when this extra body heat is not adequately released from the body or balanced , an elevation in body temperature is inevitable . as heat accumulates
sweating is critical in managing body thermal load via evaporative cooling , especially while exercising in a hot environment .
sweating reduces body fluid volume , resulting in dehydration , which increases physiological strain and reduces physical capacity .
these adverse effects can be greater as the fluid deficit increases over exercise intensities . to minimize the negative effects of dehydration , voluntary drinking to ensure a hydrated state prior to and during exercise is recommended .
rehydration by drinking is beneficial for reducing thermal and cardiovascular strain , stabilizing metabolic function , and decreasing fatigue .
thus , rehydrating to maintain body fluid volume is a concern when exercising in a hot environment .
although rehydration during exercise is recommended , individual variations in volume consumption and retention exist . to date ,
several factors influencing the volume of fluid consumption and volume retention have been proposed , including the contents of carbohydrates and electrolytes within the fluid [ 7 - 10 ] , heat acclimation status , age , and gender .
however , subjects in these previous studies were tested without experiencing both exercise stress and heat acclimation at once and/or were examined only in a laboratory setting . for example , voluntary fluid intake was measured during exercise   and recovery , and the volume intake was not controlled .
these subjects were encouraged to drink , but the volume was not manipulated . in real life , frequent drinking during exercise and heat acclimation may occur .
no reported study has investigated the impact of systemic fluid consumption training during short - term exercise and heat acclimation with a prescribed volume intake .
although maintaining an adequate hydration level is believed to minimize thermoregulatory instability and maximize circulatory and metabolic capability while exercising in a hot environment , it has not been clearly demonstrated whether an active habitual drinking practice during training could modify voluntary drinking during subsequent exercise in a hot environment .
this study was designed to investigate the impact of habitual fluid drinking practices while exercising and to evaluate the impact of heat stress on subsequent drinking volume while exercising in a hot environment .
it was hypothesized that an active drinking practice would elicit greater fluid consumption and sweat rate versus a non - active drinking condition when fluid intake was allowed ad libitum .
subjects participated in two experiments with identical procedures but different drinking interventions : 1 ) active hydration ( ah ) and 2 ) passive hydration ( ph ) .
each experiment consisted of three sequential phases : 1 ) pre - testing , 2 ) 10-day training period , and 3 ) post - testing .
, subjects ran on a treadmill in a hot environment . to achieve the study goal , they were required to drink different volume during two conditions .
subjects were required to drink 1.5 times their body weight loss while exercising during ah and 0.5 times their body weight loss during ph .
the two experiments were separated by at least 3 weeks , and the experiments were conducted during the late fall and winter .
based on medical history , they were considered free from any pulmonary , metabolic , and orthopedic complications and had no heat illnesses .
full study documentation and verbal explanations about the procedures involved were provided to all subjects , and they signed informed consent forms .
the subjects physical characteristics including age , height , weight , and body fat content were measured using the bioimpedance method ( inbody 520 ; biospace , korea ) . during at least three separate occasions prior to the testing , the subjects were familiarized with running on a treadmill . using the modified bruce protocol ,
the maximal oxygen uptake ( vo2max ) of the subjects was measured while they ran on a treadmill ( q65 , quinton instrument co. , bothell , wa , usa ) .
they exercised until volitional exhaustion while breathing through a breath - by - breath gas analysis system ( k4b , cosmed srl , rome , italy ) .
after completion of the vo2max test , they rested for at least 30 min or until they returned to their resting heart rate ( hr ) .
then , they ran once again to identify the speed of the treadmill that matched the range of each individual s exercise intensity ( 55 - 60% of the vo2max ) .
initially , the speed of treadmill was set to elicit ~50% of the vo2max prediction ; additionally , it was adjusted every 3 min to match the target oxygen uptake ( vo2 ) .
when they could steadily match their target vo2 for at least 56 min , the treadmill belt speed was recorded . that speed was used during testing and training . at 3 days after the pre - screening session
their naked body weight was measured using a scale ( 150a , cas , seoul , korea ) , and a rectal probe ( barnant co. , barrington , il , usa ) was inserted 15 cm beyond the anal sphincter .
