), moderate hypohydration (701–899 mOsmol·kg−1), and severe hypohydration (≥900 mOsmol·kg−1) (22).

Sweat patches worn during exercise were analyzed for [Na+] as previously described (11), using a flame photometer (PFP7 Flame Photometer; Jenway, Essex, United Kingdom). Data were adjusted by 35% to account for the ∼30–40% overestimation of sweat (electrolyte) using the closed-pouch method (19).

Fingertip blood samples were analyzed immediately with a GEM Premier TM 4000 containing a 30-day disposable cartridge (GEM Premier 4000 PAK; Lexington, MA, USA) to determine blood [Na+]. Within-day CV is reported as 0.5% and between-day CV is reported as 0.5% (2). Blood [Na+] was interpreted as hyponatremia (<135 mmol·L−1), normonatremia (135–145 mmol·L−1), and hypernatremia (>145 mmol·L−1) (11).

Ambient temperature and relative humidity were measured using a digital weather station (Oregon Scientific, Buckinghamshire, United Kingdom). The weather station was placed out of direct sunlight, temperature and relative humidity were recorded 60 minutes before kickoff, at kickoff, halftime, and postmatch for match-play, with the mean reported. The temperature was measured at the start and end of training, again the mean was reported.

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Statistical Analyses

Data are presented as mean ± SD. Preliminary analyses were conducted to check for normality with Kolmogorov-Smirnov tests performed on the data set to check for normality. A repeated-measures analysis of variance (ANOVA) was used to determine any significant differences between match-play and mean training data for urine osmolality, fluid intake, sweat loss, sweat loss rate, percentage body mass change, blood [Na+] change, and sweat [Na+]. A 2-way (activity [match-play vs. training] and time [pre vs. post]) repeated-measures ANOVA was used to determine any significant differences for blood [Na+]. A repeated-measures ANOVA was used to determine differences between fluid intake time points (i.e., prematch, first-half, halftime, and second-half) with Bonferroni corrections for multiple comparisons. Cohen's d effect sizes with 95% confidence intervals (CI) were calculated and interpreted using a modified effect size scale of 0–0.2 considered to be a trivial effect, 0.2–0.6 a small effect, 0.6–1.2 a moderate effect, 1.2–2.0 a large effect, and >2.0 a very large effect (7). Where the CI crossed 0, the effect was interpreted as unclear. Pearson product-moment correlations were calculated to determine correlations for pre, post, and Δblood [Na+] against percentage body mass change, total fluid intake, sweat loss, and arrival urine osmolality. Thresholds for correlations were interpreted as <0.1 (trivial), 0.1–0.3 (small), 0.3–0.5 (moderate), 0.5–0.7 (large), 0.7–0.9 (very large), and >0.9 (extremely large) (8). SPSS version 20.0 was used to conduct analysis with all statistical significance set at p ≤ 0.05.

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Results

The ambient temperature and relative humidity during match-play and training was 24° C and 44% and 17.4 ± 6.5° C, and 47 ± 4%, respectively.

There was a significant difference in arrival urine osmolality between match-play and training (382 ± 302 vs. 667 ± 260 mOsmol·kg−1, p < 0.001; moderate d = −1.01 [CI −0.04 to −1.90]). Percentage body mass change, fluid intake, urine output, and sweat loss are presented in Table 1. The difference between match-play and training were all unclear and not significant for percentage body mass change, sweat loss, urine output, and total fluid intake. Sweat loss rate was significantly greater and moderate during training than match-play (d = 0.85).

Table 1

Table 1

During match-play observations, there was a significant difference (p = 0.001) among time points for fluid intake. Fluid intake was significantly lower prematch (0.135 ± 0.169 kg) than halftime (0.441 ± 0.234 kg, p < 0.001; d = 1.50 [CI 0.45–2.42]) and during match-play (aggregated first and second-half; 0.629 ± 0.329 kg, p = 0.035; d = 1.89 [CI 0.77–2.85]).

Sweat [Na+] and blood [Na+] are presented in Table 2. There was no significant effect of activity (match-play vs. training), time (preexercise vs. postexercise) or activity *time on blood [Na+]. The difference between match-play and training for sweat [Na+] and Δblood [Na+] were all unclear and not significant.

Table 2

Table 2

The relationship between blood [Na+] and fluid balance measures are presented in Table 3 of which no significant relationships were observed. Small relationships were observed for preexercise blood [Na+] against arrival urine osmolality and fluid intake, postexercise blood [Na+] against arrival urine osmolality, and change in blood [Na+] against fluid intake. All other relationships were trivial.

Table 3

Table 3

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Discussion

This study is the first to explore habitual hydration status on arrival, sweat loss, fluid intake, sweat [Na+], and Δblood [Na+] in female rugby league players. The study shows that on average (based on urine osmolality) players habitually arrived euhydrated to match-play and training. During match-play and training, players experienced a modest change in body mass (due to a lower fluid intake in comparison to sweat loss), and blood [Na+] seemed well regulated preexercise to postexercise, despite losses of Na+ in sweat and electrolyte-free fluid consumption.

