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Below PR, Mora-Rodriguez R, Gonzalez-Alonso J, Coyle EF. Fluid and carbohydrate ingestion independently improve performance during 1h of intense exercise. Med Sci Sports Exerc. 1995; 27 (2): 200-210.

PubMed ID: 7723643
Study Design: 
Randomized crossover trial
A - Click here for explanation of classification scheme.
Quality Rating: 
NEUTRAL: See Quality Criteria Checklist below.
Research Purpose: 
The purpose of this study is to test the individual and combined effects of carbohydrate ingestion and fluid replacement on performance, core temperature and cardiovascular responses during intense exercise lasting one hour (80% of VO2max).
Inclusion Criteria: 
Endurance-trained males.
Exclusion Criteria: 
No mention of exclusion criteria.
Description of Study Protocol: 
  • Recruitment: Eight endurance-trained subjects completed the study, gave written informed consent and were financially compensated
  • Design: Randomized crossover trial. Experimental trials were chosen to fit a factorial design to evaluate the main effects of two factors and the potential interaction between them.
  • Blinding used: Double-blind. Prior to participation, the subjects were told they would be recieving four different performance-enhancing treatments. Experimenter did not know which treatment the subjects were getting at each of the four trials.


Preliminary Testing
Subjects underwent preliminary testing on a cycle ergomenter to determine VO2max and blood lactate threshold (LT). All subjects also completed a heat acclimation protocol consisting of four one-hour cycling bouts performaed in a warm environment over a 10-day period. The final acclimation bout was completed at a work rate which elicited a VO2 of 5% above LT and was used to faliliarize the subjects with the experimental procedures and to measure the individuals' sweat rate for calculation of fluid replacement volumes during trials.

Experimental Design

  • Subjects reported to the lab at the same time of day, fasted and euhydrated
  • Subjects consumed similar diets the day before each trial and refrained from exercise
  • Upon arrival, subjects had esophageal and rectal thermometers inserted in addition to a catheter into an antecubital vein
  • Subjects cycled continuously on a stationary ergometer for 50 minutes in a warm room at a constant work rate that elicited a VO2 of 5% above their LT on four different occasions that were separated by at least 72 hours
  • During these exercise bouts, the subjects randomly received
    • Fluid and carbohydrates in the form of a 6% CHO electrolyte solution (Gatorade)
    • Fluid in the form of a water-electrolyte solution
    • Carbohydrates in the form of a 40% maltodextrin electrolyte solution
    • Placebo capsles containing electrolytes.
  • Carbohydrates given in each of the CHO trials was 79±4g
  • Electrolyte content for each trial averaged 619±29mg of sodium and 141±7mg of potassium.

Performance Testing

  • Follwing the 50-minute period of constant work, subjects began a cycling performance test, which required the completion of a set amount of work in the shortest time possible
  • Subjects had to maintain an intensity elicing a VO2 of 10% above his LT.

Statistical Analysis

  • Data collected before treatments were administered were analyzed using one-way ANOVA with repeated measures
  • Data of the exercise bouts collected and repeated over time were analyzed using a three-way ANOVA with repeated measures
  • Measurements made only once used a two-way ANOVA with repeated measures
  • Giesser-Greenhouse or Box adjustment procedures were applied to all F tests
  • Differences between means were uncovered with Tukey's HSD post-hoc procedure
  • P<0.05.
Data Collection Summary: 

Timing of Measurements:

  • Immediately Pre and Post - Nude weight was measured
  • Drinking schedule was immediately before, at 15 minutees, at 25 minutes, and 34 minutes.
  • During the 50 min steady state test each of the following was measured every 10 min: skin temp, esophageal temp, heart rate, RPE
  • During the 50 min steady state test each of the following was measured every 5 min: oxygen consumption, RER
  • The cardic output was measured in triplicate during an 8min period beginning at 34 min of exercise
  • A 10 ml blood sample was drawn at 7,25, and 50 min of exercise
  • Immediately upon cessation of the performance test the following were measured: skin temp, esophageal temp, rectal temp, HR, 10 mL blood sample drawn

Dependent Variables

  • Body Temperature: Esophageal termistor, Rectal thermistor, Surface Thermistor
  • Oxygen Consumption: Daniels valve and dry gas meter
  • Cardiac Output: CO2 rebreathing technique of Collier using the Fick equation
  • Sweat Rate: Loss of nude body weight
  • Blood analysis: Serum sodium, plasma glucose, plasma lactate, plasma insulin

Independent Variables:

  • During these exercise bouts, thh subjects randomly recieved
    • Fluid and Carbohydrate in the form of a 6% CHO electrolyte solution (gatorade)
    • Fluid in the form of a water-electrolyte solution
    • Carbohydrate in the form of a 40% maltodextrin electrolyte solution
    • Placebo capsles containing electrolytes
    • Carbohydrate given in each of the CHO trials was 79+ 4g
    • Electrolyte content for each trials averaged 619 + 29 mg of sodium and 141 + 7 mg of potassium
  • Control Variables:

    • Temperature of the room and % of VO2 exercise intensity

  • Description of Actual Data Sample: 
    • Initial N: Eight endurance-trained males
    • Attrition (final N): Eight
    • Age: 23±1
    • Ethnicity: Not mentioned.


