Hydration and Physical Activity
Citation:
Millard-Stafford ML, Sparling PB, Rosskopf LB, Snow TK. Should carbohydrate concentration of a sports drink be less than 8% during exercise in the heat? Int J Sports Nutr Exerc Metab. 2005; 15 (2): 117-130.
PubMed ID: 16089271Study Design:
Randomized crossover trial
Class:
A - Click here for explanation of classification scheme.
Quality Rating:

Research Purpose:
To determine if commercially available 6% sucrose-glucose and 8% fructose-glucose-maltodextrin carbohydrate-electrolyte drinks would differentially affect physiological responses, relative fluid uptake and run performance, compared to water placebo in the heat.
Inclusion Criteria:
- Highly-trained male distance runners
- Aged less than 40 years
- Training of 110km per week for the last three months with no anticipated change in training throughout the duration of the study
- Use of programmed drinking during training
- Completion of a 32-kilometer training run in the month before the study.
Exclusion Criteria:
Excluded if not included above.
Description of Study Protocol:
- Recruitment: Methods not specified; all were from the Georgia Tech cross-country team
- Design: Randomized crossover trial
- Blinding used: Double-blind.
- Subjects ran 32km while ingesting a placebo, 6% or 8% carbohydrate-electrolyte solutions
- At 15km, a 250ml drink labeled with deuterium oxide was ingested
- Trial was separated by 14 days
- 400ml consumed 15 minutes before the run and 250ml was consumed every five km.
Statistical Analysis
- A two-way ANOVA with repeated measures was used to determine differences between trials
- ANCOVA was utilized on run performance data to account for different heat indices on different testing days.
Data Collection Summary:
Timing of Measurements
- Heart rate, ratings of perceived exertion and core temperature were measured throughout the run
- Respiratory gases collected at five, 15, 21 and 27km
- Blood samples were obtained before exercise, at selected intervals during exercise (not mentioned) and immediately after exercise.
Dependent Variables
- Heart rate was measured using telemetry
- Core temperature was measured with a telethermometer
- RPE was measured with a Borg scale
- Respiratory gases were collected using a Douglas bag
- Blood samples were analyzed for serum glucose, electrolytes, hemoglobin, lactate, hematocrit.
Independent Variables
Placebo, 6% or 8% carbohydrate-electrolyte solutions.Control Variables
Runners were asked to follow a standard-normal mixed diet (55% CHO, 15% protein, 30% fat) and to duplicate training for three days prior to testing.Description of Actual Data Sample:
- Initial N: 10 men
- Attrition (final N): 10
- Age: Mean, 23.7±3.6 years
- Ethnicity: Not mentioned
- Location: Georgia.
Summary of Results:
Other Findings
- Environmental conditions were not similar on all testing days
- Blood glucose and respiratory exchange ratio were significantly higher for the 6% and 8% carbohydrate-electrolyte drinks, compared to placebo (P<0.05)
- There were no significant effects of the treatments on relative change in plasma volume, plasma osmolality or serum electrolytes
- Rectal temperature at 32km was higher for the 8% compared to placebo, but similar to the 6% solution
- Deuterium oxide accumulation was not different among trials
- Run performance was 8.5% faster for the 8% carbohydrate-electrolyte solution (1,062±31 seconds, P<0.05), compared to placebo (1,154±56 seconds) and tended to be faster (7%) for the 6% carbohydrate-electrolyte solution (1,078±33 seconds). There were no differences in performance between the two carbohydrate-electrolyte solutions.
Author Conclusion:
- Congruent with ACSM guidelines, we observed an 8% CHO sports drink (containing 3.3% fructose) is clearly within the range for optimal CHO content and can enhance performance during competitive distance running in the heat
- No meaningful differences were observed in the relative fluid uptake or other measures reflecting the state of hydration between 6% and 8% commercially available sports drinks during running
- Moreover, these data suggest that among acclimatized distance runners, CHO availability (vs. achieving a similar peak core temperature) limits prolonged running performance in the heat.
Funding Source:
Industry: |
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Reviewer Comments:
Authors note the possibility of Type II errors resulting from uncontrolled environmental conditions.
Quality Criteria Checklist: Primary Research
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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? | 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? | ??? | |
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? | 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? | 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? | 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.) | 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? | ??? | |
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? | ??? | |
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? | Yes | |
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? | ??? | |
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? | 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)? | N/A | |
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? | 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 | |