Hydration and Physical Activity


Singh R, Brouns F, Kovacs E.  The effects of rehydration on cycling performance after exercise-induced dehydration.  Southeast Asian J Trop Med Public Health 2002; 33 (2): 378-88.

Study Design:
Non-Randomized Controlled Trial
C - Click here for explanation of classification scheme.
Quality Rating:
Neutral NEUTRAL: See Quality Criteria Checklist below.
Research Purpose:
To examine the effect of rehydration on fluid balance, metabolic function, and cycling performance following exercise-induced dehydration; either a high-Na CES or a placebo was used.
Inclusion Criteria:
healthy male athletes; informed consent; no others mentioned
Exclusion Criteria:
none mentioned
Description of Study Protocol:

Recruitment not mentioned


Design experimental


Blinding used (if applicable) none mentioned


Intervention (if applicable) high-Na CES  or placebo beverage given during a rehydration period (after a period of exercise-induced dehydration and before a timed cycling trial).


Statistical Analysis Comparisons between the two beverages was made using the overall mean over ime; further analyzed by the Wilcoxon signed rank test.  Significant differences between beverages determined using Wilcoxon signed rank test; Significance level of P<0.05.


Data Collection Summary:

Timing of Measurements

Preliminary maximal physical work capacity determined on cycle. 

Day of testing:  Pre-test:  Void bladder (volume collected); body weight measured.  Seated for placement of venous catheter for blood sampling; initial blood sample taken.

90-min Exercise-induced dehydration: Environmental chamber (28C, 63% RH) at 50% Wmax for 90 min to dehydrate 3% body weight; blood samples taken at 1 minute and 90 min of exercise; 10 min cool-down, take seated position, 4th sample obtained after 30 min (post-ex), body weight measured, urine sample collected.

Two-hour Rehydration:  Seated, drank one of twon test drinks (7.6% CES vs sweetened placebo) in volume equal to mass (g) of 50% fluid lost.  After 30 min, drank 40% of fluid lost; remaining 30% (to replace 120% lost) taken at 60 min.  Blood smaples collected every 30 min during rehydration; body weight measured at tend of rehydration.

Time-trial:  time taken to complete target amount of work (J) on cycle (based on Wmax) = 0.75*Wmax*3600;  final body weight taken after time trial, final urine sample collected.

Dependent Variables

  • Hgb (cyanmethemoglobin)
  • Hct (microcentrifugation)
  • Pvol (Dill & Costill method)
  • PNa and PCl (electrolyte analyzer)
  • Posm & Uosm (freezing point depression)
  • Pglu and PFFA (COBAS)
  • PInsulin (radioimmunoassay)
  • Urine Na, K, Cl (flame photometry); Mg, Ca (photometric method-COBAS)
  • Urine output & Net fluid balance (based on body mass loss, volume fluid ingested, urinary volume)
  • Body mass loss
  • Percent of weight lost regained, %rehydration (as index of whole-body rehydration, methods of Gonzalez-Alonso et al, 1992; Mitchell et al, 1994)


Independent Variables

 High-Na 7.6% CES (690 mg/l Na; 303 mOsm/kg)  vs. Placebo beverage (41mOsm/kg); time

Control Variables

 Room temperature during dehydration (28C, 63% RH), reydration (20C); standardized meal intake before testing; timing of beverage ingestion; volume fluid ingested

Description of Actual Data Sample:


Initial N: 13 healthy male competitive cyclists, triathletes, recreational athletes.

