Hydration Status, Fluid Intake, Sweat Rate, and Sweat Sodium Concentration in Recreational Tropical Native Runners

im: The purpose of this study was to evaluate hydration status, uid intake, sweat rate, and sweat sodium concentration in recreational tropical native runners. Methods: A total of 102

Loo Lin School of Medicine, National University of Singapore, Singapore 117456, Singapore 10 Singapore Institute for Clinical Sciences, Agency for Science, Technology and Research (A*STAR), Singapore 117609, Singapore *Correspondence: juthamard.sur@mahidol.edu Abstract: Aim: The purpose of this study was to evaluate hydration status, uid intake, sweat rate, and sweat sodium concentration in recreational tropical native runners. Methods: A total of 102 males and 64 females participated in this study. Participants ran at their self-selected pace for 30–100 min. Age, environmental conditions, running pro les, sweat rates, and sweat sodium data were recorded. Differences in age, running duration, distance and pace, and physiological changes between sexes were analysed. Ap-value cut-off of 0.05 depicted statistical signi cance. Results: Males had lower relative uid intake (6 6 vs. 8 7 mL kg 1 h 1 ,p< 0.05) and greater relative uid balance de cit ( 13 8 mL kg 1 h 1 vs. 8 7 mL kg 1 h 1 ,p< 0.05) than females. Males had higher whole-body sweat rates (1.3 0.5 L h 1 vs. 0.9 0.3 L h 1 ,p< 0.05) than females. Mean rates of sweat sodium loss (54 27 vs. 39 22 mmol h 1 ) were higher in males than females (p< 0.05). Conclusions: The sweat pro le and composition in tropical native runners are similar to reported values in the literature. The current uid replacement guidelines pertaining to volume and electrolyte replacement are applicable to tropical native runners. Keywords: recreational running; tropical climate; sweat electrolyte; uid replacement; hydration plan 1. Introduction Sweat evaporation is important for the dissipation of metabolic heat production, which may increase ten- to twenty-fold during exercise [1]. In hot environments, evaporative sweat cooling is the main avenue of heat loss, preventing rapid rises of core body tempera- ture [2–5]. Hypohydration, experienced as a result of sweat loss, increases physiological strain and perception of effort, which can decrease endurance exercise performance [6,7]. In addition, sweat loss during exercise can also result in electrolyte imbalance such as hyponatremia. Thus, it is important to replace electrolyte losses as part of the rehydration process after exercise [3,8,9]. Nutrients2021,13,

core body tempera- ture [2–5]. Hypohydration, experienced as a result of sweat loss, increases physiological strain and perception of effort, which can decrease endurance exercise performance [6,7]. In addition, sweat loss during exercise can also result in electrolyte imbalance such as hyponatremia. Thus, it is important to replace electrolyte losses as part of the rehydration process after exercise [3,8,9]. Nutrients2021,13, 1374.

Nutrients2021,13, 1374 2 of 12 Running is a common form of exercise as it can be easily performed and does not require any specialised equipment. There are an estimated ve to eight million individuals participating in running events globally. A 50% increase in participation in running has been tracked over the last decade. This growth has, in part, been driven by increased participation in Asia [10]. In tropical warm, high humidity environments, the evaporation of sweat may be compromised, leading to lower rates of body heat dissipation [11]. Thus, it is reasonable to suggest that appropriate hydration may be even more important for endurance running when in tropical Asian countries. An athlete's sweat rate and sweat electrolyte concentration vary depending on indi- vidual characteristics, type and intensity of exercise, clothing, equipment worn as well as environmental conditions [12–20]. Therefore, the assessment of individual sweat rate and sweat electrolytes losses for speci c exercise and environmental conditions is rec- ommended to create individualised hydration strategies. This approach may reduce the risk of heat illness and optimise performance [21]. Despite the individuality in the re- sponse to dehydration, current guidelines advise limiting uid de cits to no more than 2% body mass loss during exercise to avoid compromised cognitive function and aero- bic exercise performances [21]. Further decrements in performance are associated with increasing levels of hypohydration (3–10%), particularly in hot weather typical in tropical climates [12,22]. Individualised drinking plans may also reduce the risk of over-drinking and exercise-associated hyponatremia [23–28]. Tropical natives are likely to be more heat-acclimatised than athletes who live in temperate or cool environments. Heat-acclimatised individuals have thermoregulatory adaptations such as lowered core body temperature, lowered heart rate, earlier onset of sweating and higher sweat rate [29–31]. Although variable, mean sweat sodium con- centrations ([Na + ]) are reported to be approximately 50 mmol L 1 [32]. To the authors' knowledge, there are limited data on sweat rate and sweat composition of tropical native athletes, which may impact on hydration strategies during and after exercise in this popu- lation. Knowledge of sweat responses and sweat composition of tropical