thermocouples ( 692 - 0230 ; barnant co. , barrington , il , usa ) for measuring the subjects skin temperature were attached to three sites ( chest , triceps , and thigh ) using surgical tape ( micropore ; 3 m , st .
an electronic hr monitor ( s610 ; polar electro oy , finland ) was strapped around the subjects chests .
once all the equipment was adjusted , they entered a climate controlled chamber , with the temperature maintained at 39  1 with a relative humidity of 65  3% , and the subjects rested in a seated position for 23.3  7.6 min . after they adjusted to the environment , a blood sample was taken from the antecubital vein using a disposable syringe .
then , they were asked to stand on the treadmill and begin running for 90 min at their previously determined treadmill belt speed .
they were allowed to drink ad libitum during the testing , and they occasionally breathed through a mask for respiratory gas analysis .
when they completed their exercise , all the measuring equipment was detached from their bodies , and they were dried completely . after their urine samples were collected , their naked weight was taken .
subjects walked outside the chamber and recovered for 3 h at ambient temperature ( 25  2 ) without any vigorous physical activity or food consumption . during recovery , their naked body weight , blood samples , and urine samples were collected during the 1 and 3 hours .
beginning 2 days after pre - testing , subjects participated in a 10-day training period where they exercised in an environment and at an exercise intensity identical to that in pre - testing . during the 90-min exercise ,
each subject had his own bottle filled with an assigned volume of fluid prepared by the investigators .
the fluid volume to be consumed during the training period was determined during pre - testing .
subjects in ah were asked to consume a sports beverage ( gatorade thirst quencher ) at 150% of their body weight loss while subjects in ph consumed 50% of their body weight loss .
one - third of the target fluid volume was provided every 30 min of training .
when they did not consume the amount provided within the time period , the leftover was discarded .
although the amount and time frame for beverage consumption was decided for each subject , they were encouraged to drink continuously . within 2 days after training period completion
after the experiments were completed , subjects underwent a second screening session , and the vo2max was evaluated . from height and weight , the body mass index ( kgm ) and body surface area ( m ) were calculated .
all body weights were measured with accuracy of 10 g. all urine samples were used for volume measurement and urine specific gravity ( usg ) using a digital refractometer ( ug ; atago , tokyo , japan ) . rectal temperature ( tre ) and each skin site temperature ( tsk ) were monitored continuously using a thermocouple scanner ( barnant co. , barrington , il , usa ) and were recorded at 5-min intervals .
the mean tsk was calculated by appropriately weighing the individual s tsk , 0.43 ( chest tsk ) + 0.25 ( triceps tsk ) + 0.32 ( thigh tsk ) .
the mean body temperature ( tb ) was calculated as 0.73 tre + 0.27 mean tsk . during the testing
the target volume of fluid consumption during the training was determined during pre - testing of each experiment . for this , the body mass difference between the baseline and at the end of the exercise was calculated with a correction for fluid volume consumed and voided . from this value , the relative amount of the individual beverage volume was calculated , and the value was considered as sweat loss . during testing , the fluid volume consumed , volume voided , and weight changes were calculated to estimate the sweat rate ( sr ) , expressed in mlkgh .
blood samples were collected using a 10-ml sterilized disposable plastic syringe and were immediately transferred to a clot activator tube ( green vac - tube ; green cross ms corp .
the serum was separated using a centrifuge ( ha-1000 - 3 ; hanil science co. , incheon , korea ) for 7 min and was analyzed for total proteins using a blood chemistry analyzer ( ektachem dt60 ii ; johnson & johnson , new brunswick , nj , usa ) .
serum osmolality was measured in triplicate by using the freeze - point depression osmometer ( micro - osmometer model 210 ; fiske associates , norwood , ma , usa ) , and the average value of the three reading was taken . during exercise in the testing , the rating of perceived exertion ( rpe ; 11-point scale , 010 ) , thirst sensation ( 11-point scale ; 0 : no sensation to 10 : extremely thirsty ) , and heat sensation ( 11-point scale ; 0 : no sensation to 10 : extremely hot ) were asked and evaluated at the 15 and 90 minutes of exercise .
the hr and vo2max were analyzed at the 15 , 40 , 65 , and 90 minutes during the testing .
all variables are expressed as means  standard deviation . a comparison of values between the ah and ph was made using student s t - test . when comparing conditions ( ah vs. ph ) to the testing period ( pre- vs. post - testing ) , within - subjects factorial analysis of variance with repeated measures was employed .
subjects participated in two experiments with identical procedures but different drinking interventions : 1 ) active hydration ( ah ) and 2 ) passive hydration ( ph ) .
each experiment consisted of three sequential phases : 1 ) pre - testing , 2 ) 10-day training period , and 3 ) post - testing .
