Hydration and Physical Activity

Study Design:
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Quality Rating:
Research Purpose:

To examine whether atheletes, who typically replace only ~ 50% of their fluid losses during moderate-duration enduration exercise, should attempt to replace their sodium (Na+) losses to maintain extracellular fluid (ECF) volume.

Inter-compartmental fluid shifts with partial fluid replacement and complete electrolyte replacement were studied in six male cyclists who performed three, 90-minute rides at 65% of Peak O2 in a 32oC environment and ingested either no fluid (NF), or 1.2 1 of water (W) or saline (S) containing 10 mmol NaCl/l.

Inclusion Criteria:

Subjects were:

  • male
  • cyclists
  • routinely rode at least 100 km/week
Exclusion Criteria:

Not discussed


Description of Study Protocol:

Recruitment --Not discussed


Design --Randomized cross-over; 3 experimental trials each separated by 1 week


Blinding used (if applicable)--N/A


Intervention (if applicable)--

Each subject performed three 90 minute rides at 65% of peak VO2PEAK uptake in a 320 C laboratory environment and at random ingested either no fluid (NF), 1.2 l of water (W), or saline (S) containing 100 mmol of NaCl·1-1 to replace electrolyte losses. Each ride was separated by 1 week and were undertaken at the same time of day after 24 hours of no physical activity.

During 24 hours prior to each trial, subjects consumed a high CHO (>8g /kg body weight) with generous water; no caffeine or alcohol. On the morning of each trial, 2 hours before coming to the laboratory, subjects consumed a standardized breakfast consisting of 2-4 slices toast (~100 g CHO) and 250 ml H2O.

For each trial, subjects entered an environmental chamber that was set at ambient temperature of 32oC, a relative humidity of 55% and a wind speed of 4 km/ hr. The subjects cycled at 65% of their previously determined VO2peak for 90 minutes. During one trial, the subjects either consumed no fluid or they ingested 400 ml of water at the start of the exercise and then 100 ml of water every 10 min until 10 minutes before the end of the ride (1.21 l total). In one fluid trial, the subjects drank only tap water and, in the other fluid trial, they also ingested gelatin capsules containing NaCl to increase the final Na+ of the ingested solution to 100 mEq / l-1.


Statistical Analysis

Differences between trials were assessed with a Latin Square analysis of variance (ANOVA)

Within-treatment differences over time were assessed using a two-way ANOVA

Fluid-cation balance in the three trials was compared with the Wilks' Lambda statistic

Results are presented as means ± SEM

p<0.05 was considered statistically significant


Data Collection Summary:

Timing of Measurements

Preliminary--Each subject's height and weight were collected using standardized techniques.  VO2peak was determined 1 week before the experimental tirals in an incremental exercise test to exhaustion on an electronic cycle.

Baseline-- Each subject was weighed and a urine sample was collected. A three-lead electrocardiogram was attached to monitor heart rate during the trials

During exercise--Sweat was collected into a electrolyte-free bag attached to alternate forearms every 20 minutes during exercise. Blood samples were collected at rest and every 10 minutes during exercise.

At the end of each trial--Each subject towelled dry and weight, urine and blood samples were collected using standardized techniques.

Dependent Variables

  • Change in heart rate during trials
  • Change in plasma volume (PV)--hematocrit and hemoglobin used to estimate changes in PV during exercise
  • Change in plasma Na+, Cl-, and protein concentrations--changes were calculated after the first 10 minutes of the trials 
  • Inter-compartmental fluid shifts (interstitial [ISF] and intracellular [ICF] volumes)--calculated from Cl- movements, falls in PV, and total body water losses at between 10 and 90 minutes of the 3 trials; calculated after the first 10 minutes of the trials to eliminate the fluid shift in the transition from rest to exercise that is unrelated to dehydration. 
  • Change in fluid and electrolyte balances

Independent Variables

  • Ingestion of no fluid (NF), or water (W), or saline containing 100 mEq Na+/liter (S) 

Control Variables


Description of Actual Data Sample:

Initial N: 6 males

Attrition (final N): 6 males

Age: 24 ± 2 y (mean ± SEM)

Ethnicity: not discussed

Other relevant demographics: not discussed

Anthropometrics: (mean ± SEM): height 180±1 cm; weight 78±2kg; predicted surface area 1.97±0.33 m2; estimated PV 321.5±44 ml; VO2peak 4.6±0.2 l/min

Location: South Africa


Summary of Results:


Changes in heart rate and plasma volume following trials:

  • NF final heart rates were ~10 beats/min higher (168±2 beats/min) than with W and S (159±2 and 157±1 beats/min, respectively) (p < 0.005).
  • Higher heart rates in NF were associated with significant falls in plasma volume (PV) compared to W and S (p < 0.05). In the NF trial, PV fell by ~10% in the first 10 minutes of exercise and then continued to decline to almost -14% at the end of the trial (p < 0.005). In contrast, in the W and S trials, PV only decreased during the first 10 minutes of exercise and remained constant at around -10% at the end of the trials.

Plasma sodium, chloride, and protein concentrations during exercise:

  • The maintenance of PV in the W trial appeared to prevent further rises in plasma Na+, Cl- and protein concentrations after 10 minutes of exercise.
  • Final increases in plasma concentrations of Na+ and Cl- in the NF and S trials (~4 mEq/ l) and rises in the plasma concentrations of protein in the NF trial were significant (p < 0.005).

Fluid and electrolyte balances at the end of exercise:

  • Differences in plasma electrolyte concentrations had little influence on the volumes and electrolyte content of sweat and urine in the three trials.
  • The main effect of W and S were on body fluid shifts. During the NF trial, PV and interstitial fluid (ISF) and intracellular fluid (ICF) volumes decreased by ~0.1, 1.2, and 1.0 l, respectively. In the W trial, 1.2 l fluid and 120 mEq Na+ losses contracted the ISF fluid, and in the S trial, the movement of water from the ICF maintained ISF volume.  



Author Conclusion:

The main effects of water and saline ingestion were on fluid and Na+ balances at the end of the trials. Water and saline ingestion decreased final fluid deficits from around 2.4 l to 1.2 l, and saline ingestion reduced net Na+ losses from around 120 mEq to 25 mEq.

Since water and saline trials were equally effective in maintaining PV, Na+ ingestion may not be of much advantage to athletes who typically replace only ~50% of their fluid losses during competitive endurance exercise.

Funding Source:
Government: SA MRC, Foundation of Research and Development (South Africa)
Bromor Foods (South Africa)
Food Company:
University/Hospital: University of Cape Town
Reviewer Comments:

Limitations include:

  • Controlled environment may not reflect variable field conditions under which cyclists perform
  • Little information provided about subjects other than group means on age, weight, and height. Only 6 subjects.
  • Questionable generalizability; subjects were male endurance competitive cyclists; results may not apply to other athletes or age group
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? ???
  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? No
  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) No
  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? ???
  4.1. Were follow-up methods described and the same for all groups? No
  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%.) N/A
  4.3. Were all enrolled subjects/patients (in the original sample) accounted for? ???
  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? N/A
  5.1. In intervention study, were subjects, clinicians/practitioners, and investigators blinded to treatment group, as appropriate? N/A
  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? ???
  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? ???
  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)? Yes
  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? Yes
  10.1. Were sources of funding and investigators' affiliations described? Yes
  10.2. Was the study free from apparent conflict of interest? Yes