EAL

NAP: Competition (2007)

Citation:

Febbraio MA, Stewart KL. CHO feeding before prolonged exercise: effect of glycemic index on muscle glycogenolysis and exercise performance. J Appl Physiol. 1996; 81 (3): 1,115-1,120.

PubMed ID: 8889742

Study Design:

Randomized crossover trial

Class:

A - Click here for explanation of classification scheme.

Quality Rating:

NEUTRAL: See Quality Criteria Checklist below.

Research Purpose:

To determine whether pre-exercise CHO ingestion with differing glycemic indices affects exercise metabolism, in particular muscle glycogen utilization and exercise performance, during prolonged cycling.

Inclusion Criteria:

Endurance-trained men.

Exclusion Criteria:

None specifically mentioned.

Description of Study Protocol:

Recruitment: Methods not described
Design: Randomized crossover trial
Blinding used: Not used; lab tests.

Intervention

Subjects performed three exercise trials separated by at least seven days on a cycle ergometer after ingesting either a high-glycemic index, low-glycemic index or placebo meal 45 minutes prior to exercise
During each trial, subjects cycled at a workload of 70% peak VO₂ for 120 minutes, followed by a 15-minute performance cycle, where total work was measured.

Statistical Analysis

Data from three trials analyzed using two-factor (time and treatment) ANOVA with repeated measures
A one-way ANOVA was used to compare work output during the performance cycle
Simple main effects analyses and Newman-Keuls post-hoc tests were used to locate differences when ANOVA revealed a significant interaction.

Data Collection Summary:

Timing of Measurements

Heart rate, oxygen uptake and RER measured at 15-minute intervals during submaximal cycle
Blood samples obtained basally, at 15, 30 and 45 minutes post-prandial and at 20-minute intervals during submaximal cycle.

Dependent Variables

Heart rate
Oxygen uptake
RER
Muscle biopsies obtained from vastus lateralis in five subjects
Blood samples analyzed for glucose, insulin, ammonia, lactate and FFA.

Independent Variables

Low-glycemic index (lentils, one gram CHO per kg, GI=29), high-glycemic index (potatoes, one gram CHO per kg, GI=80) or placebo meal (diet jelly) 45 minutes prior to exercise
Overnight fast, abstinence from alcohol, caffeine, tobacco and strenuous exercise for at least 24 hours
Subjects completed food and activity logs.

Description of Actual Data Sample:

Initial N: Six males
Attrition (final N): Six
Age: Mean, 29±2 years
Ethnicity: Not mentioned
Location: Australia.

Summary of Results:

Other Findings

Neither VO₂ nor heart rate were different throughout submaximal exercise between trials, but RER (P<0.01) and CHO oxidation (P<0.01) were lower during the placebo meal, compared to the low- and high-glycemic index meals
Ingestion of the high-glycemic index meal resulted in an elevated (P<0.01) blood glucose concentration 15 minutes after ingestion, compared with the low-glycemic index and placebo meals, but there were no differences in the glycemic responses to the meals thereafter, despite plasma insulin concentration being higher (P<0.01) throughout the rest period after ingestion of high-glycemic index meal, compared to low-glycemic index and placebo meals
After the commencement of exercise, there was no difference in plasma insulin, glycogen or lactate concentrations among the three trials
Plasma FFA concentrations were lower (P<0.05) throughout exercise after the high-glycemic index meal, compared to the low-glycemic index and placebo meals
In addition, concentrations of this metabolite were lower (P<0.05) during the first hour of exercise in the low-glycemic index condition, compared with placebo
Despite these results, no differences were observed in either the rate of muscle glycogen utilization during submaximal exercise or work output during the performance cycle when the three trials were compared.

Author Conclusion:

In summary, these data demonstrate that the pre-exercise ingestion of CHO, while increasing CHO oxidation irrespective of the glycemic index, does not influence the rate of muscle glycogen utilization or exercise performance
It is likely that the increase in CHO oxidation results from an increase in cellular uptake and subsequent oxidation of blood-borne glucose, mediated by elevated insulin and lower circulating FFA.

Funding Source:

University/Hospital:

Royal Melbourne Institute of Technology

Reviewer Comments:

Inclusion and exclusion criteria and recruitment methods were not well-defined
No power calculations done.

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?	No
	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?		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?	N/A
	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?	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?		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