, subjects ran on a treadmill in a hot environment . to achieve the study goal , they were required to drink different volume during two conditions .
subjects were required to drink 1.5 times their body weight loss while exercising during ah and 0.5 times their body weight loss during ph .
the two experiments were separated by at least 3 weeks , and the experiments were conducted during the late fall and winter .
based on medical history , they were considered free from any pulmonary , metabolic , and orthopedic complications and had no heat illnesses .
full study documentation and verbal explanations about the procedures involved were provided to all subjects , and they signed informed consent forms .
the subjects physical characteristics including age , height , weight , and body fat content were measured using the bioimpedance method ( inbody 520 ; biospace , korea ) . during at least three separate occasions prior to the testing , the subjects were familiarized with running on a treadmill . using the modified bruce protocol ,
the maximal oxygen uptake ( vo2max ) of the subjects was measured while they ran on a treadmill ( q65 , quinton instrument co. , bothell , wa , usa ) .
they exercised until volitional exhaustion while breathing through a breath - by - breath gas analysis system ( k4b , cosmed srl , rome , italy ) .
after completion of the vo2max test , they rested for at least 30 min or until they returned to their resting heart rate ( hr ) .
then , they ran once again to identify the speed of the treadmill that matched the range of each individual s exercise intensity ( 55 - 60% of the vo2max ) . initially , the speed of treadmill was set to elicit ~50% of the vo2max prediction ; additionally , it was adjusted every 3 min to match the target oxygen uptake ( vo2 ) .
when they could steadily match their target vo2 for at least 56 min , the treadmill belt speed was recorded . that speed was used during testing and training .
at 3 days after the pre - screening session , pre - testing was conducted . on the day of testing , subjects reported to the laboratory after a 10-h overnight fast .
their naked body weight was measured using a scale ( 150a , cas , seoul , korea ) , and a rectal probe ( barnant co. , barrington , il , usa ) was inserted 15 cm beyond the anal sphincter .
thermocouples ( 692 - 0230 ; barnant co. , barrington , il , usa ) for measuring the subjects skin temperature were attached to three sites ( chest , triceps , and thigh ) using surgical tape ( micropore ; 3 m , st .
an electronic hr monitor ( s610 ; polar electro oy , finland ) was strapped around the subjects chests .
once all the equipment was adjusted , they entered a climate controlled chamber , with the temperature maintained at 39  1 with a relative humidity of 65  3% , and the subjects rested in a seated position for 23.3  7.6 min .
after they adjusted to the environment , a blood sample was taken from the antecubital vein using a disposable syringe .
then , they were asked to stand on the treadmill and begin running for 90 min at their previously determined treadmill belt speed .
they were allowed to drink ad libitum during the testing , and they occasionally breathed through a mask for respiratory gas analysis .
when they completed their exercise , all the measuring equipment was detached from their bodies , and they were dried completely . after their urine samples were collected , their naked weight was taken .
subjects walked outside the chamber and recovered for 3 h at ambient temperature ( 25  2 ) without any vigorous physical activity or food consumption . during recovery , their naked body weight ,
beginning 2 days after pre - testing , subjects participated in a 10-day training period where they exercised in an environment and at an exercise intensity identical to that in pre - testing . during the 90-min exercise ,
each subject had his own bottle filled with an assigned volume of fluid prepared by the investigators .
the fluid volume to be consumed during the training period was determined during pre - testing .
subjects in ah were asked to consume a sports beverage ( gatorade thirst quencher ) at 150% of their body weight loss while subjects in ph consumed 50% of their body weight loss .
