Hydration and Physical Activity


Wemple RD, Morocco TS, Mack GW. Influence of Sodium Replacement on Fluid Ingestion Following Exercise-Induced Dehydration. Int J Sport Nutr. 1997 Jun;7(2):104-16.

PubMed ID: 9189781
Study Design:
Randomized Controlled Trial
A - Click here for explanation of classification scheme.
Quality Rating:
Neutral NEUTRAL: See Quality Criteria Checklist below.
Research Purpose:
  • To investigate the hypothesis that supplemental NaCl in a carbohydrate-electrolyte beverage, in the range of 20 to 50 mmol/L NaCl, would promote fluid ingestion and improve fluid balance following dehydration induced by intense, continuous, upright exercise in warm environment
  • To clarify the independent effects of exercise mode and fluid palatability (sweetening) on fluid replacement following dehydration - this study was compared to earlier experiments in which cycling was the mode of exercise and tap water or flavored, sweetened water was used for hydration.
Inclusion Criteria:

Not described - just stated that "trained" volunteers participated.

Exclusion Criteria:
none described
Description of Study Protocol:



  • Each subject participated in 3 identical dehydration/rehydration protocols designed to reduce body weight 3.0±0.2%
  • Each dehydration/rehydration trial was separated by at least 1 week and the order of the trials was randomized
  • All rehydration fluids were similarly flavored (lemon-lime), sweetened, and served at a temperature of ~15-17°C.
  • VO2max was determined 1-2 weeks before the subject's first trial, and the appropriate treadmill speed and grade needed to elicit 70% VO2max were verified during a 15-min steady-state run conducted on a separate day.
  • Day of experiment:
    • Subjects voided upon waking then drank 400ml water.
    • Reported to the lab having fasted the previous 12 hours.

    Results from this study were compared to earlier experiments in which cycling was the mode of exercise and tap water or flavored, sweetened water was used for rehydration

    Blinding used (if applicable)

    Double Blind Study

    Intervention (if applicable)

    • Once inside the environmental chamber (27°C), esophageal thermocouple and venus catheter control were placed.
    • Blood, urine samples and thirst rating were collected
    • Subjects moved to a second environmental chamber (32°C with <30% relative humidity)
    • After standing on a treadmill for 30 minutes, standing control blood and urine samples were collected and body weight was measured
    • Dehydration was induced by 90 minutes of continuous treadmill exercise at 70% VO2max
    • After exercise, subjects rested in 1st chamber for 30 minutes then recovery blood sample, urine sample, body weight and thirst rating were taken every hour for 3 hours
    • Subjects rehydrated ad libitum for 3 hours with one of the three drinks

    Statistical Analysis

    • Treadmill experiments used repeated-measures ANOVA with two within-subject factors (time and drink)
    • Paired t tests with Bonferroni correction were made to compare individual time points
    • Significance was set at p<0.05
  • Data Collection Summary:

    Timing of Measurements

     Between mid-February and mid-June

    Dependent Variables

    • Heart Rate-measured using telemeter heart rate monitors
    • Body core temperature - monitored with an esophageal thermocouple every 30 seconds
    • Sodium and Potassium concentration measured by sweat samples collected during the first and last 20 minutes of exercise using filter paper disks
    • Blood samples - measured in triplicate for hematocrit, hemoglobin, total protein, plasma osmolality, sodium and potassium - taken at 15, 30, 60 and 90 minutes of exercise
    • Urine volume - to calculate urine flow rate, urine osmolality, sodium and potassium
    • thirst rating - measured by a visual analog scale

    Independent Variables

    3 drinks were offered:

    1. Flavored and sweetened (aspartame) water (H20-R) 
    2. 6% sucrose (CHO) with 25 mmol/L NaCl (LNa+-R)
    3. 6% sucrose with 50mmol/L NaCl (HNa+-R)

    Control Variables

    Two environmental chambers were used

    • First one- temperature of 27°C - used to collect control blood and urine samples
    • Second one -Temperature of 32°C (<30% relative humidity) for intervention
    Description of Actual Data Sample:


    Initial N: 6 trained volunteers

    Attrition (final N):

    Results are reported as mean ± standard deviation for 6 subjects in the treadmill study, 8 subjects in the preliminary study and 7 subjects in the study of Nose et al.

    Age: Mean: 20.7years  Range: 19-21years

    Ethnicity: not described

    Other relevant demographics:

    none described


    Weight (kg): Mean=71.258; Range = 67.524-76.560

    VO2 max (ml·kg-1·min-1):  Mean=63.63; Range=57.17-70.02


     John B. Pierce Laboratory and the Department of Epidemiology and Public Health at Yale School of Medicine, New Haven, CT

    Summary of Results:



    Treatment Group

    Measures and confidence intervals

    Control group

    Measures and confidence intervals

    Statistical Significance of Group Difference

    Total Body Weight Deficits (ml/kg BW)

     LNa+-R: 28.5±1.3

    HNa+-R: 29.8±1.4

     H2O-R:   30.2±1.7


    Total Cation (Na+, K+) loss during dehydration     (mEq/kg BW)

