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Hydration and Physical Activity


Davis JM, Burgess WA, Slentz CA, Bartoli WP, Pate RR.  Effects of ingesting 6% and 12% glucose/electrolyte beverages during prolonged intermittent cycling in the heat.  Eur J Appl Physiol 1988; 57: 563-69.

PubMed ID: 3396573
Study Design:
Randomized Crossover Trial
C - Click here for explanation of classification scheme.
Quality Rating:
Neutral NEUTRAL: See Quality Criteria Checklist below.
Research Purpose:

To examine the effects of ingesting a 12% CES on fluid absorption, circulatory and thermoregulatory function and work performance during prolonged exercise in a warm environment.  A 6% CES was also tested in order to confirm previous findings under different exercise conditions.

Inclusion Criteria:

Well-trained, male cyclists; informed consent

Exclusion Criteria:

none mentioned.

Description of Study Protocol:


 not mentioned.


 Randomized crossover, 3 conditions (double-blind, counter-balanced assignment)

Blinding used (if applicable)

 double-blind, counterbalanced assignment of conditions

Intervention (if applicable)

 275 ml of 1 of 3 beverages administered every 20 min during cycling session;  first drink contained 20 ml D2O


6% CES (6% glu, 20 mmol/l Na, 3.5 mmol/l K)

12% CES (12% glu, 20 mmol/l Na, 3.5 mmol/l K)

Statistical Analysis

 Repeated measures (type of beverage, time of measurement) ANOVA, with randomized block design (subjects as blocking factor).  Multiple comparisons among individual means using orthogonal contrasts; questionnaire data analyzed with Chi Square.  Sig level P<0.05

Data Collection Summary:

Timing of Measurements

 Prior to testing:  Peak O2 uptake determined

Test sessions: after 10 h fast, weighed; rectal thermometer inserted; catheter inserted for blood sampling.

Exercise session included:

2 exercise bouts, E1, E2; 3 Performance bouts, P1, P2, P3:

E1 = work performed during 1h continuous cycling at 65% VO2max; P1 = at end of E1, pedaled additional 270 revolutions as quickly as possible (est 90% VO2max).  After 5 min rest period, cycle repeated (E2, P2).  20 min rest period after P2, then 5 min warm-up followed by another performance ride (P3).  Expired gass collected at 1, 10, 30, 50 min;

Consumed 275 ml of 1 of 3 beverages (water, 6% CES, 12% CES) every 20 min during cycling.  First drink had 20 ml D2O tracer. 

End of testing: body weight recorded.

Dependent Variables

  • RER, VO2 max form Expired gas (meteorological balloons)
  • Inspired air vol (Parkinson Cowan gasometer)
  • Body weights
  • Hct, Hgb (cyanmethemoglobin method)
  • Pvol change (Dill & Costill)
  • Plasma glu (enzymatic analyzer)
  • Sweat rates (body weight loss corrected for fluid intake and other losses)
  • Plasma D2O (infrared analyzer)
  • Palatability questionnaire for beverages
  • Subjective ratings of incidence of GI distress

Independent Variables

 3 drink conditions administered during exercise (see above)

Control Variables

 Lab environment 26.6-27.7 C; 67-68% RH; resistance/workload/ RPMs maintained; timing of measurements.

Description of Actual Data Sample:


Initial N: N=15 M,  well-trained cyclists

Attrition (final N): N=15 M

Age: 20-31 yr

Ethnicity: not mentioned

Other relevant demographics: none mentioned.


63.0±6.0 ml/kg/min peak aerobic power; 73.5±9.7 kg;  no others mentioned.


 Exercis eBiochemistry Lab, College of Health, Univ SC, Columbia, SC.

Summary of Results:

Mean Performance Times from subjects who completed all tasks under all drink conditions (N=7) 



6% CES

12% CES

Statistical Significance of Group Difference

P1 (s)





P2 (s)





P3 (s)





 Several subjects were unable to maintain required work output during E2 under some of the treatments, and their performance times were not included in the analysis.

Other Findings

 NS differences between the 3 drink conditions for HR, VO2, RER temp, plasma volume, performance .

Plasma glu: significant differences (P<0.05) between water and both 6 and 12% CES between 30 and 60 min of E1, between 20 and 60 min E2, and at P3. 

Body weight loss and Sweat rate: Sweat rate:  1.75, 1.68, 1.67 l/h for water, 6%, and 12% CES, respectively (NS).  Body weight loss: 3.49±0.2, 3.35±0.18, 3.34±0.19 kg for water, 6% and 12%CES, repsectively.

D2O:  main effects for drink and drink*time sig (P<0.05):  sig slower with 12% CES than 6% CES or water from 30 min of E1 until 10 min E2. 

Subjective ratings:  12% CES sig higher (P<0.05) in stomach upset, nausea and fullness ratings than 6% CES or water. 

Author Conclusion:

Beverages similar to 12% CES are absorbed into body fluids more slowly than water or a 6% CES.  because these beverages remain in the stomach longer, they may increase the risk of GI distress.

Funding Source:
University/Hospital: University of South Carolina
Reviewer Comments:
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? N/A
  2.3. Were health, demographics, and other characteristics of subjects described? ???
  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) Yes
  3.2. Were distribution of disease status, prognostic factors, and other factors (e.g., demographics) similar across study groups at baseline? N/A
  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? Yes
  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? 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)? 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