variable, mean sweat sodium con- centrations ([Na + ]) are reported to be approximately 50 mmol L 1 [32]. To the authors' knowledge, there are limited data on sweat rate and sweat composition of tropical native athletes, which may impact on hydration strategies during and after exercise in this popu- lation. Knowledge of sweat responses and sweat composition of tropical native athletes will allow us to understand if consensus recommendations on hydration are also relevant to heat-acclimatised athletes. Therefore, the purpose of the present study was to evaluate the hydration status, uid intake, sweat rate, and sweat [Na + ] in recreational tropical native runners. 2. Materials and Methods 2.1. Study Design and Participants This study adopted an observational cohort study design. A total of 102 males and 64 females were recruited to participate in this study. All measurements were made on a single day of practice sessions in various running groups. Ethics approval was obtained from the Centre of Ethical Reinforcement for Human Research, Mahidol University (MU- CIRB 2018/208.2311 and protocol no. 2018/198.0910). All participants provided written informed consent before participation. 2.2. Experimental Protocol Resting heart rate, blood pressure and aural temperature were measured before and after running. Heart rate and blood pressure were measured using an upper arm blood pressure monitor (BM 28, Beurer GmbH, Ulm, Germany). Aural temperature was mea- sured using an ear thermometer (FT 78, Beurer GmbH, Ulm, Germany). Participants with high resting blood pressure and/or high resting aural temperature (systolic blood pres- sure>180 mmHgand/or diastolic blood pressure >110 mmHg and/or aural temperature >38 C) were excluded from the study. Six running sessions were completed on separate days. Each session involved a warm-up of 10–15 min, followed by 30–70 min of running, and ended with 10–15 min of cool-down. Participants ran at their individual pace, with most participants running at a light to moderate intensity. Two running sessions were conducted in the morning,

Nutrients2021,13, 1374 3 of 12 between 6 a.m. and 8 a.m., while four running sessions were conducted in the evening, between 5 p.m. and 8 p.m. The rst ve running sessions were conducted at Lumphini Park, Bangkok, Thailand while the sixth running session was conducted in a park within Mahidol University, Nakhon Pathom, Thailand. All six sessions were held in a public park with a 2.5 km running track. A water station was provided. Runners consumed plain water (Aqua na, PepsiCo, Harrison, NY, USA)ad libitumfrom individual water bottles. Water bottles were weighed before and after each running session to record the volume of uid intake during running. Before each running session, all participants voided their bladders. Mid-stream urine samples were collected to measure urine speci c gravity (USG). Pre-exercise body mass was measured using a bench scale (N.V. Mettler-Toledo S.A., Zaventem, Belgium) while minimally clothed (T-shirts, shorts or tights, and without shoes), and recorded to the nearest 0.10 kg. For participants who needed to urinate during the run, body mass was measured before and after the excretion to estimate urine output. To collect sweat, the right or left forearm was cleaned with an alcohol pad (3M, Minneapolis, MN, USA), rinsed with distilled water, and dried with electrolyte-free gauze. An absorbent patch (9 cm 10 cm) (3M™Tegaderm™+ Pad Film Dressing with Non-Adherent Pad, 3M, Minneapolis, MN, USA) was then applied to the mid-forearm [33]. After each running session, the participants towel-dried themselves and post-exercise body mass was measured. The same bench scale was used and participants wore the same attire as during the pre-exercise body mass assessment. The absorbent patch was then removed from the forearm, placed in the barrel of a plastic syringe using clean forceps and squeezed with a plunger to collect sweat. Sweat samples were analysed for sweat [Na + ] and sweat potassium concentration ([K + ]). 2.3. Measurement of Environmental Conditions Ambient temperature and relative humidity were measured and recorded at 10-min intervals using a data logger (QUESTemp 34, 3M, Minneapolis, MN, USA) during each running session, and the mean value was calculated. 2.4. Urine Speci

