Voluntary drinking and hydration in trained, heat-acclimatized girls exercising in a hot and humid climate

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1 Eur J Appl Physiol (2008) 103: DOI /s ORIGINAL ARTICLE Voluntary drinking and hydration in trained, heat-acclimatized girls exercising in a hot and humid climate Anita M. Rivera-Brown Farah A. Ramírez-Marrero Boguslaw Wilk Oded Bar-Or Accepted: 11 January 2008 / Published online: 5 February 2008 Springer-Verlag 2008 Abstract This study examined the evects of beverage composition on the voluntary drinking pattern, body Xuid balance and body temperature responses of heat-acclimatized trained girls exercising intermittently in outdoor conditions (WBGT = C). Twelve trained, heatacclimatized girls (age = years) performed three 3-h sessions, each consisting of four 20-min cycling bouts at 60% VO 2 max, alternating with 25-min rest. One of three beverages was assigned: unxavored water (W), Xavored water () or Xavored water plus 6% carbohydrate and 18 mmol/l NaCl (). Drinking was ad libitum. Total intake was similar among conditions (W = ; = ; = g). A mild hypohydration occurred during the three conditions (W = 1.12%; = 0.95%; = 0.74% BW, P > 0.05). Sweat loss, higher than previously reported for sedentary girls, was not diverent among conditions (W = 1, ; = ; = 1, g). The average amount of urine produced (W = ; = ; = g) was 73 and 68% higher during and W, respectively, compared to A. M. Rivera-Brown (&) Center for Sports Health and Exercise Sciences at the Albergue Olímpico, Department of Physical Medicine, Rehabilitation and Sports Medicine, University of Puerto Rico School of Medicine, P.O. Box 2004, Salinas, Puerto Rico aniriver@coqui.net F. A. Ramírez-Marrero Department of Physical Education and Recreation, University of Puerto Rico, San Juan, Puerto Rico B. Wilk O. Bar-Or Children s Exercise and Nutrition Centre, McMaster University, Hamilton, ON, Canada ( vs., P < 0.05). The increase in rectal temperature, heart rate and all perceptual variables did not diver among conditions. In conclusion, Xavoring of the water and addition of 6% carbohydrate plus 18 mmol/l NaCl do not prevent mild hypohydration in trained, heat-acclimatized girls with high sweating rates. However, there is a tendency towards a greater Xuid retention with the beverage. Keywords Children Sweating Voluntary dehydration Body temperature Tropical Introduction Children, similar to adults, voluntarily do not drink enough to replenish Xuid losses when overed only water during prolonged exercise in hot environments and show a progressive dehydration (Bar-Or et al. 1980; Rivera-Brown et al.1999; Rodríguez Santana et al. 1995; Wilk and Bar-Or 1996). Following prolonged intermittent exercise in hot and humid environments, hypohydration levels of 1 2% have been reported in both unacclimatized sedentary boys (Bar- Or et al. 1980; Wilk and Bar-Or 1996) and in trained, heatacclimatized boys with high sweating rates (Rivera-Brown et al. 1999) when water drinking was ad libitum. Impairment in aerobic performance (Wilk et al. 2002) and a greater increase in core temperature compared to adults (Bar-Or et al. 1980) have been observed in children at these mild hypohydration levels, thus underscoring the importance of optimal hydration during exercise. The current Xuid replacement guidelines for children during exercise in the heat (American Academy of Pediatrics 2000) state that a child who weighs 40 kg should drink 150 ml of cold tap water or Xavored salted beverage every 20 min and an adolescent who weighs 60 kg should drink 250 ml. We

2 110 Eur J Appl Physiol (2008) 103: have shown that heat-acclimatized, trained boys (Rivera- Brown et al. 1999) demonstrate a considerably higher sweating rate than non-acclimatized sedentary boys (Wilk and Bar-Or 1996) and therefore need to drink more Xuid than that recommended for their sedentary counterparts in order to remain well hydrated during exercise. A major limitation in literature is a paucity of data regarding the Xuid losses and hydration patterns of girls who participate in outdoor sports such as soccer, long distance running, and tennis in tropical environments. This population may be at an increased risk for dehydration because they may have high sweating rates as a result of repeated exposure to the combined stress of exercise and environmental heat and humidity. They may also be at risk if they train and compete frequently while exposed to high levels of heat stress and do not drink suycient amount of Xuid due to inadequate guidelines, insuycient Xuid breaks and/or unavailability of Xuids in the training sessions. One strategy to increase Xuid intake and help maintain body hydration during exercise in boys has been the addition of Xavor, carbohydrate and sodium