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Hydration and Physical Activity


Cheuvront, AN and Haymes, EM.  Ad libitum fluid intake and thermoregulatory responses of female distance runners in three environments.  Journal of Sports Sciences. 2001;19:845-854.

PubMed ID: 11695506
Study Design:
Crossover design with random assignment to treatment order
C - Click here for explanation of classification scheme.
Quality Rating:
Neutral NEUTRAL: See Quality Criteria Checklist below.
Research Purpose:
To determine the effect of ad libitum fluid intakes on the thermoregulatory responses of women distance runners during prolonged exercise in three commonly encountered race environments (wet bulb temperature = 25 C, 17 C and 12 C, corresponding to hot, moderate and cool conditions, respectively).
Inclusion Criteria:

Not provided

Subjects included in the study were female, marathon runners.

Exclusion Criteria:
None specified
Description of Study Protocol:


Subjects were recruited from the Gulf Winds Track Club and surrounding Tallahassee Florida community.  No other details provided


Design Crossover design with random assignment to treatment groups


Blinding used (if applicable)

Not applicable


Intervention (if applicable)

 Each day of trial, subjects completely voided bladders and a urine sample was collected.  Aferwards a nude weight was obtained.  A rectal and skin thermistors were fitted.  Venous blood drawn after 15 minutes (standing position).  Participants entered a environmental chamber maintained under hot (29.9 C, 55% relative humidity, wet bulb globe temperature =25 C), moderate (20.4 C, 54% relative humidity, wet bulb globe temperature =17 C), and cool (14.3 C, 64% relative humidity, wet bulb globe temperature =12 C) conditions in random order.  All exercise trials consisted of a 30 km treadmill run at an exercise intensity of each individual's best marathon race pace (mean relative exercise intensity of 71+ 7% VO2max.  Water was offered every 5 km in cups filled to 120 ml (simulated long feeding station in a marathon race).  Subjects were instructed to consume fluids ad libitum just as they would under race conditions.

Statistical Analysis


Data Collection Summary:

Timing of Measurements

Rectal temperature, mean skin temperature, heart rate and chamber temperature were measured at 10 min intervals throughout exercise.  Exercise gas exchange was measured for 3 min every 5 km beginning at 4 km. 

Venous blood drawn at baseline (after 15 minutes in standing position before entering environmental chamber) and in the standing position within 5-10 min of the end of exercise.

Final urine volume were collected following exercise. 

Body weight prior and following each exercise session.

 Dependent Variables

  • Exercise heart rate
  • Oxygen consumption
  • Ad libitum fluid consumption
  • % Relative rehydration= total amount of fluid consumed/total sweat loss
  • Total body mass loss:  [(pre-run nude body mass - post-run nude body mass) + ingested fluid volume - urine volume
  • Sweat loss = Total body mass loss - respiratory water loss -metabolic mass loss (CO2- O2 exchange)
  • Serum osmolality
  • Plasma volume
  • Rectal temperature (Tre) , change in rectal temp (post - pre)
  • Mean Skin Temperature using 4-site weighting formula of Ramanathan (1964)
  • Body temperature Tb
  • Thermal gradient =   Tre- skin temperature (Tsk C)
  • Thermal balance expresed as energy storage (S W m-2)) = M (metabolic energy produced during exercise) - W (energy used to perform external work) + R (radiation) + C (convection)- E (evaporative heat loss)

Independent Variables

3 environmental conditions:  HOT (29.9 C, 55% relative humidity, wet bulb globe temperature =25 C), MODERATE (20.4 C, 54% relative humidity, wet bulb globe temperature =17 C), and COOL(14.3 C, 64% relative humidity, wet bulb globe temperature =12 C). 


Control Variables

To control for potential trial to trial differences in nutritional and hydration status, 3 day diet and training records were recorded before trial 1 and their replication was required before trials 2 and 3.  Participants were tested in the follicular phase of their menstual cycle to control for diurnal rhythm and menstrual phase changes in basal body temperature.

Description of Actual Data Sample:

 Initial N:  8 females started and completed the study

Attrition (final N):  0

Age: 37+ 4 years

Ethnicity: No reported

Other relevant demographics: Female 

Anthropometrics:  Body Mass 53.3 + 7.8 kg; Height 1.58 + 0.09 m; mean body surface area 1.52 + 0.15 m2, percent body fat 15.7 + 5.2%; VO2max 50.2 + 5.0 ml/kg/min


Florida State University 

Summary of Results:





Hot Trial (25° C)

Measures and standard deviation

Moderate Trial (17° C)

Measures and standard deviation

Cool Trial (12° C)

Measures and standard deviation

Total Body Mass Loss (kg)