    LNa+-R: 3.04±0.35

    HNa+-R: 2.99±0.30

    H2O-R:   3.32±0.32


    Exercise-induced dehydration

    Posm: 292±1 mOsm/kg H20

    PNa: 154.4±0.5 mEq/kg H20

    Posm: 285±1 mOsm/kg H20

    PNa: 150.0±1.4 mEq/kg H20

    Plasma Volume (%) due to exercised-induced dehydration LNa+-R: 5.78±1.25

    HNa+-R: 4.46±1.48

    H2O-R:   5.87±1.37

    Thirst Rating (%) by the end of 30-min recovery period 61±3 13±2 r=.79, p<0.05 (increased in proportion to Posm)
    Fluid intake (ml/kg BW by 180 min) LNa+-R: 46.2±7.3

    HNa+-R: 38.8±5.8

    H2O-R:   37.6±5.7

    Cumulative Fluid intake of water lost during dehydration (%) LNa+-R: 163±25

    HNa+-R: 133±22

    H2O-R:   123±16  n/a
     Net fluid gain by end of 3-hr rehydration period (%) LNa+-R: 130±19

    HNa+-R: 105±15

     H2O-R:   91±13  n/a


    Other Findings

    • Cumulative fluid intake during LNa+-R was significantly greater than for H2O-R by 120 min of rehydration.
    • Net fluid gain increased sharply over the first hour and leveled off during H2O-R but continued to rise slowly during LNa+-R from 120 to 180 min.
    • Net fluid gain was greater in LNa+-R than H2O-R at 120 and 180 minutes, with a trend toward higher fluid gain compared to HNa+-R at 180 min (0.05<p<.1)
    • During rehydration with HNa+-R, PNa+ was higher than during LNa+-R or H2O-R at all time points.
    • During H2O-R, Posm decreased sharply and remained lower than Posm during HNa+-R and LNa+-R at 120 and 180 min.
    • Plasma Volume
      • was restored more quickly and to a greater extent during HNa+-R, returning to the control level within the first hour (change in PV=0.5±1.9%) and exceeding control levels at 120 min (change in PV=2.8±2.3%)and 180 min (change in PV=2.7±1.9%)
      • In LNa+-R, it rose from its reduced level following dehydration, reaching the control level by 120 min
      • In H2O-R, restoration was smaller and delayed; by 180 min it remained well below the control level


     This study protocol was compared to other exercise protocols for fluid intake, urine output and net fluid gain:

      Mild Intermittent Cycle Ergometer Exercise in heat at 50% VO2 max (CEX-50); rehydration with flavored, sweetened water. (Data from previous, unplublished work) Mild Intermittent Cycle Ergometer Exercise in heat at 40% VO2 max (CEX-40), rehydration with tap water. (Data from Nose et al) Intense treadmill exercise in heat at 70% VO2 max (TEX), rehydration with flavored sweetened water (H2O-R)
    Total Body Water Lost (ml/kg BW) 25.3±1.7 24.6±1.7 30.2±1.7
    Average Cation Loss (mEq/kg BW) 1.94±0.22 1.38±0.15 3.32±0.32
    Cumulative fluid intake over 3 hours following exercise-induced dehydration (ml/kg BW) 26.6±2.4 16.1±2.9 37.6±5.7
    Net fluid gain(ml/kg BW)


    (or 45±6%)


    (or 64±13%)


    (or 91±13%)






















    . .

    Author Conclusion:
    • The addition of 25 mmol/L NaCl to a sweetened/flavored rehydration beverage significantly enhanced fluid intake and effectively restored lost body fluids compared to flavored water or a drink containing 50 mmol/L NaCl. 
    • This study also demonstrates a lack of involuntary dehydration following intense treadmill running in the heat. These data suggest that the distribution of water losses from various fluid compartments, which is influenced by the dehydration procedure, has a profound impact on rehydration patterns in humans.


    Funding Source:
    Reviewer Comments:

    Note: These studies were supported by a grant from the Gatorade Sports Sciences Institute

    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? ???
    2. Was the selection of study subjects/patients free from bias? ???
      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? ???
      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? ???
    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) ???
      3.2. Were distribution of disease status, prognostic factors, and other factors (e.g., demographics) similar across study groups at baseline? Yes
      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? Yes
      4.1. Were follow-up methods described and the same for all groups? Yes
      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%.) Yes
      4.3. Were all enrolled subjects/patients (in the original sample) accounted for? Yes
      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? No
      6.6. Were extra or unplanned treatments described? No
      6.7. Was the information for 6.4, 6.5, and 6.6 assessed the same way for all groups? Yes
      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? Yes
    8. Was the statistical analysis appropriate for the study design and type of outcome indicators? Yes
      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)? ???
      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? Yes
      9.1. Is there a discussion of findings? Yes
      9.2. Are biases and study limitations identified and discussed? Yes
    10. Is bias due to study's funding or sponsorship unlikely? ???
      10.1. Were sources of funding and investigators' affiliations described? Yes
      10.2. Was the study free from apparent conflict of interest? ???