chloride (NaCl) to the hydrating beverage (Rivera-Brown et al. 1999; Wilk and Bar-Or 1996). Wilk and Bar Or (1996) showed that voluntary Xuid intake increased by 45% when Xavor was added, and with further addition of carbohydrate and NaCl the boys drank almost twice as much, compared to unxavored water. We conwrmed this Wnding by showing that a Xavored beverage containing carbohydrate and NaCl increased voluntary drink intake by 32% (compared to unxavored water) in trained, heat-acclimatized boys who experienced much higher sweating rates during prolonged exercise (Rivera-Brown et al. 1999). In contrast, a recent study by Wilk et al. (2007) showed that beverage Xavoring, but not the addition of NaCl, enhanced voluntary drinking in non-acclimatized girls exercising in the heat and that even thirst-guided drinking of plain water was suycient to maintain euhydration in these girls. It is not known if beverage Xavoring and addition of carbohydrate plus NaCl will increase Xuid intake and prevent hypohydration in trained, heat-acclimatized girls who exhibit high sweating rates and as a result may be at risk for incurring greater dehydration, and if this has a modulating evect on thermal strain. Therefore, the purpose of this study was to examine the evects of beverage Xavor and composition on voluntary drinking pattern, body Xuid balance and body temperature responses in trained, heat-acclimatized girls, indigenous to a tropical climate, during prolonged intermittent exercise in hot and humid outdoor conditions. In these girls, a high sweating rate is benewcial because it contributes to enhanced evaporative cooling, but it can also be detrimental by increasing the probability of a negative Xuid balance. Our previous study with trained, heat-acclimatized boys (Rivera-Brown et al. 1999) showed that the ad libitum intake of a carbohydrate electrolyte drink helps prevent a negative Xuid balance during prolonged intermittent outdoor exercise in a hot and humid environment. Therefore, based on our previous Wndings in boys, we hypothesized that Xuid intake would be higher and hypohydration level would be lower with both a Xavored drink and a Xavored, carbohydrate electrolyte drink compared to unxavored water alone. Methods Subjects Twelve healthy, 9 12-year-old trained girls, residents of Puerto Rico, volunteered for this study. All of the subjects had participated in an exercise-training program for at least 6 months prior to the study. Their sports specialties were: diving (n = 3), gymnastics (n = 4), wrestling (n =1), track and Weld (n = 1), swimming (n = 2) and tae kwon do (n =1). They had been training for a mean ( SD) of min/day, days/week and for months. They were exposed to solar radiation during physical activity outdoors for min/day. Puerto Rico is a tropical island with few seasonal variations in temperature. The average temperature during the year is 28 C and the average relative humidity is 77%. Therefore, our subjects were considered to be acclimatized to hot and humid conditions by their prolonged exposure to this tropical weather. All subjects and their parents were informed of the general purpose and procedures of the study and gave written informed consent. The study was approved by the University of Puerto Rico Medical Sciences Campus Institutional Review Board. Mean ( SD) age, height, body weight, body fat% and VO 2 max of the subjects were: years, cm, kg, % and ml kg 1 min 1, respectively. Their maturational level ranged from pre-to mid-puberty (Tanner stages I III) and all were premenarcheal. Study design and procedures Familiarization and baseline data Each girl served as her own control. Subjects had three preparatory sessions and three experimental sessions. During the Wrst visit, their health status was assessed by a physician who also determined their maturational stage from pubic hair growth according to Tanner (Tanner 1962). During this visit, age, height and weight were recorded and skinfold thickness (subscapular and triceps) were measured for the assessment of body fat%. Maximal aerobic power (VO 2 max) was determined with a metabolic cart (AMETEK, Applied Electrochemistry, model S-3A oxygen analyzer and model CD-3A