3.17 + 0.76*^

2.43 + 0.60

2.26 + 0.61

Total Sweat Loss (kg)

 2.91 + 0.73*^

 2.11 + 0.56

 1.91 + 0.58

Fluid Intake (l/h)

 0.699+ 0.31^

 0.541 + 0.26

 0.470 + 0.13

Total Fluid Intake (kg)

1.84 + 0.84^

1.41 + 0.57

1.29 + 0.43


33.28 + 1.04*^

30.53 + 0.87

29.27 + 1.33


37.37 + 0.23*^

36.72 + 0.26

36.47 + 0.31

Thermal Gradient (Tre-Tsk)

5.27 + 1.15*^

7.84 + 0.85^

9.08 + 1.43

W (energy used to perform external work)

10.5 + 1.54^

10.7 + 1.59

10.7 + 1.56

R (radiation)

15.7 + 4.73*^

47.7 + 3.00^

70.6 + 10.54

C (convection)

42.9 + 12.98*^

130.6 + 8.16^

193.4 + 28.83

E (evaporative heat loss)

314.7 + 56.24*^

205.9 + 56.75^

127.2 + 73.57

S (body energy storage)

18.7 + 9.50*^

11.6 + 10.02

11.9 + 10.79

 *P<0.05 compared with 17°C. ^P<0.05 compared to 12°C

Other Findings

  • HR values for all 10 minutes intervals between 10-120 minutes for the hot trial were signficantly different (P=0.002) from the moderate and cool trials. 
  • Oxygen consumption:  cool trial significantly different from from hot trial at 15, 20, 25, and 30 km; Moderate trial significantly different from hot trial at 25 and 30 km and cool trial significantly different from moderate trial at 20, 25 and 30 km (P<0.05).
  • Similar volumes of ad libitum fluid intakes were recorded across 5 km intervals for all 3 trials.  Total fluid intake was significantly higher (P=0.023) in the hot versus the cool trial but not in the moderate trial. 
  • % relative rehydration was not significantly different between trials.
  • Neither post-exercise serum osmolarity nor changes in plasma volume were different between the 3 trials.
  • Significant main effect of time (P<0.001) for rectal temperature measured at 10 min intervals throughout out exercise
Author Conclusion:
Ad libitum fluid intakes representing ~60-70% of sweat losses allow women distance runners to maintain plateau in rectal temperature below values commonly linked to heat-stress fatigue and heat injury (39 C) during extended exercise in a range of compensable climates.  These fluid volumes are less than the 100% replacement of sweat loss often strived for and indicate that modest, voluntary rehydration during prolonged exercise is adequate to maintain temperature homeostasis provided that heat loss is not limited by the environment.
Funding Source:
University/Hospital: Florida State University
Foundation associated with industry:
Reviewer Comments:
Limitations:  Because investigators wanted to test women at the same time in their monthly menstrual cycle, the 3 exercise trials were separated by 28-32 days, covering over 3 months.  In the study there was a brief mention of a significant finding in work rate which was higher in the cool weather vs. hot weather trials owing to a "slight change in mean body mass between trials"  There was no other mention, nor any data or tables indicating how body weight changed over the course of the study.  The authors point out that the participants fluid intake in this study was higher than previously observed in female marathon runners.  One factor that may have contributed to that is right before each fluid break subjects were tested with the respiratory metabolic system, and they may have consumed more fluids to counteract the dry mouth from this testing.  Thus testing may have altered the ad libitum fluid intake in the lab vs in the field setting.
Quality Criteria Checklist: Primary Research
Relevance Questions
  1. Would implementing the studied intervention or procedure (if found successful) result in improved outcomes for the patients/clients/population group? (Not Applicable for some epidemiological studies) Yes
  2. Did the authors study an outcome (dependent variable) or topic that the patients/clients/population group would care about? Yes
  3. Is the focus of the intervention or procedure (independent variable) or topic of study a common issue of concern to dieteticspractice? Yes
  4. Is the intervention or procedure feasible? (NA for some epidemiological studies) Yes
Validity Questions
1. Was the research question clearly stated? Yes
  1.1. Was (were) the specific intervention(s) or procedure(s) [independent variable(s)] identified? Yes
  1.2. Was (were) the outcome(s) [dependent variable(s)] clearly indicated? Yes
  1.3. Were the target population and setting specified? Yes
2. Was the selection of study subjects/patients free from bias? No
  2.1. Were inclusion/exclusion criteria specified (e.g., risk, point in disease progression, diagnostic or prognosis criteria), and with sufficient detail and without omitting criteria critical to the study? No
  2.2. Were criteria applied equally to all study groups? Yes
  2.3. Were health, demographics, and other characteristics of subjects described? Yes
  2.4. Were the subjects/patients a representative sample of the relevant population? No
3. Were study groups comparable? Yes
  3.1. Was the method of assigning subjects/patients to groups described and unbiased? (Method of randomization identified if RCT) Yes
