EAL

NAP: Competition (2007)

Citation:

Sparks MJ, Selig SS, Febbraio MA. Pre-exercise carbohydrate ingestion: effect of the glycemic index on endurance exercise performance. Med Sci Sports Exerc. 1998; 30 (6): 844-849.

PubMed ID: 9624641

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 compare the effects of pre-exercise ingestion of foods with high- and low-glycemic indices on metabolism during submaximal exercise of 50 minutes' duration and on performance during a 15-minute "all-out" cycle.

Inclusion Criteria:

Endurance-trained male triathletes.

Exclusion Criteria:

None specifically mentioned.

Description of Study Protocol:

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

Intervention

On three separate occasions separated by at least seven days, subjects ingested a one-gram CHO per kg of high-glycemic index (GI=80), a low-glycemic index (GI=29) or placebo meal 45 minutes before exercise and then cycled for 50 minutes at 67% VO_2max
Subjects subsequently performed a 15-minute self-paced performance ride in which total work was recorded.

Statistical Analysis

Resting and submaximal data were analyzed by two-way (time vs. treatment) ANOVA with repeated measures
The performance trial data was analyzed using a one-way ANOVA with repeated measures
When ANOVA revealed a significant interaction, simple main effects and Newman-Keuls post-hoc tests were used to locate differences.

Data Collection Summary:

Timing of Measurements

Heart rate recorded at 15-minute intervals during exercise and at completion of performance ride
Gas exchange measurements made at 15, 30 and 45 minutes during exercise
RPE obtained at 15-minute intervals during exercise
Blood samples collected immediately before the meal, every 15-minutes post-prandial and every 10 minutes during exercise.

Dependent Variables

Maximal oxygen uptake tests were conducted on a friction-braked cycle ergometer
Gas exchange measurements through Douglas bags
Heart rate measured with heart rate monitor
RPE
Blood samples analyzed for glucose, FFA, lactate, ammonia, insulin.

Independent Variables

Subjects ingested a one-gram CHO per kg of high-glycemic index (GI=80), a low-glycemic index (GI=29) or placebo meal 45 minutes before exercise
Subjects were to refrain from exercise, tobacco, alcohol and caffeine for 24 hours prior to tests.

Description of Actual Data Sample:

Initial N: Eight male triathletes
Attrition (final N): Eight
Age: Mean, 22.7±1.4 years
Ethnicity: Not mentioned
Location: Australia.

Summary of Results:

	Placebo	High-GI	Low-GI
VO₂ (ml/kg/min)	45.5±2.2	44.5±2.7	45.8±2.5
RER	0.85±0.01	0.90±0.01, P<0.01	0.86±0.01
CHO oxidation (g/min)	2.16±0.12	2.80±0.15, P<0.01	2.35±0.15
HR (beats/min)	150±5	152±7	149±5
RPE	13.5±0.8	12.8±0.6	12.6±0.8

Other Findings

Mean VO₂, heart rate and RPE did not differ between the three trials
Plasma glucose concentrations were higher (P<0.01) after ingestion of the high-GI meal than the low-GI meal and placebo (7.53±0.64 vs. 5.55±0.21 vs. 4.65±0.14mmol per L for high-, low- and placebo, 30 minutes post-prandial), but declined at the onset of exercise and were lower (P<0.01), compared with low-glycemic index and placebo (4.03±0.31 vs. 4.64±0.24 vs. 5.09±0.16mmol per L for high-, low- and placebo) at 10 minutes of exercise
Plasma glucose remained depressed (P<0.01) until 30 minutes into exercise in the high-GI, compared with other trials
Plasma insulin concentrations were higher (P<0.01) following ingestion during rest and exercise in high-GI, compared with low-GI and placebo
Plasma FFA concentrations were lower (P<0.05) following ingestion in high- and low-GI, compared with placebo and higher (P<0.05) in low-GI, compared wtih high-GI at the start and end of exercise
RER and CHO oxidation was higher (P<0.01) in high-GI, compared with low-GI and placebo during submaximal exercise
There were no differences in work output during the performance cycle (254±12 vs. 249±15 and 253±10kJ for placebo, high-GI and low-GI).

Author Conclusion:

In conclusion, the data from this study indicate that pre-exercise ingestion of a high-GI food results in hyperglycemia and subsequent hyperinsulinemia before exercise
As a consequence, an insulin-mediated hypoglycemia and reduced FFA availability was accompanied by an increased CHO oxidation in this trial. In contrast, ingestion of a low-GI food resulted in a lower glycemic response, reduced insulin secretion, an attenuated fall in FFA and, subsequently, a lower rate of CHO oxidation, such that the rate of CHO oxidation was not different, compared with placebo.
Despite these changes, work output during a 15-minute performance trial was not different when comparing the three trials
Research examining glucose kinetics and muscle glycogenolysis during exercise preceded by CHO ingestion with differing glycemic indices warrants further investigation.

Funding Source:

University/Hospital:

Victoria University of Technology, University of Melbourne (Australia)

Reviewer Comments:

Inclusion criteria, exclusion criteria and recruitment methods were not well-described.

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?		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?	N/A
	6.6.	Were extra or unplanned treatments described?	Yes
	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?		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