3 Eur J Appl Physiol (2008) 103: carbon dioxide analyzer) during a continuous exercise test to volitional exhaustion on an electromechanical cycle ergometer (Monark, model 829E) using the McMaster protocol (Bar-Or and Rowland 2004). The power load required to achieve 60% of VO 2 max was identiwed for each subject, for later use in the heat exposure sessions. During a second visit, the subjects completed a 20-min submaximal exercise bout outdoors pedaling at 60% VO 2 max. Oxygen consumption was measured in order to ensure that the load previously chosen was the correct intensity for the experimental sessions. A second purpose of this exercise session was to habituate the girls to the experimental procedures and to identify any with abnormal responses to exercise in hot and humid conditions. Tasting protocol During a third visit to the laboratory, a tasting session was conducted to determine drink Xavor preference of each child. This individualized Xavor was used during subsequent and experimental sessions. This test also included ratings of the perception intensity of sweetness, saltiness and sourness, which were assessed by visual analog scales (Meyer et al. 1995). Experimental sessions The three experimental sessions were conducted outdoors at the same time of the day (10:00 am to 1:00 pm), one week apart during the summer months (May to July). The subjects drank at least two glasses ( ml) of water, milk or juice the night before and one glass of water ( ml) at least 30 min before the start of the Wrst exercise bout. This was done in order to increase the likelihood of full hydration prior to each testing session and to standardize feelings of thirst. The mean ( SD) preexercise body weights were similar among the sessions (W = ; = ; = kg) suggesting a similar hydration status before the experimental sessions. One of the following beverages was assigned to each session in a Latin Square design: unxavored water (W), Xavored water () or Xavored 6% carbohydrate and 18 mmol/l NaCl solution (), with the and having the same Xavor and with subjects and investigators blinded to the content. During each session, a bottle with the assigned cool (8 10 C) beverage was placed within arm s reach and the subjects were informed that they could drink when they wanted. The subjects were not informed of the content of the bottle and were not encouraged or reminded by the investigators to drink. They did not know that drink volume was monitored periodically. Exercise protocol In each experimental session, subjects completed four 20-min exercise bouts (60% VO 2 max) alternating with 25-min rest periods (sitting on a chair) for a total time of 180 min. They were exposed to direct sunlight during each session. During the resting periods, subjects sat on a chair exposed to the sun. This protocol is intermittent and of long duration and thus similar, although not identical, to the type of activity experienced by girls who practice for most team sports (periods of high intensity exercise followed by resting periods). The primary reason for using this protocol was that we wanted to make a comparison of the responses in these trained, heat-acclimatized girls with those of trained, heatacclimatized boys studied previously by us (Rivera-Brown et al. 1999) and with non-acclimatized untrained boys (Wilk and Bar-Or 1996) and girls (Wilk et al. 2007), studied using an identical protocol. The subjects wore shorts, bikini top and athletic shoes and a sweat-proof sunscreen when needed. Oxygen consumption was measured during min 10 of bout 2 and 4 of each session. Beverage preference during training and competition At the end of the last experimental session, the subjects completed a questionnaire designed to examine the use of sports drinks and their preference for water or sports drinks during their normal training routine and competition. They were also asked if they liked the Xavor of the beverage that they had been given. Measured and calculated variables Wet bulb, dry bulb, and black globe temperatures were measured using a heat stress monitor (Imaging and Sensing Technology, model RSS-214) during min 8, 10 and 18 of each exercise bout and during min 2, 10 and 20 of each rest period. These values were used to calculate the wet bulb globe temperature (WBGT) heat stress index as follows: (wet bulb temperature 0.7) + (black globe temperature 0.2) + (dry bulb temperature 0.1; Yaglou and Minard 1957). Heart rate (HR), rectal (T re ) and skin (T sk : thigh, back, forearm) temperatures were determined while sitting on the cycle ergometer outdoors 5 min prior to exercise, during each exercise bout (min 4, 9, 14 and 19), and during each rest period (min 5, 15 and 24) using a heart rate monitor (Vantage XL, Polar), a rectal thermistor (YSI, model 702A) inserted 8 cm past the anal sphincter, and an infrared skin thermometer (Mikron Instruments, model M101), respectively. The three T sk sites were weighted equally. All values for environmental, HR, T re and T sk temperatures were averaged to represent the total testing session. The change in T re for each session was calculated as T re at 19 min of the last exercise bout minus T re at baseline. The change in T sk for each session was taken as the arithmetic mean of T sk at 19 min of the last exercise bout minus mean T sk at baseline. Heat storage (S) was calculated using the

