EAL

NAP: Competition (2007)

Citation:

DeMarco HM, Sucher KP, Cisar CJ, Butterfield GE. Pre-exercise carbohydrate meals: application of glycemic index. Med Sci Sports Exerc. 1999; 31 (1): 164-170.

PubMed ID: 9927025

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 compare post-prandial glycemic, insulinemic and physiologic responses to pre-exercise meals calculated to have low and moderately high glycemic indices.

Inclusion Criteria:

Subjects were accustomed to cycling to fatigue over prolonged periods (over two hours)
Before they were accepted into the study, subjects underwent an initial screening, which included a health history, fasting blood analysis, urine analysis, a complete physical and a resting electrocardiogram, as well as glucose tolerance test.

Exclusion Criteria:

Excluded if not included above.

Description of Study Protocol:

Recruitment: Recruited from local cycling teams.
Design: Randomized crossover trial. After an overnight fast, subjects consumed one of the test meals or water 30 minutes before cycling for two hours at 70% VO_2max, followed by cycling to exhaustion at 100% VO_2max.
Blinding used: Not used; lab tests.
Intervention: Low- (GI: 36) or high-glycemic index (GI: 69.3) pre-exercise meals or water, consumed 30 minutes prior to testing. Each meal provided three foods totaling 1.5g CHO per kg. Tests conducted one week apart.
Statistical analysis: The data from the three trials were analyzed using two-way ANOVA for repeated measures (meal x time). When ANOVA indicated significant main effects, Tukey post-hoc tests were used to isolate significantly different means (P<0.05).

Data Collection Summary:

Timing of Measurements

Blood samples taken before the meal and at 15 and 30 minutes after the meal
Oxygen consumption, RER, RPE and blood samples obtained every 20 minutes during the first two hours of exercise and at exhaustion.

Dependent Variables

Blood samples analyzed for plasma glucose and insulin
Oxygen consumption and RER
RPE based on Borg's scale.

Independent Variables

Low-glycemic index or high-glycemic index meals or water consumed 30 minutes prior to exercise
For two days before each trial, subjects did not engage in strenuous activity and maintained a standardized carbohydrate intake (six to eight kg).

Description of Actual Data Sample:

Initial N: 10 cyclists, all male
Attrition (final N): 10
Age: Mean, 30.7±4.3 years
Ethnicity: Not mentioned
Location: California.

Summary of Results:

	High-GI Meal	Low-GI Meal
Available CHO (grams)	113	113
Protein (grams)	18	33
Fat (grams)	6	9
Dietary Fiber (grams)	5	57
Total energy; kJ per meal	2,418	2,782
Total energy; kcal per meal	578	665

Other Findings

Oxygen consumption and percentage VO_2max remained stable throughout the endurance period and were similar among trials
At 15 and 30 minutes post-meal, plasma glucose levels were higher in the high-glycemic index meal, but it was not statistically significant
There was a sharp decline in plasma glucose concentrations in both glycemic index test meals 20 minutes into exercise, which were significantly lower than the control trial of water only
Plasma insulin levels were significantly lower (P<0.05) after the low-glycemic index meal than after the high-glycemic index meal through 20 minutes of exercise
Significantly higher (P<0.05) respiratory exchange ratios were observed after high-glycemic index meals than low-glycemic index meals until two hours of exercise
At two hours of exercise, plasma glucose levels were significantly higher and RPE were lower (P<0.05) after low-glycemic index meals, compared with after high-glycemic index
Time to exhaustion was 59% longer after low-glycemic index (206.5±43.5 seconds) than after high-glycemic index (129.5±22.8 seconds) and longer than controls (120.0±31.4 seconds).

Author Conclusion:

In conclusion, a significant improvement in maximal exercise performance time, following endurance exercise, was observed when a calculated low-glycemic index meal was consumed 30 minutes prior to exercise, as compared with the consumption of a meal calculated to have a high glycemic index
These results may support the use of low-glycemic index meals for individuals participating in early morning competitive events
Increasing the availability of blood glucose may supplement endogenous stores adequately enough to enhance performance during maximal exercise
Nevertheless, further research is needed, testing a number of mixed carbohydrate meals of differing computed glycemic indices to assure that the results observed in the present study results from the glycemic index itself rather than from any potential uniqueness of the two test meals.

Funding Source:

University/Hospital:

San Jose State University VA Health Care System

Reviewer Comments:

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?	Yes
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?	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?		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