plunger to collect sweat. Sweat samples were analysed for sweat [Na + ] and sweat potassium concentration ([K + ]). 2.3. Measurement of Environmental Conditions Ambient temperature and relative humidity were measured and recorded at 10-min intervals using a data logger (QUESTemp 34, 3M, Minneapolis, MN, USA) during each running session, and the mean value was calculated. 2.4. Urine Speci c Gravity (USG) USG from mid-stream urine samples were measured using a hand-held refractometer (PAL-10S, ATAGO ® , Saitama, Japan). USG was assessed in duplicates and the average value was used for recording. USG was used an indicator of hydration status, with USG >1.020 indicating hypohydration and USG >1.030 indicating severe hypohydration [34]. 2.5. Whole-Body Sweat Loss (WBSL) and Whole-Body Sweat Rate (WBSR) WBSL and WBSR were calculated using Equations (1) and (2) respectively: WBSL (L) = (Pre-exercise body mass (kg) Post-exercise body mass (kg)) + Fluid intake (L) Urine output (L), (1) WBSR (L h 1 ) = WBSL (L)/Exercise duration (h). (2) 2.6. Whole-Body Sweat Sodium Concentration Sweat [Na + ] and sweat [K + ] were analysed via ion-selective electrode (ISE) technology using Na + (LAQUAtwin Na-11, HORIBA Advanced Techno Co., Ltd., Kyoto, Japan) and K + analysers (LAQUAtwin K-11, HORIBA Advanced Techno Co., Ltd., Kyoto, Japan). Whole- body sweat [Na + ] (mmol L 1 ) and whole-body sweat Na + loss (mmol) were calculated using Equations (3) and (4) respectively [33]: Predicted whole-body sweat [Na + ] (mmol L 1 ) = 0.57 (forearm sweat Na + ) + 11.05, (3) Whole-body sweat Na + loss (mmol) = WBSL (L) Predicted whole-body sweat [Na + ] (mmol L 1 ). (4)

Nutrients2021,13, 1374 4 of 12 Sweat [Na + ] were classi ed into three groups: low ([Na + ] <30 mmol L 1 ), mod- erate ([Na + ] = 30–60 mmol L 1 ), and high ([Na + ] >60 mmol L 1 ) [35,36]. Addition- ally,12 sampleswere randomly selected and analysed using the gold standard high- performance liquid chromatography (HPLC) method (Dionex ICS-5000, Thermo Fisher Scienti c, Inc., Waltham, MA, USA). 2.7. Statistical Analysis Statistical analysis was conducted using IBM SPSS Statistics version 19.0 (IBM, Ar- monk, NY, USA). Descriptive data were generally expressed as mean standard deviation (SD). All biochemical data were log-transformed to reduce non-uniformity of error. The data were back transformed before being expressed as parametric mean SD. Normality was determined using the Shapiro–Wilk test. Differences in age, running characteris- tics, body mass loss, uid intake, and net uid balance between males and female run- ners were analysed using independentt-test. Pearson correlation coef cient was used to analyse the correlation between parameters, including the correlation of whole-body sweat [Na + ] between ISE and HPLC methods. Correlation coef cients were interpreted based on the following thresholds:r 0.35 = weak, 0.36 r 0.67 = moderate, and 0.68 r 1.0 = strong [37]. For all analyses, ap-value of < 0.05 was considered signi cant. 3. Results 3.1. Number of Subjects and Environmental Conditions for Each Running Session There were 166 participants in total (102 males and 64 females). The number of participants for each running session is shown in Table, together with the mean am- bient temperature and mean relative humidity. The mean (range) ambient tempera- ture and relative humidity across the six running sessions were 29.6 (28.0–31.5) C and 70 (55–87)% respectively. Table 1.Time of day, time, environmental conditions and number of participants during each running session. Session Time of Day Time Mean Ambient Temperature ( C) Mean Relative Humidity (%) Participants Male Female 1 Morning 7.00 a.m. to 8.00 a.m. 28.5 75 9 6 2 Morning 6.00 a.m. to 8.00 a.m. 30 86 10 4 3 Evening 7.00 p.m. to 8.00 p.m. 29.5 63 16