4 112 Eur J Appl Physiol (2008) 103: equation of Craig et al. (1954). The rating of perceived exertion was assessed by using the 6 20 Borg Scale (Borg 1972) during min 4 and 19 of each exercise period, and thirst perception (0 10; analog scale), stomach fullness (0 4; category scale) and thermal perception (0 6; category scale) were assessed pre-exercise and during the last minute of each exercise and resting period. Body weight (BW) was measured before and at the end of each exercise bout and at the end of the Wnal resting period with an electronic scale accurate to 20 g (model UMC555, Ancaster). The weight of each subject s clothes (shorts, bikini top, underwear, socks) was measured at the beginning and end of each session with an electronic scale accurate to 0.1 g (model XP 3000, Denver Instruments). Bottles containing the beverage were weighed at the beginning of each exercise bout and at the end of each resting period to calculate the cumulative drink intake. The total Xuid intake for each session was calculated and expressed as a percent of the initial body weight (% IBW). Subjects emptied their bladder before the beginning of the session. Urine was collected at the end of each session after the subject was weighed, or at any time during the session when the subject needed to urinate (interim urine). The Wnal body weight was calculated as BW at the end of each testing session minus the urine output. Sweating rate was calculated from the change in body weight plus the change in clothing weight plus total Xuid intake minus urine output and respiratory water loss. Respiratory water loss was calculated according to Mitchell et al. (1972). Total Xuid loss was calculated as sweat loss plus urinary output. Statistical analysis Data are presented as the mean and standard error of the mean (SEM) for each variable. A two-way (drink time) analysis of variance (ANOVA) with repeated measures was used to assess the evect of drink on changes in dependent variables over time. General linear model was used when missing data occurred. A one-way ANOVA was used to compare climatic conditions, total Xuid loss and Xuid intake variables and thermoregulatory responses between conditions. If signiwcant main evects were found, Tukey s critical diverence procedure was used to locate diverences among means. An alpha level of P < 0.05 was considered to be signiwcant. Table 1 Ambient heat stress components during the three experimental drinking conditions Values are means SEM WBGT wet bulb globe temperature bulb, wet bulb or black globe temperatures, the WBGT index or relative humidity among the three experimental sessions. The dry bulb temperature and relative humidity values are typical of tropical environments in the summer months. It is noteworthy that the climatic conditions were stable from one day to another as judged from the means SEM. Body Xuid balance W Dry bulb ( C) Wet bulb ( C) Black globe ( C) WBGT ( C) Relative Humidity (%) Cumulative Xuid intake is presented in Fig. 1. Drink intake was similar among conditions throughout the whole exercise session (W = ; = 1, ; = g). A progressive hypohydration occurred after min of each exercise session (Fig. 2) and reached 1.12, 0.95 and 0.74% of the initial body weight by the end of the session for W, and, respectively (P > 0.05). There was a tendency for water intake ( g ) to be lower during the resting periods when compared with ( g) and ( g), but this did not reach statistical signiwcance (W vs., P = 0.10; W vs., P =0.17). (g) Intake Drink Cumulative W Results Climatic heat stress The climatic heat stress during the experimental sessions is presented in Table 1. No diverences were found in dry Fig. 1 Cumulative drink intake of trained, heat-acclimatized girls during exercise when drinking unxavored water (W), Xavored water () and carbohydrate electrolyte solution (). Values are means SEM

5 Eur J Appl Physiol (2008) 103: (% IBW) Loss Weight Cumulative Fig. 2 Cumulative weight loss of trained, heat-acclimatized girls during exercise when drinking unxavored water (W), Xavored water () and carbohydrate electrolyte solution (). Values are means SEM Fluid losses through sweat and respiration were similar between the sessions (Table 2). Sweating rate, higher than previously reported for sedentary girls of similar age and maturational status (Wilk et al. 2007), was not diverent among conditions (W = ; F.W = ; = g m 2 min 1 ). However, the total amount of urine produced (W = ; = ; = g) was 73 and 68% lower during compared to and W, respectively, ( vs., P < 0.05, vs. W, P =0.06; Fig.3). Fluid intake was 29, 23 and 22% lower than Xuid losses for W, and, respectively. Consequently, a negative body Xuid balance was observed with the three beverages. Body temperature responses Rectal temperature increased by 0.55 C with