one - third of the target fluid volume was provided every 30 min of training .
when they did not consume the amount provided within the time period , the leftover was discarded . although the amount and time frame for beverage consumption was decided for each subject , they were encouraged to drink continuously . within 2 days after training period completion
after the experiments were completed , subjects underwent a second screening session , and the vo2max was evaluated .
from height and weight , the body mass index ( kgm ) and body surface area ( m ) were calculated .
all body weights were measured with accuracy of 10 g. all urine samples were used for volume measurement and urine specific gravity ( usg ) using a digital refractometer ( ug ; atago , tokyo , japan ) . rectal temperature ( tre ) and each skin site temperature ( tsk ) were monitored continuously using a thermocouple scanner ( barnant co. , barrington , il , usa ) and were recorded at 5-min intervals .
the mean tsk was calculated by appropriately weighing the individual s tsk , 0.43 ( chest tsk ) + 0.25 ( triceps tsk ) + 0.32 ( thigh tsk ) .
the mean body temperature ( tb ) was calculated as 0.73 tre + 0.27 mean tsk . during the testing ,
the target volume of fluid consumption during the training was determined during pre - testing of each experiment . for this , the body mass difference between the baseline and at the end of the exercise was calculated with a correction for fluid volume consumed and voided . from this value , the relative amount of the individual beverage volume was calculated , and the value was considered as sweat loss . during testing , the fluid volume consumed , volume voided , and weight changes were calculated to estimate the sweat rate ( sr ) , expressed in mlkgh .
blood samples were collected using a 10-ml sterilized disposable plastic syringe and were immediately transferred to a clot activator tube ( green vac - tube ; green cross ms corp . ,
the serum was separated using a centrifuge ( ha-1000 - 3 ; hanil science co. , incheon , korea ) for 7 min and was analyzed for total proteins using a blood chemistry analyzer ( ektachem dt60 ii ; johnson & johnson , new brunswick , nj , usa ) .
serum osmolality was measured in triplicate by using the freeze - point depression osmometer ( micro - osmometer model 210 ; fiske associates , norwood , ma , usa ) , and the average value of the three reading was taken . during exercise in the testing ,
the rating of perceived exertion ( rpe ; 11-point scale , 010 ) , thirst sensation ( 11-point scale ; 0 : no sensation to 10 : extremely thirsty ) , and heat sensation ( 11-point scale ; 0 : no sensation to 10 : extremely hot ) were asked and evaluated at the 15 and 90 minutes of exercise .
the hr and vo2max were analyzed at the 15 , 40 , 65 , and 90 minutes during the testing .
all variables are expressed as means  standard deviation . a comparison of values between the ah and ph was made using student s t - test . when comparing conditions ( ah vs. ph ) to the testing period ( pre- vs. post - testing ) , within - subjects factorial analysis of variance with repeated measures was employed .
all subjects completed all procedures required in the study . during testing and training , they ran at a speed of 8.0  0.5 kmh for a distance of 11.6  1.4 km .
the calculated exercise intensity during testing and training was 51  3.9% of vo2max . while subjects were requested to drink the full - strength beverage , most subjects preferred half the strength of the original beverage .
the subjects baseline physical conditions during pre - testing in the two conditions did not differ ( table 1 ) .
body weight and body mass index did not change between pre- and post - testing during ah and ph ( p > 0.05 ) , but body fat content decreased in both conditions between pre- and post - testing ( p < 0.05 , student s t - test ) .
the target amount of beverage intake in training during ah was approximately three times greater than that during ph ( table 2 ) .
the average volume consumed in training during ah was short of the target amount , but it was greater than 100% of the subjects body weight loss . during ph ,
when the actual beverage consumption was normalized by body weight , their consumption during ah was approximately three times greater than during ph . while exercising in post - testing during ah , the beverage consumption was approximately two times greater than during pre - testing ( p < 0.05 , table 2 ) .
however , the beverage consumption while exercising during ph was not statistically significantly different between pre- and post - testing . during recovery ,
the amount of beverage consumption was neither different between pre- and post - testing nor between the ah and ph conditions .