    • Weight: 70.6±3.0kg
    • VO2: 4.44±0.08 1*min-1


    • Texas.
    Summary of Results: 




    Fluid Fluid + Carbohydrate

    Statistical Significance of Group Difference

    Oxygen Consumption (l*min -1)





    No significant difference

    Hydration Status Sweat Rate (l/h)



    1.35±0.08 1.32±0.07

    No significant difference

    Performance Test



    9.93±0.28 10.22±0.27

    CHO significantly faster than NO CHO and the fluid + CHO was significantly faster than the carbohydrates-alone

    Cardiovascular Response (HR post b/min-1) 179±3
    Author Conclusion: 
    • The main finding is that both fluid replacement and carbohydrate ingestion individually, improve high-intensity cycling performance
    • Performance times were improved by an equal magnitude when subjects consumed either fluid or carbohydrates, in comparison to a placebo. Furthermore, when both fluid and carbohydrates were combined, performance was improved two-fold in comparison to the placebo.
    • Therefore, the ingestion of carbohydrates and fluid can significantly improve cycling performance in comparison to the ingestion of only water.
    Funding Source: 
    Gatorade Sports Science Institute, Exercise Physiology Laboratory
    Food Company:
    University/Hospital: University of Texas at Austin
    Reviewer Comments: 
    • Inclusion/exclusion criteria and recruitment methods not well defined.
    • A good effort was taken to control the study but still maintain a real life situation. I think that these studyies should use a 24 hour recall or some other method to assess overall carbohydrate consumption.

    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)
      2. Did the authors study an outcome (dependent variable) or topic that the patients/clients/population group would care about?
      3. Is the focus of the intervention or procedure (independent variable) or topic of study a common issue of concern to dieteticspractice?
      4. Is the intervention or procedure feasible? (NA for some epidemiological studies)
    Validity Questions
    1. Was the research question clearly stated?
      1.1. Was (were) the specific intervention(s) or procedure(s) [independent variable(s)] identified?
      1.2. Was (were) the outcome(s) [dependent variable(s)] clearly indicated?
      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?
      2.2. Were criteria applied equally to all study groups?
      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?
      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 used? (Concurrent preferred over historical controls.)
      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?
      3.5. If case control or cross-sectional 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. Criterion may not be applicable in some cross-sectional studies.)
      3.6. If diagnostic test, was there an independent blind comparison with an appropriate reference standard (e.g., "gold standard")?
    4. Was method of handling withdrawals described?
      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%.)
      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?
    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?
      5.4. In case control study, was case definition explicit and case ascertainment not influenced by exposure status?
      5.5. In diagnostic study, were test results blinded to patient history and other test results?
    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?
      6.2. In observational study, were interventions, study settings, and clinicians/provider described?
      6.3. Was the intensity and duration of the intervention or exposure factor sufficient to produce a meaningful effect?
      6.4. Was the amount of exposure and, if relevant, subject/patient compliance measured?
      6.5. Were co-interventions (e.g., ancillary treatments, other therapies) described?
      6.6. Were extra or unplanned treatments described?
      6.7. Was the information for 6.4, 6.5, and 6.6 assessed the same way for all groups?
      6.8. In diagnostic study, were details of test administration and replication sufficient?
    7. Were outcomes clearly defined and the measurements valid and reliable?
      7.1. Were primary and secondary endpoints described and relevant to the question?
      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?
      7.4. Were the observations and measurements based on standard, valid, and reliable data collection instruments/tests/procedures?
      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?
      7.7. Were the measurements conducted consistently across groups?
    8. Was the statistical analysis appropriate for the study design and type of outcome indicators?
      8.1. Were statistical analyses adequately described and the results reported appropriately?
      8.2. Were correct statistical tests used and assumptions of test not violated?
      8.3. Were statistics reported with levels of significance and/or confidence intervals?
      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)?
      8.5. Were adequate adjustments made for effects of confounding factors that might have affected the outcomes (e.g., multivariate analyses)?
      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?
    9. Are conclusions supported by results with biases and limitations taken into consideration?
      9.1. Is there a discussion of findings?
      9.2. Are biases and study limitations identified and discussed?
    10. Is bias due to study's funding or sponsorship unlikely?
      10.1. Were sources of funding and investigators' affiliations described?
      10.2. Was the study free from apparent conflict of interest?
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