W, 0.52 C with and 0.76 C with (Table 3). The absolute values and changes in T re, T sk and S did not diver among the sessions. There was a signiwcant time, but not a drink evect or interaction in T re responses throughout W, and sessions (Fig. 4). Mean values for T re pre-exercise ranged W Final (g) Loss Urine Cumulative 400 W Final * * * Fig. 3 Urine production of trained, heat-acclimatized girls during exercise when drinking unxavored water (W), Xavored water () and carbohydrate electrolyte solution (). Values are means SEM. *Higher than, P <0.05 from 37.4 to 37.5 C, increased by 0.5 C at the end of the Wrst exercise bout for all sessions and remained stable during the exercise periods thereafter in W and sessions. In the session, T re showed a further increase by 0.26 C after the second exercise bout and remained stable thereafter. Exercise intensity and perceptual variables The subjects exercised at an intensity equivalent to , and % of their VO 2 max for W, and, respectively. No inter-session diverences were found in the mean values of HR (W = ; = and = beats min 1 ), ratings of perceived exertion (W = ; and = ), thirst perception (W = ; and = ), stomach fullness (W = ; = and = ) and thermal perception (W = ; and = ) nor were there session time interactions for these outcome measures. Heart rate increased by 3 10 beats min 1 from the Wrst to the last exercise bouts in W, and (Fig. 5). * Table 2 Fluid losses through sweat, urine and respiration in three experimental drinking conditions Values are means SEM * Higher than, P <0.05 W Sweat loss (g) 1, , Sweat rate (g m 2 h 1 ) Fluid loss (g) 1, , , Respiratory water loss (g) Interim urine loss (g) * Final urine loss (g) * Total urine loss (g) *

6 114 Eur J Appl Physiol (2008) 103: Table 3 Body temperature responses of trained, heat-acclimatized girls during exercise in three experimental drinking conditions W Final T re ( C) ΔT re ( C) Final T sk ( C) ΔT sk ( C) S (KJ kg 1 ) Values are means SEM; T re and T sk, mean of values during the session T re rectal temperature; T sk skin temperature; S heat storage Rectal Temperature ( C) Fig. 4 Rectal temperature of trained, heat-acclimatized girls during exercise when drinking unxavored water (W), Xavored water () and carbohydrate electrolyte solution (). Values are means SEM H eart Rate (beats m in - 1 ) W W Fig. 5 Heart rate of trained, heat-acclimatized girls during exercise when drinking unxavored water (W), Xavored water () and carbohydrate electrolyte solution (). Values are means SEM Flavor preference and taste test results The preferred Xavors were: fruit punch (n =6), grape (n = 5), and orange (n = 1). The girls were not able to distinguish between and, rating the intensity of sweetness, saltiness and sourness of these beverages similarly (P > 0.05). Beverage preference during training and competition All of the subjects reported that they drank water frequently during training. Most of them (8 out of 12) indicated that they did not ingest sports drinks during training. The reason frequently given for not drinking sports drinks during training was that they were not provided to them. All of the subjects indicated that they liked the Xavored beverages that they had been given in the study. Discussion The majority of studies examining the drinking pattern and Xuid balance of children during exercise in the heat have been conducted with boys (Bar Or et al. 1980; Rivera- Brown et al. 1999; Wilk and Bar-Or 1996). There is a paucity of information regarding the body temperature responses and body Xuid balance of trained, heat-acclimatized girls exercising in natural outdoor conditions, and it is not clear if hydration guidelines developed from studies with boys exercising in hot and humid conditions are applicable to girls. Therefore, we designed this study to examine the evects of beverage Xavor and composition on voluntary drinking pattern, body Xuid balance and body temperature responses in trained, heat-acclimatized girls indigenous to a tropical climate, during prolonged intermittent exercise in hot and humid outdoor conditions. The major Wndings of the present study were that: (1) Xavoring of the water or addition of 6% carbohydrate and 18 mmol/l NaCl did not prevent mild hypohydration in girls with high sweating rates during exercise in a tropical climate; (2) there was a tendency towards a lower hypohydration with the carbohydrate electrolyte drink; and (3) urine volume was lower when the girls drank the carbohydrate electrolyte drink, and thus Xuid retention was greater. Similar to previous reports in unacclimatized boys (Wilk and Bar-Or 1996) and in heat-acclimatized boys (Rivera- Brown 1999) using a comparable exercise protocol, the girls in the present study showed a progressive hypohydration when they were overed unxavored water and drank guided by their thirst. However, we found several diverences in the hydration/dehydration patterns of the girls in the present study compared to year-old trained, heatacclimatized boys previously studied by us (Rivera-Brown