the total amount of beverage consumption during testing was greater in post - testing than in pre - testing during ah ( p < 0.05 ) , but not during ph ( p > 0.05 ) .
both sweat loss and sweat rate were greater in post - testing compared with pre - testing during the exercise and recovery period of ah ( p < 0.05 , table 3 ) .
however , these variables were only greater in post - testing compared with pre - testing during the recovery period of ph ( p < 0.05 ) .
the final tre during exercise in post - testing was reduced compared with that in pre - testing in both experiments , but it was not statistically significant ( fig .
the magnitude of the tre elevation during exercise was 3.0  0.7 , 2.8  0.4 , 3.2  0.8 , and 2.8  0.8 in pre - testing during ah , post -testing during ah , pre - testing during ph , and post - testing during ph , respectively , and no statistically significant difference was noted .
the tsk and tb were also not signi - ficantly different among testing and experiments .
the hrs also decreased during exercise in post - testing compared with those in pre - testing in both experiments , but the difference was not statistically significant ( fig .
the magnitude of reduction of the final hr from pre - testing to post - testing was 14.7  14.6 beatsmin during ah and 9.6  16.4 beatmin during ph ( p > 0.05 ) .
the usg , serum osmolality , and total protein were not different at baseline or at 3 h of recovery among testing and experiments ( table 4 ) .
no psychometric variable during exercise was different among testing and experiments ( table 5 ) .
dehydration during exercise and heat induces performance deterioration , mainly due to thermoregulatory and cardiovascular strain .
fluid intake helps restore body fluid and fluid volume regulatory function , thereby improving physical capacity [ 1 - 3 ] .
however , voluntary drinking during exercise , aimed at restoring fluids and regulatory function , does not always guarantee meeting the adequate volume requirement .
we hypothesized that habitual drinking practices while exercising in a hot environment may help increase voluntary volume intake during subsequent exercise and heat stress . to our knowledge , this is the first reported study examining the effects of drinking a prescribed volume during exercise with short - term heat acclimation on fluid volume regulation .
the most significant finding of the present study was that active habitual drinking practice raised the volume of voluntary drinking as well as increased the sweat rate while exercising in a hot environment .
nevertheless , no thermoregulatory or circulatory benefit from the active drinking practice compared with the non - active practice was supported statistically . in this study ,
subjects participated in 10 days of exercise training in a hot environment , and they ran for 90 min while being required to drink 50% ( during ph ) or 150% ( during ah ) of their body weight loss during exercise . during the 10-day program ,
subjects drank approximately 84% ( during ph ; 42% of their weight loss ) and 83% ( during ah ; 123% of their weight loss ) of their target volume . after the program
, subjects were tested and allowed to drink ad libitum while exercising in a hot environment .
the volume of voluntary drinking and the sweat rate increased significantly during ah while no change was observed during ph .
in both conditions , the tre and hr decreased , but no significant difference between the conditions was noted .
our findings suggest that short - term exercise training in a hot environment while drinking , generally , may help reduce thermoregulatory and circulatory strain .
these responses were not affected by drinking volume during exercise training , at least under the conditions used in this study .
however , an increased active volume drinking practice during training caused an increased voluntary drinking volume and sweat rate .
some studies have reported that short - term heat exposure with exercise increases voluntary fluid intake .
greenleaf et al .   reported that voluntary water intake increased from 450 ml / h on day 1 of exercise and heat stress increased to about 1,000 mlh on days 58 . in their study , subjects were free from a target drinking volume , which was used in the present study .
also reported an increased daily fluid intake while exercising in a warm environment . in the present study
, subjects drank about 570 mlh voluntarily in pretesting and 1,061 mlh in post - testing during ah . by comparing the volume consumed , the active drinking practice seemed to be no more beneficial over a non - target voluntary drinking volume .
although 150% of weight loss was the target volume during ah , most subjects could not meet the requirement . despite the fact that thirst sensation increased by exercise - induced dehydration , driven by blood osmolality and angiotensin ii , this may not be sufficient to promote voluntary fluid intake to match water loss .
studies have shown that voluntary drinking replaced , at most , 30 - 70% of water loss during exercise even when fluid was readily available [ 9 , 18 - 20 ] .