Hydration status before match-play seems comparable with their male rugby league counterparts (396 ± 252 mOsmol·kg−1; (17)). Despite a significant difference for mean arrival urine osmolality between match-play and training, this does not precisely indicate that players were either more or less hydrated per se, as values were indicative of euhydration (<700 mOsmol·kg−1; (22)). Practitioners should be aware that a more dilute urine sample is not indicative of being more hydrated. The precision of urine osmolality to accurately determine hydration status, when fluid is available, is also questionable (18); however, an alternative practical field measure does not exist.

This study provided sweat loss data for female rugby league players, which was less than male rugby league players (reported as 32.64 ± 13.0 ml·min−1 (17); calculated as 1.958 kg·h−1). This may be due to differences in exercise intensity, although to the author's knowledge, no data are available to support this. Alternatively, the findings of this study supports previous observations demonstrating larger sweat losses in males than females, although when expressed relative to body mass, the magnitude of difference between female (in this study) and male rugby league players is small (0.29 ± 0.09 vs. 0.34 ± 0.10 ml·min−1·kg−1; (17)). Another confounding variable when making comparisons between the findings of this study and that by O'Hara et al., (17) is environmental temperature (12.1 ± 5.3° C and 70.5 ± 11.4% relative humidity), which was greater in this study. It would be anticipated that if environmental conditions were the same, a greater magnitude of difference would be observed between male and female rugby league players due to the association of sweat rates and environmental conditions.

On average, during match-play, female rugby league players consumed less fluid than male rugby league players (1.56 ± 0.57 L; (17)), which seems to reconcile with sweat loss data, (i.e., if players sweat less they need to consume less fluid). The body mass change during match-play was modest in comparison with male rugby league players (−1.32% (17)) and similar to female soccer players (12). At no point during this study, did body mass loss exceed 2% for any player. To the authors knowledge, no study has shown that a body mass loss of <2% has been associated with a decrease in exercise performance, whereas a loss >2% has (3). Observations of this cohort suggest that severe dehydration per se during match-play or training does not seem to be of concern for international female rugby league players, when habitual hydration strategies are adopted.

Mean sweat [Na+] was similar between match-play and training, and also similar to data reported for international female soccer players during 2 training sessions (43.9 ± 13.8 and 46.2 ± 7.9 mmol·L−1; (12)). Despite female rugby league players losing Na+ in sweat and consuming Na+ free sports drink or water, mean blood [Na+] was well regulated, thus it would be assumed that overall Na+ supplementation would not be required. The small, trivial negative correlation between change in blood [Na+] against total fluid intake and sweat loss would also suggest that neither the ingestion of electrolyte-free solution nor sweat loss directly disturbed tightly regulated blood [Na+] in this cohort.

The findings of this study show that sweat loss for international female rugby league players during match-play and training is variable, ranging from 0.988–2.448 kg to 0.002–3.014 kg, respectively. Despite the large observed range in sweat loss, the limited change in body mass (ranging from −1.3 to 0.0% for match-play and −1.7 to 0.6% for training) is due to the habitual adjustments in fluid intake (0.502–1.801 kg for match-play and 0.064–3.786 kg for training). The findings of this study suggest that fluid intake during match-play and training was sufficient to offset sweat loss, without intervention or prescription, thus during exercise, thirst may be the most suitable individualizing hydration strategy. It should be acknowledged that some interplayer variability may be due to the fact the phase of menstrual cycle was not acknowledged or recorded. During the luteal phase of the menstrual cycle, females have a higher rate of water turnover (4). Contrary, it has previously been reported that this may have a minimal effect on fluid balance per se (23) or renal water and electrolyte retention with fluid replacement after exercise (14).

In conclusion, international female rugby league players seem to habitually arrive adequately hydrated to match-play and training, and consume adequate fluid in comparison with their sweat loss, causing a modest change in body mass, less than previously reported in male rugby league players (17). Mean blood [Na+] seems to be well regulated despite losses of Na+ in sweat and electrolyte-free fluid consumption. For the duration of the study, players did not experience a body mass loss (dehydration >2%) indicative of a reduction in exercise performance.

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Practical Applications

The findings of the study show that overall female rugby league players arrive to training and match-play hydrated. Furthermore, they remained hydrated during training and match-play, despite varying sweat loss rates. Therefore, strength and conditioning coaches may not need to intervene above the habitual hydration strategies when working with female rugby players. The consumption of only water (with the absence of sodium; i.e., sports drink) is sufficient at maintaining blood [Na+], thus players do not seem to be at risk of developing hyponatremia or hypernatremia. Practitioners should first evaluate the individual habitual hydration status of an athlete to determine whether interventions above habitual strategies are warranted. This study showed that overall habitual hydration strategies were adequate, although one player arrived hypohydrated (urine osmolality >900 mOsmol·kg−1) to training and match-play, whereas one player arrived to training hyponatremic (blood sodium <135 mmol·L−1), which may suggest hyperhydration. At no point during exercise, did body mass loss exceed 2% nor was hyponatremia or hypernatremia observed post exercise. As such, in practice, the hydration strategies of players should be determined on a case-by-case basis, with an emphasis on their arrival state. Furthermore, the findings of this study should be applied to mild environmental conditions, as training or matches played in more challenging environmental conditioning (i.e., >30° C) may require specific intervention.

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Acknowledgments

The authors would like to thank for cooperation of the players, coaching, medical, and administrative staff at the Rugby Football League.

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Keywords:

electrolyte; nutrition; performance

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