EAL

NAP: Competition (2007)

Citation:

Kirwan JP, Cyr-Campbell D, Campbell WW, Scheiber J, Evans WJ. Effects of moderate and high glycemic index meals on metabolism and exercise performance. Metabolism, 2001; 50 (7): 849-855.

PubMed ID: 11436193

Study Design:

Randomized crossover trial

Class:

A - Click here for explanation of classification scheme.

Quality Rating:

POSITIVE: See Quality Criteria Checklist below.

Research Purpose:

To determine whether pre-exercise ingestion of meals with moderate- and high-glycemic indexes affects glucose availability during exercise and exercise performance time.

Inclusion Criteria:

Healthy, active men screened for exercise training and smoking status
All participants had normal responses to 75g oral glucose tolerance test.

Exclusion Criteria:

None specifically mentioned.

Description of Study Protocol:

Recruitment: Subjects volunteered for study
Design: Randomized crossover trial
Blinding used: Double-blind; only one researcher aware of what was consumed.

Intervention

Subjects ingested 75g carbohydrate in the form of two different breakfast cereals: Rolled oats (moderate-GI=61) or puffed rice (high-GI=82) or water alone
Meals were ingested 45 minutes prior to cycling exercise at 60% peak VO₂ to exhaustion
Trials separated by seven to 10 days.

Statistical Analysis

Differences between dependent variables examined by repeated measures ANOVA
Specific mean differences identified with Newman-Keuls post-hoc test.

Data Collection Summary:

Timing of Measurements

Blood samples drawn before the test meal, 15, 30 and 45 minutes after the meal and at 30-minute intervals during exercise until exhaustion
Muscle biopsies obtained before eating test meal and after exercise
Gas exchange measurements determined for first 30 minutes of exercise and at 30-minute intervals during exercise.

Dependent Variables

Blood samples analyzed for glucose, FFA, glycerol, insulin, epinephrine and norepinephrine concentrations
Muscle biopsies obtained from vastus lateralis for muscle glycogen
Gas exchange and RER determined by indirect calorimetry.

Independent Variables

Rolled oats (moderate-GI=61) or puffed rice (high-GI=82) or water alone
Subjects lived in General Clinical Research Center for two days and three nights prior to each exercise trial.

Description of Actual Data Sample:

Initial N: Six active men
Attrition (final N): Six
Age: Mean, 22±1 years
Ethnicity: Not mentioned
Location: Pennsylvania.

Summary of Results:

Other Findings

Before exercise, both test meals elicited significant hyperglycemia and hyperinsulinemia (P<0.05), compared with water alone
The glycemic response was higher (P<0.05) at the start of exercise after the high-glycemic index meal than after the control
During exercise, plasma glucose levels were higher at 60 and 90 minutes after the moderate-GI meal than after the high-GI meal or control (P<0.05)
Total CHO oxidation was greater during the moderate-GI trial than in control (P<0.05) and was directly correlated with exercise performance time (R=0.95, P<0.0001)
Pre-exercise plasma FFA levels were suppressed (P<0.05) 30 and 45 minutes after ingestion of the high-glycemic index meal and 45 minutes after ingestion of the moderate-GI meal, compared with control
At 30, 60 and 120 minutes of exercise, FFAs remained suppressed (P<0.05) for both test meals, compared with control
At exhaustion, plasma glucose, insulin, FFA, glycerol, epinephrine and norepinephrine levels and muscle glycogen use were not different for all trials
Heart rate, VO₂ and RER were not different between the trials
Exercise time was prolonged (P<0.05) after the moderate-GI meal, compared with control, but the high-GI meal was not different from control (moderate-GI, 165±11; high-GI, 141±8; control, 134±13 minutes).

Author Conclusion:

In conclusion, ingestion of a moderate-glycemic index meal containing 75g of carbohydrate 45 minutes before cycling exercise at 60% of peak VO₂ increased exercise duration, compared with a control trial
In contrast, ingestion of a high-glycemic index meal did not improve performance
Before exercise, the moderate-GI meal elicited an attentuated glycemic response, compared wtih the high-GI meal
During exercise, the higher level of glycemia and greater total carbohydrate oxidation after the moderate-GI meal may have contributed to the enhanced performance
The results suggest that individuals who wish to perform exercise or physically demanding work for a prolonged period may benefit from eating a moderate-GI meal before the activity.

Funding Source:

Government:

NIH

Industry:

Quaker Oats Inc

Food Company:

Not-for-profit

Foundation associated with industry:

Reviewer Comments:

Subjects lived in GCRC prior to testing in order to control diet and physical activity
Subjects underwent preliminary testing prior to study entrance.

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?		Yes
	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?		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?	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?	N/A
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