Hydration and Physical Activity

Study Design:
- Click here for explanation of classification scheme.
Quality Rating:
Research Purpose:
To investigate the effect of different sodium concentrations in the replacement fluids consumed during prolonged exercise on hematological variables and endurance performance.
Inclusion Criteria:
Well trained, healthy female runners that had participated in several long distance races (half marathon, marathon, 100km races, or ultrarunning)
Exclusion Criteria:
None mentioned
Description of Study Protocol:


Article does not mention how these runners were recruited, only that consent was obtained from the local ethics committee


  • 3 trials held on 3 consecutive Saturdays at 10:30am. Trials took place on a 400m track (altitude 510m above sea level)
  • Subjects were randomly assigned to one of three groups in which the liquids were consumed in different orders.  

Blinding used (if applicable)

 Not mentioned

Intervention (if applicable)

  • They performed no exercise on the day before each of the 3 trials
  • Each recorded their intake of food and drink during the 24 hours before the first trial
  • They were asked to replicate their intake for trials 2 and 3
  • Subjects were asked to run as many kilometers as possible during the 4 hour trial
  • They received one of the 3 different sodium concentrations at each trial: 1) High Sodium Concentration plus semisolid (680 mg/l) 2) Low Sodium Concentration (410 mg/l) 3) Water plus semisolid
  • Subjects consumed 0.33 liters of the fluid and semisolids immediately before and every 20 minutes during the trials, which resulted in an intake of 1 liter/h during the trial
  • Nothing was consumed after the trials until they had been weighed and blood samples taken for analysis of sodium plasma, packed cell volume, and mean corpuscular volume.

Statistical Analysis

 Analysis of Variance performed:

  • for sodium plasma before the run to ensure comparable starting conditions
  • to detect significant differences in changes in sodium plasma, weight, packed cell volume, mean corpuscular volume (MCV)
  • to detect performance for the 3 liquids and trials

Kruskal-Wallis test was used if there was no normal distribution in the measured variables

Bonferroni post hoc tests were performed for statistically significant analysis of variance

Linear correlation coefficients were calculated (significance set at p<0.05)






Data Collection Summary:

Timing of Measurements

  • Each trial took place on a Saturday at 10:30am
  • Blood samples were taken by routine venipuncture within 30 minutes before and after the trials and analyzed within 24 hours

Dependent Variables

  • Blood levels used to determine changes in sodium plasma, packed cell volume, mean corpuscular volume
  • performance
  • change in body weight

Independent Variables

  • Weather conditions varied between the 3 trials
    • Trial 1: 5.3 degrees Celcius and snow
    • Trial 2: 19.0 degrees Celcius and sunny
    • Trial 3: 13.9 degrees Celcius and precipitation
  • 3 different sodium concentrations at each trial:
    • 1) High Sodium Concentration plus semisolid (680mg/l)
    • 2) Low Sodium Concentration (410 mg/l)
    • 3) Water plus semisolid

Control Variables

  • Nutrition protocol - each participant recorded food intake and drink during 24 hours prior to trial 1 and were asked to repeat it before trials 2 and 3. Protocols were checked before each trial
  • Same track was used for each trial
Description of Actual Data Sample:

Initial N:


Attrition (final N):

No explaination given for attrition

  • Trial H (n=10)
  • Trial L (n=12)
  • Trial W (n=12)


Median age 39.6 years (range 52.1-64.4)


not described

Other relevant demographics:

none listed


mean weight was 57.7 kg (range 52.1-64.4)


 Trials took place on a 400m track (altitude 510m above sea level)


Summary of Results:



High Sodium Concentration Trial

Measures and confidence intervals

Low Sodium Concentration Trial

Measures and confidence intervals

Water Trial


Measures and confidence intervals

Statistical Significance of Group Difference

Prerun Sodium Plasma (mmol/l)





Post run Sodium Plasma (mmol/l)





Change in Sodium Plasma (mmol/l) -2.5±2.5 -4.4±2.5 -6.2±2.1* p=0.001
Change in Sodium Plasma/kilometer (mmol/km) -0.06±0.06 -0.11±0.05 -0.15±0.05* p=0.001
Performance (km) 39.91±4.26 42.03±4.78 40.55±5.21 NS
Change in body weight (kg) 2.1±1 1.8±1.2 1.9±1.9 NS
Change in packed cell volume -0.012±0.024 -0.018±0.024 -0.015±0.019 NS
mean corpuscular volume (fl) -0.81±1.10 -0.85±0.90 -0.58±1.42 NS

 *Differed significantly from High Sodium Concentration Trial




Trial 1

Measures and confidence intervals

Trial 2

Measures and confidence intervals

Trial 3

 Measures and confidence intervals

Statistical Significance of Group Difference

Prerun Sodium Plasma (mmol/l)





Postrun Sodium Plasma (mmol/l)





Change in Sodium Plasma (mmol/l) -3.5±2.8 -5.3±2.3 -4.3±3.1 NS
Change in Sodium Plasma/kilometer (mmol/km) -0.10±0.08 -0.12±0.05 -0.10±0.07 NS
Performance (km) 36.43±2.81 43.2±3.91** 42.86±4.01** p<0.0001
Change in body weight (kg) 2.4±1.0 1.3±1.1** 2.0±1.2 p<0.03
Change in packed cell volume -0.006±0.024 -0.028±0.017** -0.010±0.020 p<0.03
mean corpuscular volume (fl) 0.08±0.90 -2.00±0.39** -0.32±0.69*** p=0.0001

 ** Differed significantly from trial 1

***Differed significantly from trial 2

Other Findings
































Author Conclusion:


  • At least 680 mg/h sodium for endurance exercise of 4 hours or more in women who are in a state of fluid overload to minimize the risk of hyponatraemia
  • Fluid intake of less than 1 liter/hour for women during prolonged exercise because consuming 1 liter/hour during a four hour run was sufficient to cause fluid overload in most subjects in the study
Funding Source:
ECR Pharma GmbH (Switzerland)
Pharmaceutical/Dietary Supplement Company:
Reviewer Comments:

Other fluid recommendations noted in article introduction

  • American College of Sports Medicine:
    • 0.6-1.2 liters/hour for shorter distances
  • Burke (ref 11):
    • 0.5-1 liter/hour in ultradistance events
  • Noakes (ref 12):
    • 0.5 liters/hour for less competitive athletes during prolonged exercise
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? 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? ???
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? ???
  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? ???
  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? ???
  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? 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)? 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? 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? No
  10.2. Was the study free from apparent conflict of interest? ???