EAL

NAP: Competition (2007)

Citation:

Wee SL, Williams C, Gray S, Horabin J. Influence of high- and low-glycemic index meals on endurance running capacity. Med Sci Sports Exerc. 1999; 31 (3) :393-399.

PubMed ID: 10188743

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 examine the influence of high- and low-GI carbohydrate meals, containing two grams CHO per kg body weight, ingested three hours before exercise on subsequent endurance-running capacity.

Inclusion Criteria:

Active subjects; all were involved in endurance-running at a frequency of at least four times a week
Female subjects were pre-menopausal non-oral contraceptive users.

Exclusion Criteria:

None of the subjects had diabetes mellitus
Female subjects were not on any estrogen-replacement therapy.

Description of Study Protocol:

Recruitment: Methods not described
Design: Randomized crossover trial
Blinding used: Single-blind; lab tests.

Intervention

Subjects ran on a treadmill at about 70% VO_2max to exhaustion on two occasions separated by seven days
Three hours before the run after an overnight fast, each subject was given an isoenergetic meal (850±21kcal, 67% carbohydrate, 30% protein, 3% fat) of high-GI or low-GI carbohydrate foods providing 2.0g CHO per kg body weight.

Statistical Analysis

One-way repeated-measures ANOVA used to evaluate changes over time for time-dependent variables
Tukey post-hoc test used to locate any significant difference
Differences between the two trials were evaluated by comparing areas under the time-variable curve using Student's T-tests for paired observations.

Data Collection Summary:

Timing of Measurements

Fasting blood and expired air samples collected before test meal consumed. Air samples collected every 30 minutes and blood samples every 15 and 30 minutes and at 1, 2, and 3 hours after the meal, as well as immediately before exercise. Expired air, RPE and blood samples collected every 20 minutes during exercise.

Dependent Variables

Blood samples analyzed for glucose, lactate, hemoglobin, hematocrit, plasma glycerol and plasma volume, FFA, insulin, cortisol, sodium and potassium electrolytes
Gas exchange measurements
Heart rate monitored through short-range telemetry
RPE.

Independent Variables

Three hours before the run, subjects were given an isoenergetic meal (850±21kcal, 67% carbohydrate, 30% protein, 3% fat) of high-GI or low-GI carbohydrate foods providing 2.0g CHO per kg body weight
Subjects followed same training schedule three days prior to testing
Subjects refrained from strenuous exercise and consumption of alcohol or caffeine 24 hours prior to testing
Females underwent testing during the last half of their monthly menstrual cycle
Subjects kept three-day weighed food records.

Description of Actual Data Sample:

Initial N: Eight active subjects (five male, three female)
Attrition (final N): Eight
Age: Mean, 33.2±3.8 years for males, 27.0±1.6 years for females
Ethnicity: Not mentioned
Location: United Kingdom.

Summary of Results:

Other Findings

There were no differences in the daily energy intake nor macronutrient composition of the subjects during the three days before each trial
Ingestion of the high-GI meal resulted in a 580% and 330% greater incremental area under the three-hour blood glucose and serum insulin response curves, respectively (both P<0.001)
Performance times were not different between the high-GI and low-GI trials (113±4 minutes and 111±5 minutes, respectively)
During the first 80 minutes of exercise in the low-GI trial, CHO oxidation was 12% lower and fat oxidation was 118% higher than in the high-GI trial
Although serum insulin concentrations did not differ between trials, blood glucose at 20 minutes into exercise in the high-GI trial was lower than that during the low-GI trial at the same time (3.6±0.3mmol per L vs. 4.3±0.3mmol per L, P<0.05)
During exercise, plasma glycerol and serum FFA concentrations were lower in the high-GI trial than in the low-GI trial
Exercise time to exhaustion was similar for both treatments and no difference was found when time to exhaustion for the treatments were analyzed by order.

Author Conclusion:

In summary, the results of the present study show that ingestion of a high- or low-GI CHO three hours before exercise results in similar endurance running capacity
Compared with a low-GI meal, ingestion of a high-GI meal results in a shift in substrate utilization from fat to CHO during exercise
In terms of carbohydrate nutrition, the metabolic challenge is to maintain carbohydrate supply to the muscles but to slow its depletion by relying optimally on fat as a fuel. However, it is difficult to optimize the availability of glucose to muscle and liver before exercise and still maintain high concentrations of FFA during exercise.

Funding Source:

Industry:

Gatorade Sports Science Institute,

Food Company:

University/Hospital:

Loughborough University

Not-for-profit

Reviewer Comments:

Authors note that a study limitation is the lack of a fasting condition as a control.

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?	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?	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