7 Eur J Appl Physiol (2008) 103: ). The girls demonstrated lower sweating rates (298 g m 2 h 1 ) than the boys (398 g m 2 h 1 ), but a higher degree of hypohydration (1.12 vs. 0.94%) when drinking unxavored water. This is probably due to the fact that although the amount of unxavored water the girls drank replaced a similar amount of the sweat loss compared to the boys (about 90% of the Xuid loss compared to 86% in boys), they produced twice as much urine ( g) as the boys ( g). Studies in collegiate athletes (Poule and Volpe 2001) and recreational adult exercisers (Stover et al ) have suggested that females maintain total body water more evectively than males and therefore start exercise in a better state of hydration, as evidenced by lower pre-exercise urine speciwc gravity measurements. Thus, it is possible that the girls in the present study may have arrived for experimental testing in a more hydrated state than the boys previously studied by us, and, consequently, the girls experienced a higher urine production when drinking unxavored water. Another diverence between girls in the present study and heat-acclimatized boys was that, while the Xavored, carbohydrate electrolyte drink elicited a 32% higher Xuid intake in the boys (more than enough to replace all the Xuid lost in sweat), the Xavored drink elicited only a 7% increase in intake and the intake of the carbohydrate electrolyte drink was lower than their Xuid losses. Thus, the addition of Xavor or carbohydrate and electrolytes to the drink did not stimulate Xuid intake above that observed with unxavored water in these trained, heat-acclimatized girls. The reasons why the carbohydrate electrolyte drink was not more evective than water in stimulating voluntary Xuid intake in these heat-acclimatized girls are not clear. Baker et al. (2005) reported a similar Wnding in older women and men exercising intermittently at a moderate intensity in the heat. In that study, men drank signiwcantly more of a carbohydrate electrolyte solution than unxavored water, while women drank equal amounts of both beverages. Furthermore, the women drank more unxavored water, relative to their body weight, than did the men. These authors concluded that the diverences in Xuid intake between men and women are most likely due to diverences in behavior; women make a greater evort to drink the Xuid that is available because they are more health and safety conscious. Our Wndings are in contrast to those of Wilk et al. (2007) who, using a similar exercise protocol to ours but a lower exercise intensity, found that unacclimatized girls with lower sweating rates (170 g m 2 h 1 ) than girls in the present study increased their Xuid intake by 24 and 38% when overed Xavored water and a carbohydrate-electrolyte drink, respectively, and maintained euhydration. However, in that study, even thirst-guided drinking of unxavored water was suycient for the girls to maintain euhydration, whereas these authors previously reported voluntary dehydration with unxavored water in non-acclimatized boys (Wilk and Bar-Or 1996). Thus, our observations of sex-based diverences in drinking behavior during exercise in trained, heatacclimatized children is consistent with the literature on untrained, unacclimatized children (Bar-Or et al. 1980; Wilk et al. 2007) and older adults (Baker et al. 2005). Social, cultural and psychological factors may contribute to ad libitum Xuid intake. Verbalis (1991) has indicated that cognitive factors, including learning mechanisms, play a role in the control of voluntary Xuid intake. It is possible that in the present study the intake of the carbohydrate electrolyte drink was not higher than that of water because of the girls habitual drinking pattern during training. While all of the subjects reported drinking water frequently during training, most of them indicated that they do not ingest sports drinks during training because they are not provided to them. Therefore, our results are in agreement with those of Baker et al. insofar as the sex-based diverences in voluntary Xuid intake during exercise in the heat are likely a result of socio-cultural diverences. However, we cannot rule out the possibility that physiological mechanisms may also contribute to the observed diverences in drinking behavior between boys and girls. The lack of an enhanced voluntary drink intake by the addition of carbohydrate and NaCl to the Xavored water might suggest that girls, compared with boys, are less sensitive to an osmotically related mechanism of thirst