time, environmental conditions and number of participants during each running session. Session Time of Day Time Mean Ambient Temperature ( C) Mean Relative Humidity (%) Participants Male Female 1 Morning 7.00 a.m. to 8.00 a.m. 28.5 75 9 6 2 Morning 6.00 a.m. to 8.00 a.m. 30 86 10 4 3 Evening 7.00 p.m. to 8.00 p.m. 29.5 63 16 8 4 Evening 7.00 p.m. to 8.00 p.m. 28 87 31 13 5 Evening 7.00 p.m. to 8.00 p.m. 29.8 56 12 12 6 Evening 5.00 p.m. to 6.00 p.m. 31.5 55 24 21 3.2. Participants' Age, Running Pro le, and Body Mass Change across Running Participants were aged between 21–68 years with running experience ranging from 6 monthsto more than 10 years. All runners were native to Thailand and had been within the country for 6 months, exercising in hot and humid environments, prior to the trial. The running durations of all participants during the running sessions were between 30–100 min. Male runners ran a further mean distance and at a faster mean pace than female runners (p< 0.05) (Table). However, the mean running duration did not differ between sexes. Both mean WBSL and mean WBSR among the male runners were greater than among the female runners (p< 0.05) (Table). While six male and four female runners had >2% body mass loss after the running session, percentage body mass loss did not differ between sexes (p> 0.05) (Table).

Nutrients2021,13, 1374 5 of 12 Table 2. Mean age, running pro le, body mass change, sweat rate, and uid intake of male and female runners across all running sessions. Data are presented as mean SD (range). Male (n= 102) Female ( n= 64) Age (years) 36 9 (21–68) 34 9 (22–62) Running duration (min) 43.7 14.8 (33–97) 43.6 13.6 (43–100) Running distance (km) 6.4 1.1 (2.5–12.5) 5.3 1.1 * (2.5–10) Running pace (min km 1 ) 6.8 3.7 (3.5–10.0) 8.2 3.9 * (4.3–11.0) Pre-running body mass (kg) 70.8 10.6 (46.9–99.7) 56.7 9.7 * (42.3–93.2) Post-running body mass (kg) 70.2 10.6 (46.6–99.0) 56.4 9.6 * (42.3–92.8) Percentage body mass loss (%) 1.3 0.5 (0.2–3.6) 1.2 0.5 (0.1–3.7) Whole-body sweat loss (WBSL) (L) 0.9 0.3 (0.2–2.6) 0.6 0.3 * (0.1–1.9) Whole-body sweat rate (WBSR) (L h 1 ) 1.3 0.5 (0.2–3.8) 0.9 0.3 * (0.1–2.2) Fluid intake (L) 0.3 0.3 (0–1.1) 0.3 0.2 (0–1.1) *p< 0.05, compared to male participants. 3.3. Urine Speci c Gravity USG data were absent from 18 male and four female runners due to insuf cient urine samples. Mean USG of runners who ran in the morning and evening were1.015 0.008 and 1.013 0.007, respectively. There was no difference between the mean USG of runners from the morning and evening sessions (p> 0.05). The number of hypohydrated participants (USG > 1.020) did not differ between the morning or evening running session (p> 0.05). However, a greater percentage of participants were hypohydrated (USG > 1.020) before the run when the running session was conducted in the morning (28%) than in the evening (15%) (Table). A greater number of runners were severely hypohydrated (USG > 1.030) before the run when the session was conducted in the morning (4%) than in the evening (1%). In addition, when the running session was conducted in the morning, there is a moderate positive correlation between pre-exercise USG and uid intake (p< 0.05,r= 0.42), and a moderate negative correlation between pre-exercise USG of male runners and WBSL (p< 0.05,r= 0.54). Table 3. Level of dehydration between sexes (male vs. female) and time of day of