the increase in volume loading due to exercising in a hot environment may stimulate the cardiopulmonary baroreceptor , resulting in suppression of the drinking desire . in his review , noakes   noted that a high rate of fluid intake ( > 1 lh ) can be difficult during running and may lead to feelings of abdominal discomfort possibly due to the accumulation of unabsorbed fluid in the intestines .
he also reported that most do not ingest fluid equal to the amount of fluid loss ; subsequently , he proposed that drinking practices during training may reduce the severity of these symptoms .
the present study supported this notion because the increased active volume drinking practice resulted in an increased volume of voluntary drinking .
as mentioned above , the exercise - heat stress may have reduced cardiopulmonary baroreceptor sensitivity leading to increased fluid intake .
one notable observation was that subjects were reluctant to drink the full - strength sports drink while running in the heat .
numerous studies have shown that carbohydrate - electrolyte solutions are more effective than plain water for stimulating voluntary fluid intake , and promoting rapid intestinal absorption [ 25 - 27 ] , although it is not always consistent in some populations .
although a solution containing about 6% of carbohydrates is considered a sport drink , this concentration seemed to be too strong for the voluntary drinking conditions in the present study s subjects .
a high concentration of a carbohydrate solution has been shown to decrease the rate of gastric emptying   and absorption . additionally , drinking the full - strength beverage might have been uncomfortable for the present study s subjects and possibly reduced gastric emptying .
osterberg et al .   reported that volume retention between beverages containing 36% carbohydrate was not different .
their usual drinking solution during exercise was plain water , instead of a full strength sports drink .
although active drinking practices induced a higher drinking volume and sweat rate while exercising in a hot environment , it was not advantageous over passive drinking for thermo - regulatory or circulatory adjustments or psychometric responses . the general recommendation to prevent dehydration during exercise and heat stress is to prevent excessive dehydration ( > 2% body water deficit ) and excessive disturbance of the electrolyte balance .
one reason why we did not observe any difference between the two conditions may have been the severity of the dehydration . during ph , although full recovery from weight loss by exercise was not noticed , more than 40% of the subjects body weight loss was replaced by voluntary drinking .
this volume consumption may contribute to physiological and psychometric response adjustments and may subsequently allow at least a minimum adjustment to prevent dehydration .
studies on more prolonged exercise interventions and/or severe volume restrictions during exercise may be warranted .
active drinking practice while exercising in a hot environment induced greater voluntary fluid intake and sweat loss .
although they were required to drink 150% of their weight loss , they actually consumed 123% of their weight loss .
there was no additional benefit from the increased volume intake in terms of thermoregulatory and circulatory adjustments or psychometric parameters , which may be due to the severity of body weight loss while exercising under the two conditions .
 [ purpose]to examine the effects of active drinking practices on fluid consumption and sweat rate while exercising in a hot environment.[methods]nine men completed two experiments . 
 each consisted of 3 phases : pre - testing ( pre ) , training period , and post - testing ( post ) . during testing , the subjects ran on a treadmill at a moderate intensity for 90 min at 39  1 followed by a 3-h recovery . 
 they drank ad libitum . during training 
 , they ran for 90 min for 7 days while either drinking actively ( ah , 150% of weight loss ) or passively ( ph , 50% of weight loss).[results]the actual volume consumed in training was three times greater during ah than during ph . in post during ah , the volume of drinking was two times greater than pre ( 1592  953 and 855  551 ml , respectively ; p < 0.05 ) . 
 no difference in volume consumption during ph between pre and post was found . the sweat loss during exercise was greater in post ( 1377  956 ml ) than in pre ( 558  642 ml ) during ah ( p < 0.05 ) , but not during ph . 
 rectal temperature and heart rate decreased after training . 
 serum osmolality following exercise were not different than the baseline or between the conditions.[conclusion]active drinking practices while exercising in a hot environment induced greater voluntary fluid intake and sweat loss . 
INTRODUCTION
METHODS
Study design
Subjects
Pre-screening session
Pre- and post-testing
Training period
Post-screening session
Measurements and calculations
Statistical analyses
RESULTS
DISCUSSION
CONCLUSIONS