NAP: Competition (2007)

Citation:

Febbraio MA, Keenan J, Angus DJ, Campbell SE, Garnham AP. Pre-exercise carbohydrate ingestion, glucose kinetics, and muscle glycogen use: effect of the glycemic index. J Appl Physiol. 2000; 89: 1,845-1,851.

PubMed ID: 11053335
 
Study Design:
Randomized crossover trial
Class:
A - Click here for explanation of classification scheme.
Quality Rating:
Neutral NEUTRAL: See Quality Criteria Checklist below.
Research Purpose:
To further examine the influence of pre-exercise ingestion of CHO with differing glycemic index on substrate metabolism.
Inclusion Criteria:
Endurance-trained men.
Exclusion Criteria:
None specifically mentioned.
Description of Study Protocol:
  • Recruitment: Methods not specified
  • Design: Randomized crossover trial 
  • Blinding used: Not used; lab tests. 

Intervention (if applicable)

Subjects cycled at 70% peak oxygen uptake for 120 minutes, followed by a 30-minute performance cycle after ingesting either a high-glycemic index, low-glycemic index or placebo meal 30 minutes before exercise.

Statistical Analysis

  • Data from three trials were compared by using two-factor (time and treatment) ANOVA with repeated measures
  • One-way ANOVA was used to compare work output during the performance cycle
  • Newman-Keuls post-hoc tests were used to locate differences when ANOVA revealed significant interaction.
Data Collection Summary:

Timing of Measurements

  • Subjects reported to the lab on three different occasions separated by at least seven days
  • Heart rate, oxygen uptake, RER measured at 15-minute intervals
  • Blood samples taken basally, 10, 20 and 30 minutes post-prandial and at 15-minute intervals during exercise.

Dependent Variables

  • VO2peak determined using electrically braked cycle ergometer
  • Work output
  • Gas measurements completed using Douglas bag
  • Muscle biopsies from vastus lateralis muscle
  • Blood samples analyzed for plasma glucose, insulin, FFA and lactate.

Independent Variables

  • High-glycemic index (mashed potatoes, GI not given), low-glycemic index (muesli, GI=52) or placebo meal (diet jelly)
  • Amount of CHO for both low- and high-glycemic index meals was one gram CHO per kg
  • Overnight fast, abstinence from alcohol, caffeine, tobacco and strenuous exercise for at least 24 hours.
Description of Actual Data Sample:
  • Initial N: Eight males
  • Attrition (final N): Eight
  • Age: Mean, 26±6 years
  • Ethnicity: Not mentioned
  • Location: Australia.
Summary of Results:

Other Findings

  • Neither VO2 nor heart rate throughout submaximal exercise was different between the three trials
  • Ingestion of the high-glycemic index meal resulted in an elevated blood glucose concentration at 10, 20 and 30 minutes post-ingestion, compared to the low-glycemic index meal and placebo (P<0.01)
  • At the onset of exercise, blood glucose fell (P<0.05) such that it was lower (P<0.05) after the high-glycemic index meal, compared with low-glycemic index and placebo meals at 15 and 30 minutes during exercise
  • Plasma insulin concentration was higher (P<0.01) at 10, 20 and 30 minutes after ingestion of high-glycemic index, compared with low-glycemic index and placebo
  • Plasma FFA concentrations were lower (P<0.05) at 20 and 30 minutes post-ingestion and throughout exercise in high-glycemic index, compared with low-glycemic index and placebo
  • Carbohydrate oxidation was higher (P<0.05) throughout exercise, whereas glycogen use tended (P=0.07) to be higher in high-glycemic index, compared with low-glycemic index and placebo
  • No differences were observed in work output during the performance cycle when comparing the three trials.
Author Conclusion:
  • In summary, these data demonstrate that pre-exercise ingestion of CHO foods with different glycemic index causes an alteration in glycemic, insulinemic and lipolytic responses during exercise.
  • The ingestion of a high-glycemic index meal resulted in hyperinsulinemia, which increased glucose uptake and decreased FFA availability. As a result, the rate of CHO oxidation after pre-exercise CHO ingestion is influenced by the GI, with the ingestion of low-glycemic index food providing a more stable metabolic response.
  • Pre-exercise feeding with a high-glycemic index, but not a low-glycemic index, meal augments carbohydrate utilization during exercise, but does not affect exercise performance. 
Funding Source:
Industry:
Nestle Research Center
Food Company:
Reviewer Comments:
  • Inclusion and exclusion criteria and recruitment methods were not well-defined
  • No power calculations done
  • Glycemic index of high-glycemic index meal not provided.
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? ???
  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? N/A
  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? Yes
  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? ???
  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? ???
  9.1. Is there a discussion of findings? Yes
  9.2. Are biases and study limitations identified and discussed? No
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