  3.2. Were distribution of disease status, prognostic factors, and other factors (e.g., demographics) similar across study groups at baseline? No
  3.3. Were concurrent controls or comparisons used? (Concurrent preferred over historical control or comparison groups.) Yes
  3.4. If cohort study or cross-sectional study, were groups comparable on important confounding factors and/or were preexisting differences accounted for by using appropriate adjustments in statistical analysis? N/A
  3.5. If case control study, were potential confounding factors comparable for cases and controls? (If case series or trial with subjects serving as own control, this criterion is not applicable.) N/A
  3.6. If diagnostic test, was there an independent blind comparison with an appropriate reference standard (e.g., "gold standard")? N/A
4. Was method of handling withdrawals described? ???
  4.1. Were follow-up methods described and the same for all groups? N/A
  4.2. Was the number, characteristics of withdrawals (i.e., dropouts, lost to follow up, attrition rate) and/or response rate (cross-sectional studies) described for each group? (Follow up goal for a strong study is 80%.) No
  4.3. Were all enrolled subjects/patients (in the original sample) accounted for? ???
  4.4. Were reasons for withdrawals similar across groups? N/A
  4.5. If diagnostic test, was decision to perform reference test not dependent on results of test under study? N/A
5. Was blinding used to prevent introduction of bias? Yes
  5.1. In intervention study, were subjects, clinicians/practitioners, and investigators blinded to treatment group, as appropriate? Yes
  5.2. Were data collectors blinded for outcomes assessment? (If outcome is measured using an objective test, such as a lab value, this criterion is assumed to be met.) Yes
  5.3. In cohort study or cross-sectional study, were measurements of outcomes and risk factors blinded? N/A
  5.4. In case control study, was case definition explicit and case ascertainment not influenced by exposure status? N/A
  5.5. In diagnostic study, were test results blinded to patient history and other test results? N/A
6. Were intervention/therapeutic regimens/exposure factor or procedure and any comparison(s) described in detail? Were interveningfactors described? Yes
  6.1. In RCT or other intervention trial, were protocols described for all regimens studied? Yes
  6.2. In observational study, were interventions, study settings, and clinicians/provider described? N/A
  6.3. Was the intensity and duration of the intervention or exposure factor sufficient to produce a meaningful effect? Yes
  6.4. Was the amount of exposure and, if relevant, subject/patient compliance measured? Yes
  6.5. Were co-interventions (e.g., ancillary treatments, other therapies) described? N/A
  6.6. Were extra or unplanned treatments described? N/A
  6.7. Was the information for 6.4, 6.5, and 6.6 assessed the same way for all groups? N/A
  6.8. In diagnostic study, were details of test administration and replication sufficient? N/A
7. Were outcomes clearly defined and the measurements valid and reliable? Yes
  7.1. Were primary and secondary endpoints described and relevant to the question? Yes
  7.2. Were nutrition measures appropriate to question and outcomes of concern? Yes
  7.3. Was the period of follow-up long enough for important outcome(s) to occur? Yes
  7.4. Were the observations and measurements based on standard, valid, and reliable data collection instruments/tests/procedures? Yes
  7.5. Was the measurement of effect at an appropriate level of precision? Yes
  7.6. Were other factors accounted for (measured) that could affect outcomes? Yes
  7.7. Were the measurements conducted consistently across groups? N/A
8. Was the statistical analysis appropriate for the study design and type of outcome indicators? N/A
  8.1. Were statistical analyses adequately described and the results reported appropriately? Yes
  8.2. Were correct statistical tests used and assumptions of test not violated? Yes
  8.3. Were statistics reported with levels of significance and/or confidence intervals? Yes
  8.4. Was "intent to treat" analysis of outcomes done (and as appropriate, was there an analysis of outcomes for those maximally exposed or a dose-response analysis)? N/A
  8.5. Were adequate adjustments made for effects of confounding factors that might have affected the outcomes (e.g., multivariate analyses)? N/A
  8.6. Was clinical significance as well as statistical significance reported? Yes
  8.7. If negative findings, was a power calculation reported to address type 2 error? N/A
9. Are conclusions supported by results with biases and limitations taken into consideration? No
  9.1. Is there a discussion of findings? Yes
  9.2. Are biases and study limitations identified and discussed? No
10. Is bias due to study's funding or sponsorship unlikely? N/A
  10.1. Were sources of funding and investigators' affiliations described? Yes
  10.2. Was the study free from apparent conflict of interest? No