Hydration and Physical Activity

Study Design:
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Quality Rating:
Research Purpose:

To compare the effects of two beverages with differing carbohydrate and electrolyte concentrations and an indistinguishable water placebo during endurance cycling. 

Inclusion Criteria:

Well-trained cyclists; informed consent

Exclusion Criteria:

none mentioned

Description of Study Protocol:

Recruitment not mentioned


Design randomized crossover (3 conditions)


Blinding used (if applicable) Double-blind, counter-balanced assignment of conditions


Intervention (if applicable) 3 drinks given during endurance cycling: 

flavored water placebo (50 mOsm/l);

low-carbohydrate-electrolyte solution (CES, 25 g/l glu, 10.2 mmol/l Na, 4.86 mmol/l K, 187 mOsm/l);

moderate-CES (40 g/l sucrose, 20 g/l glu, 20.4 mmol/l Na, 3.44 mmol/l K, 360 mOsm/l).


Statistical Analysis

 repeated measures ANOVA as a randomized block design (subjects as blocking factor); multiple comparisons among individual means made with Student-Newman-Keuls procedure. 

Pre-determined level of significance not stated in Analysis section of paper.

Data Collection Summary:

Timing of Measurements

VO2max determined prior to testing.

10h fast prior to testing; Weighed morning of testing.

Rectal themometer (temperature) and catheter (blood sampling) inserted

Each exercise session consisted of 2 exercise bouts (T1, T2) separated by 30 min recovery period; blood sample collected before tesing, and during rest period.  Weight recorded before and after exercise.

T1: 2h continuous cycling at 75% VO2max; 90 rpm cadence (or 10,800 pedal revolutions)

30 min rest

T2: 2700 pedal revolutions as fast as possible

275 ml of 1 of 3 drinks given during exercise every 20 min

 First drink and drink consumed during rest period labeled with 20 g 99.8 D2O

Dependent Variables

  • Performance (time to complete T1 and T2)
  • HR, temperature
  • Expired gas (Douglas bag method)
  • Hct, Hgb (cyanomethoglobin)
  • change in Pvol (from Hct and Hgb)
  • Plasma glucose (glucose analyzer)
  • Sweat rates (calculated from body weight loss corrected for fluid intake, blood sampling, respiratory water loss)
  • Plasma D2O concentration in body water (fixed-filter infrared analyzer), <2% variability intra-; <10% variability interassay

Independent Variables

 Drink condition:  water, low-CES, moderate CES:

flavored water placebo (50 mOsm/l);

low-carbohydrate-electrolyte solution (CES, 25 g/l glu, 10.2 mmol/l Na, 4.86 mmol/l K, 187 mOsm/l);

moderate-CES (40 g/l sucrose, 20 g/l glu, 20.4 mmol/l Na, 3.44 mmol/l K, 360 mOsm/l).

Control Variables

lab environment (high heat stress: 26.6-27.7C, 67-68% RH), no fans during exercise.

cadence  and resistance maintained throughout testing.


Description of Actual Data Sample:


Initial N: 19M

Attrition (final N): 19 M

Age: 16-26 yr

Ethnicity: not mentioned

Other relevant demographics: none mentioned

Anthropometrics (e.g., were groups same or different on important measures)

4.47±0.10 l/min VO2max; 70.1±2.1 kg; ht not mentioned

Location: Lab, Exercise Biochemistry Laboratory, Dept of Physical Education, University S. Carolina, Columbia, SC.


Summary of Results:






Statistical Significance of Group Difference

Sweat rate (g/m2)





Performance time, T1  (min)





Performance time, T2 (min)




 P<0.05 vs water

Combined tasks (min) 163.0±3.1 159.9±2.6 159.3±2.4 NS


Other Findings

 NS differences in HR, O2 consumption,, D2O accumulation, sweat rate, Hgb, Hct, change in Pvol. 

*Mean±SEM not given*  Plasma glucose was altered in the water vs the 2 CES conditions (glucose maintained, p<0.01), as in the water condition, glucose rose slightly during first 30-40 min, then fell back to baseline, then decreased during the rest period before increaseing again to only 86% of baseline values (P<0.05).

*Mean±SEM not given* RER significantly higher with Mod-CES than with Low-CES and water (P<0.05).

Sweat rates, though NS, corresponded to 1.6l lost during exercise session. 

*Mean±SEM not given* Subjects consumed 2.5L of fluid during each session, though they still lost an average of 2.3 kg body weight (gross weight loss of 4.8 kg). 


Author Conclusion:

Moderate CES do not adversely affect fluid replenishment during prolonged exercise when compared with water or low-CES.  Moderate CES can help maintain blood glucose and enhance performance without compromising fluid replenishment. 

Funding Source:
University/Hospital: University of South Carolina
Reviewer Comments:

Mean±SEM not reported for much of the data (is presented all in Figures).  Good design, though I had expected more from the D2O analysis.

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? 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? No
  2.4. Were the subjects/patients a representative sample of the relevant population? ???
3. Were study groups comparable? N/A
  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? 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")? 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? 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.) N/A
  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? 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? ???
  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? No
  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? ???
  8.7. If negative findings, was a power calculation reported to address type 2 error? No
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