the evening (1%). In addition, when the running session was conducted in the morning, there is a moderate positive correlation between pre-exercise USG and uid intake (p< 0.05,r= 0.42), and a moderate negative correlation between pre-exercise USG of male runners and WBSL (p< 0.05,r= 0.54). Table 3. Level of dehydration between sexes (male vs. female) and time of day of running session (morning vs. evening) based on pre-exercise urine speci c gravity (USG). Time of Day of Session Sex Urine Speci c Gravity (USG) 1.020 >1.020 >1.030 Morning Male 11 (69%) 4 (31%) 1 (6%) Female 7 (78%) 2 (22%) 0 (0%) Total 18 (72%) 6 (28%) 1 (4%) Evening Male 59 (85%) 10 (15%) 1 (1%) Female 43 (84%) 8 (16%) 0 (0%) Total 102 (85%) 18 (15%) 1 (1%) 3.4. Relative Sweat Loss and Fluid Intake during Running Mean WBSR during recreational running was higher in males than females (p< 0.05) (Table). Sweat loss relative to body mass was calculated and differences between sexes were compared. Males had higher sweat loss relative to body mass than female runners

Nutrients2021,13, 1374 6 of 12 (19 8 vs. 16 6 mL kg 1 h 1 ,p< 0.05) (Figure). With regards to uid intake during running, 15 of the 102 male runners (14.7%) did not drink any water during the run. However, only two of the 64 female runners (3.1%) did not drink any water during the run. Meanad libitum uid intake during running in males and females did not differ between sexes (p> 0.05) (Table). However, male runners had a lower uid intake relative to body mass than female runners (6 6 vs. 8 7 mL kg 1 h 1 ,p< 0.05) (Figure). Therefore, males had a greater negative uid balance relative to body mass than female runners ( 13 8 mL kg 1 h 1 or 67.4% vs. 8 7 mL kg 1 h 1 or 48.9%,p< 0.05) (Figure) . Furthermore, running pace and sweat rate were found to have a moderate negative correlation (r= 0.47,p< 0.01).Nutrients 2021, 13, x FOR PEER REVIEW 6 of 12 3.4. Relative Sweat Loss and Fluid Intake during Running Mean WBSR during recreational running was higher in males than females (p < 0.05) (Table 2). Sweat loss relative to body mass was calculated and differences between sexes were compared. Males had higher sweat loss relative to body mass than female runners (19 ± 8 vs. 16 ± 6 mL·kg −1 ·h −1 , p < 0.05) (Figure 1). With regards to fluid intake during run- ning, 15 of the 102 male runners (14.7%) did not drink any water during the run. However, only two of the 64 female runners (3.1%) did not drink any water during the run. Mean ad libitum fluid intake during running in males and females did not differ between sexes (p > 0.05) (Table 2). However, male runners had a lower fluid intake relative to body mass than female runners (6 ± 6 vs. 8 ± 7 mL·kg −1 ·h −1 , p < 0.05) (Figure 1). Therefore, males had a greater negative fluid balance relative to body mass than female runners (−13 ± 8 mL·kg −1 ·h −1

This study evaluates hydration and sweat characteristics in tropical native runners.