stimulation and Xuid intake, although this possibility will require further study. It is noteworthy that in the present study, the average urine production was lower when the girls drank the beverage that contained sodium, which is consistent with previous Wndings of a lower diuresis when drinking a beverage with sodium (Vrijens and Rehrer 1999). The tendency towards higher Xuid retention with the beverage suggests that the sodium in the beverage served as an osmotic drive to retain the ingested Xuid by reducing urine production by the kidneys. In conclusion, Xavoring of water or the addition of carbohydrate and electrolytes does not prevent voluntary dehydration in trained, heat-acclimatized girls exercising in hot and humid environmental conditions. However, there is a tendency towards a greater Xuid retention with the beverage. The level of hypohydration observed did not appear to represent a signiwcant clinical risk, as indicated by the core temperature and heart rate responses. The current hydration guidelines proposed by the American Academy of Pediatrics for exercising children (drink 150 ml every 20 min for children with a body weight of 40 kg) apply to these trained, heat-acclimatized girls during intermittent exercise, such as the one performed in this study. However, if our subjects had been exercising continuously, the total Xuid loss may have been higher and therefore the guidelines would need to be modiwed accordingly. More research is needed to identify physiological and/or

8 116 Eur J Appl Physiol (2008) 103: behavioral factors that inxuence Xuid intake and the evects of dehydration in females during actual sport training and competition. Acknowledgments The authors thank Maribel Torres, Aurea Reíllo, David Bahamundi, Reynaldo Soler, Francisco José Otero, Sandra Rodríguez, Carmen Beabraut, Dr. Enrique Amy, Dr. William Micheo and Dr. Luis Aragón for their help during testing and Dr. Brian Timmons for his critical review of the manuscript. They especially thank the girls who participated in this study and their coaches and parents for their support. This study was supported in part by the Gatorade Sports Science Institute. References American Academy of Pediatrics (2000) Policy statement: climatic heat stress and the exercising child and adolescent. Pediatrics 106: Baker LB, Munce TA, Kenney WL (2005) Sex diverences in voluntary Xuid intake by older adults during exercise. Med Sci Sports Exerc 37: Bar-Or O, Dotan R, Inbar O, Rotshtein A, Zonder H (1980) Voluntary hypohydration in year-old boys. J Appl Physiol 48: Bar-Or O, Rowland TW (2004) Pediatric exercise medicine: from physiologic principles to health-care application. Human Kinetics, IL Borg G (1972) Psychophysical bases of perceived exertion. Med Sci Sports Exerc 14: Craig FN, Garren HW, Frankel H, Blevins VW (1954) Heat load and voluntary tolerance time. J Appl Physiol 6: Meyer F, Bar-Or O, Wilk B (1995) Children s perceptual responses to ingesting drinks of diverent compositions during and following exercise in the heat. Int J Sport Nutr 5:13 24 Mitchell JW, Nadel ER, Stolwijk JA (1972) Respiratory weight losses during exercise. J Appl Physiol 32: Poule KA, Volpe SL (2001) Assessment of hydration status of collegiate athletes. Med Sci Sports Exer 33:S256 Rivera-Brown AM, Gutiérrez R, Gutiérrez JC, Frontera WR, Bar-Or O (1999) Drink composition, voluntary drinking and Xuid balance in exercising trained heat-acclimatized boys. J Appl Physiol 86:78 84 Rodríguez-Santana JR, Rivera-Brown AM, Frontera WR, Rivera MA, Mayol PM, Bar-Or O (1995) EVect of drink pattern and solar radiation on thermoregulation and Xuid balance during exercise in chronically heat-acclimatized children. Am J Hum Biol 7: Stover EA, Petrie HJ, Passe D, Horswill CA, Murray B, Wildman R (2006) Urine speciwc gravity in exercisers prior to physical training. Appl Physiol Nutr Metab 31: Tanner JM (1962) Growth at adolescence, 2nd edn. Blackwell, Oxford, p 32 Verbalis JG (1991) Inhibitory controls of drinking and satiation of thirst In: Ramsey DJ, Booth D (eds) Thirst, physiological and psychological aspects. Springer, London, Chap 19 Vrijens DM, Rehrer NJ (1999) Sodium-free Xuid ingestion decreases plasma sodium during exercise in the heat. J Appl Physiol 86: Wilk B, Rivera-Brown AM, Bar-Or O (2007) Voluntary drinking and hydration in non-acclimatized girls exercising in the heat. Eur J Appl Physiol 101: Wilk B, Bar-Or O (1996) EVect of drink Xavor and NaCl on voluntary drinking and hydration in boys exercising in the heat. J Appl Physiol 80: Wilk B, Yuxiu H, Bar-Or O (2002) EVect of body hypohydration on aerobic performance of boys who exercise in the heat. Med Sci Sports Exerc 34:S48 Yaglou CP, Minard D (1957) Control of heat casualties at military training centers. Arch Ind Health 16: