EAL

NAP: Training (2014)

Citation:

Moore LJS, Midgley AW, Thomas G, Thurlow S. McNaughton LR. The effects of low- and high-glycemic index meals on time trial performance. Int J Sports Physiol Perform. 2009; 4: 331-344.

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 investigate the influence of a low GI (glycemic index) meal vs. a high-GI meal on exercise performance that closely represented a sporting situation.

Inclusion Criteria:

Male
Cyclist who cycled at least five times per week.

Exclusion Criteria:

Not described.

Description of Study Protocol:

Design

Randomized, double-blind cross-over.
Each subject participated in two 40km time trials, separated by at least seven days, on a Velotron cyclePro ergometer after a five-minute 50-watt warm-up. Two different carbohydrate test meals were consumed 45 minutes before the onset of exercise.
Subjects reported to lab after a six-hour fast for each experiment
A pre-experiment meal was given and consumed within 10 minutes, then subjects rested for 45 minutes before beginning the exercise protocol
Ad lib water was given during the first trial and same amount was provided during second trial.

Blinding Used

Randomized, double blind.

Intervention

Low-GI or high-GI meal for 45 minutes containing 1g per kg body mass prior to exercise. The meals had a GI of 30 for the low-GI meal and 72 for the high-GI meal. Both meals contained cereal and low-fat milk; amounts were varied depending on subject body weight. The low-GI meal used bran flakes cereal and the high-GI meal used cornflakes. Effects of low-GI and high-GI meals were evaluated.

Statistical Analysis

Repeated-measures ANOVA on two factors (experimental treatment and time) was used to determine metabolic and performance differences between trials. When a significant difference was determined, a post-hoc test was used to locate the difference. Statistical significance was P<0.05 level. SPSS statistical software for Windows, version 14.0 was used for all analyses.

Data Collection Summary:

Timing of Measurements:

Baseline VO₂_max was done on all subjects on same Velotron cyclePro, at the same time of day in the laboratory, prior to experiment
Two days prior to the first trial, subjects recorded diet and exercise patterns to standardize carbohydrate intake and activity levels sot that they could be repeated before each trial
Subjects reported to the lab after a six-hour fast for each experiment:
- After sitting for five minutes, a fingertip blood sample was taken and a five-minute resting expired air sample was collected
- A pre-experiment meal was given and consumed within 10 minutes, then subjects rested for 45 minutes before beginning exercise protocol (meal and fluid intake were standardized)
- Ad lib water was given during first trial and same amount was provided during second trial
- Fingertip blood samples were obtained 15 minute post-prandial, 10 minutes before onset on exercise and immediately after exercise to measure blood glucose and lactate
- A heart rate monitor was attached to subject after the trial
- Expired air was analyzed throughout the test to determine substrate oxidation rates, energy expenditure and respiratory exchange ratio.

Dependent Variables

Variable 1: Performance time; time to complete a 40km cyling trial
Variable 2: Heart rate was measured by heart rate monitor (Polar Electro, OY, Finland)
Variable 3: Rating of perceived exertion (RPE), measured with automated open-circuit gas analysis system (Quark b²)
Variable 4: Respiratory exchange ratio (RER)
Variable 5: Carbohydrate oxidation rates
Variable 6: Fat oxidation rate
Variable 7: Energy expenditure
Variable 8: Blood glucose analyzed using YSI 2700 Stat (Yellow Springs Instrument, Yellow Springs, US)
Variable 9: Blood lactate (same analyzer as above).

Independent Variables

Age
Anthropometrics
Baseline measures of above dependent variables.

Control Variables

Pre-experiment meal
Pre-experiment and during experiment water intake
Humidity and temperature in lab
The 24-hour pre-experiment nutrition and fluid intake.

Description of Actual Data Sample:

Initial N: Eight males
Attrition (final N): All eight completed both experiments
Age: 29.4±6.4 years
Other relevant demographics: All subjects were well trained cyclists, but not elite cyclists; VO_2max was 58.2±10.1ml per kg per minute

Anthropometrics

Weight: 76.7±10.9kg
Height: 182±7.9cm.

Location

Department of Sport, Health and Exercise Science, University of Hull, Hull, East Yorkshire, UK laboratory.

Summary of Results:

Key Findings

Differences Between Low and High GI Meal Time Trials

Variables	Low GI Measures and Confidence Intervals	High GI Measures and Confidence Intervals	Statistical Significance of Group Difference
Dep var: Performance time	92.5±5.2 minutes	95.6±6.0 minutes	P=0.001
Dep var: Respiratory Exchange Ratio (RER)	0.93±0.035 throughout trial	0.90±0.04 throughout trial	P=0.001
Dep var: Carbohydrate oxidation	2.51±1.74g per minute	2.18±1.53g per minute	P=0.003
Dep var: Fat oxidation	0.15±0.15g per minute	0.29±0.18g per minute	P=0.002
Dep var: Blood glucose 15 minutes after meal	Increase from 4.6±0.4mmol to 5.5±1.7mmol per L	Increase from 4.5±0.5mmol to 7.3±2.4mmol per L	P=0.05
Dep var: Blood glucose at point of exhaustion	5.2±0.6mmol per L	4.7±0.7mmol per L	P=0.001

Other Findings

There were no significant differences between groups in heart rate during or at the end of the time trial. Both groups had a significant increase in heart rate from the beginning to the end of time trial (P=0.001). There was no significant difference between groups in blood lactate levels from the beginning to the end of the time trial. The difference in lactate levels for time was significant for both groups.
Substrate utilization:
- The respiratory exchange ratio was significantly higher throughout the time trial for the low-GI meal trial compared to the high-GI meal trial
- Carbohydrate: The carbohydrate oxidation rate was significantly higher throughout the time trial for the low-GI meal trial compared to the high-GI meal trial
- Fat: The fat oxidation rate was higher for throughout the time trial for the high-GI meal trial than for the low-GI meal trial.
The low-GI group completed the time trial in less time and had lower blood glucose 15 minutes after the meal, but higher blood glucose concentrations after 20 minutes of exercise throughout the time trial and at point of exhaustion than the high-GI group. The effect on blood glucose is thought to be the reason for improved performance in the low-GI group. The low-GI group used more carbohydrate for fuel and had a higher respiratory exchange ratio than the high-GI group, while the high-GI group used more fat for fuel than the low-GI group. These results are likely related to the fact that the low-GI group had more carbohydrate available for oxidation throughout the trial as evidenced by higher blood glucose by the end of the trial.

Author Conclusion:

The ingestion of a low-GI meal 45 minutes prior to endurance exercise significantly improved performance as compared to a high-GI meal with same amount of carbohydrate. It is possible that an increase in the availability of carbohydrate and greater carbohydrate oxidation throughout the exercise period may explain the observed improvements in time trial performance in the low-GI trial. These data support the ingestion of a meal containing 1g per kg body weight of carbohydrate of low GI content before prolonged exercise.

Funding Source:

Other:

Funding source not described.

Reviewer Comments:

This study definitely supports low-GI carbohydrate over high-GI carbohydrate intake prior to prolonged exercise. The meals provided also provided protein (16g in high-GI meal and 19g in low GI meal); the effect of protein in the meal was not discussed. Meals were reported to be similar in macronutrient content, but high GI meal was 91% of total calories, 84% of total protein and 75% of total fat in low GI meal. These differences seem like they could possibly make a difference in performance, but they were not discussed. It would be good to see the same study done with meals with identical protein, fat and energy as well as carbohydrate intake but different GI levels.
The study was described as double blind; however, it would be difficult to subjects to be blinded as to whether they were eating corn flakes or bran flakes due to difference in taste and appearance of the two cereals. It is quite possible that subjects were not told which meal was high-GI vs. low-GI, which would have blinded them to which meal they were consuming.

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")?	Yes
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?	Yes
	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?	Yes
	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?		???
	10.1.	Were sources of funding and investigators' affiliations described?	No
	10.2.	Was the study free from apparent conflict of interest?	Yes