Attrition (final N): 13 M

Age: 23±1.2 years

Ethnicity: not mentioned

Other relevant demographics: none mentioned

Anthropometrics 71.9±0.9 kg; 183.2±1.6 cm; 391.6±10.4 W (physical working capacity)

Location: Lab, Department of Human Biology, Maastricht University, The Netherlands


Summary of Results:



Hi-Na CES Group


Placebo group


Statistical Significance of Group Difference

Time trial (min)




HR (bpm)




Power output (W)




Percent Wmax (%) 70.1±2.3 70.1±2.4 ns
 Pre-Ex; Post-Dehy; Post-Time Trial      
 Pna (mmol/l)  145.6±0.4; 150.4±0.5*; 151.2±5*  145.9±0.6; 150.6±0.4*; 149.4±0.6*,^

 *P<0.01, sig diff from euhydrated state

^P<0.05, sig diff from CES

 Pcl (mmol/l)  108.1±0.4; 112.5±0.4*; 109.9±0.4*  108.2±0.5; 113.2±0.5*; 109.2±0.6  *P<0.01
 Pk (mmol/l)  4.19±0.08; 5.25±0.07*; 5.35±0.16*  4.22±0.09; 5.27±0.08*; 5.67±0.14*  *P<0.01
Plasma FFA (mmol) 0.21±0.03; 0.64±0.07*; 0.42±0.06* 0.23±0.05; 0.72±0.07*; 0.76±0.11*, ^ *P<0.01, sig diff from euhydrated state

^P<0.05, sig diff from CES

Plasma Insulin (U/l)

8.4±1.6; 49.5±11.0*; 3.2±0.9*

6.9±1.1; 5.3±0.4; 3.1±0.3 *P<0.01, sig higher than P
 Body weight loss (rehy), g  375.4±93.7  645.4±142.8*  *P<0.01


Body weight decreased by 3.2±0.1%; during rehydration, subjects drank 2.72±0.11 kg. 

Percent body weight loss that was regained at end of 2-h rehydration (%rehydration) during CES (70±3%) was sig (p<0.01) higher than placebo (60±5%).  Difference in body weight between CES and P was 270±90 g (p<0.01). 

Rehydration index was used for body weight restoration, calculated as 1.85±0.17 for CES and was sig lower (p<0.01) than P (3.30±0.78). 

After drinking a volume equal to 120% of the body fluid loss during exercise, a higher percent rehydration was observed with CES, though there was no difference in cycling performance between the two drink conditions. 

No differences in urine output between 2 conditions.  Net fluid balance was positive with CES with 1.5 hours of rehydration period (128.1±49.5 ml versus -36.5±83.8 ml, p<0.05), which then became negative at the end of the 2-hr rehydration period, but still sig (p<0.01) less negative than the net fluid balance with P. 

Posm sig higher with CES than with P throughout study (p<0.05). 

Fall in plasma glucose level during timed trial was directly proportional to time taken to complete targeted work in the timed trial, indicating the larger the fall in glucose, slower the time taken to complete work (r=0.69, p<0.01).


Other Findings


Author Conclusion:

After drinking a volume equal to 120% body fluid loss during exercise, although higher percent rehydration was observed with the CES, there was no difference in performance between the 2 drinks. 

Ingesting 120% of body fluid loss was able to retore Pvol balance to euhydrated level with both drinks, but Pvol at end of 3 hr rehydration period was sig higher with CES (P<0.01). 

Despite the more effective rehydration given by high-Na CES after exercise-induced dehydration, CES ingestion during the 2-hour rehydration period prior to a timed trial of cycling performance results in a lowering of plasma glucose and a decrease in FFA availability during vigorous exercise, which may be mediated by hyperinsulinemia.

Funding Source:
Isostar, Sandoze Nutrition
Food Company:
Pharmaceutical/Dietary Supplement Company:
Reviewer Comments:

HOW treatments were delivered to subjects (i.e. randomized, blinding; counterbalanced/within-subject/repeated measures) not adequately described. 

ASSUMING a between-subject, non-RCT.

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) N/A
  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? Yes
  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? Yes
  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? Yes
3. Were study groups comparable? ???
  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? ???
  3.3. Were concurrent controls or comparisons used? (Concurrent preferred over historical control or comparison groups.) ???
  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")? Yes
4. Was method of handling withdrawals described? N/A
  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%.) N/A
  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? ???
  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.) ???
  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? ???
  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? Yes
  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? Yes
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? 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? ???
  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? Yes
  10.1. Were sources of funding and investigators' affiliations described? Yes
  10.2. Was the study free from apparent conflict of interest? Yes