EAL

NAP: Competition (2007)

Citation:

Wee SL, Williams C, Tsintzas K, Boobis L. Ingestion of a high-glycemic index meal increases muscle glycogen storage at rest but augments its utilization during subsequent exercise. J Appl Physiol. 2005; 99: 707-714.

PubMed ID: 15831796

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 determine whether the GI of the CHO-rich pre-exercise breakfast affects muscle glycogen storage during a three-hour post-prandial period and muscle glycogen utilization during subsequent exercise.

Inclusion Criteria:

Recreational runners.

Exclusion Criteria:

None specifically mentioned.

Description of Study Protocol:

Recruitment: Methods not mentioned
Design: Randomized crossover trial
Blinding used: Not used; lab tests
Intervention: On two occasions 14 days apart, subjects ran at 71% maximal oxygen uptake for 30 minutes on a treadmill. Three hours before exercise, in randomized order, subjects consumed either isoenergetic high-GI (GI=80) or low-GI (GI=36) breakfasts that provided 3.43MJ energy, 175g CHO, 21g protein and four grams fat per 70kg body mass.

Statistical Analysis

The changes in physiological and biochemical variables were analyzed by two-way ANOVA for repeated-measures (meal x time)
For values attaining this criterion, Tukey's post-hoc test was used to locate the differences
The dietary data, incremental area under the curve, total substrate oxidation and energy expenditure were analyzed by Student's paired T-tests.

Data Collection Summary:

Timing of Measurements

Blood, gas exchange and muscle biopsies collected at rest
After meals, subjects had gas exchange measurements taken every 30 minutes and blood samples taken every 15, 30, 60 and 120 minutes
Three hours after meal, pre-exercise muscle and blood samples were obtained
Gas exchange measurements were collected every 10 minutes and blood samples were collected every 10, 20 and 30 minutes during exercise
Heart rate monitored continuously
Immediately after run completion, third muscle biopsy taken.

Dependent Variables

Muscle biopsies obtained from vastus lateralis
Heart rate measured by short-range telemetry
Gas exchange measurements collected using Douglas bag method
Blood samples analyzed for glucagon, glucose, glycerol, insulin.

Independent Variables

Subjects consumed either isoenergetic high-GI (GI=80) or low-GI (GI=36) breakfasts that provided 3.43MJ energy, 175g CHO, 21g protein and four grams fat per 70 kg body mass three hours before exercise
All subjects adhered to normal diets throughout study
Subjects were required to record training and weighed food intake, with records analyzed by RD
Subjects refrained from physical training, caffeine and alcohol the day prior to testing.

Description of Actual Data Sample:

Initial N: Seven trained men
Attrition (final N): Seven
Age: Mean, 31±4 years
Ethnicity: Not mentioned
Location: United Kingdom.

Summary of Results:

	Low-GI	High-GI
Oxygen uptake; basal	0.29±0.02	0.29±0.02
Oxygen uptake; post-prandial	0.37±0.02	0.36±0.02
Oxygen uptake; exercise	2.85±0.18	2.82±0.21
RER; basal	0.81±0.03	0.82±0.03
RER; post-prandial	0.91±0.02	0.94±0.02
RER; exercise	0.96±0.01, P<0.05	0.99±0.01
Blood lactate; basal	0.8±0.1	0.7±0.1
Blood lactate; post-prandial	1.8±0.1, P<0.05	1.1±0.1
Blood lactate; exercise	2.7±0.5, P<0.05	3.2±0.5
Heart rate; exercise	161±6	162±5

Other Findings

The incremental areas under the three-hour plasma glucose and serum insulin response curves after the high-GI meal were 3.9-fold (P<0.05) and 1.4-fold greater (P<0.001), respectively, than those after the low-GI meal
During the three-hour post-prandial period, muscle glycogen concentration increased by 15% (P<0.05) after the high-GI meal, but remained unchanged after the low-GI meal
Muscle glycogen utilization during exercise was greater in the high-GI (129.1±16.1mmol per kg dry mass), compared with the low-GI (87.9±15.1mmol per kg dry mass, P<0.01) trial
VO₂ and RER values were higher during the post-prandial period, compared with the fasted state, but were not different between trials
Overall, substrate oxidation during the post-prandial period did not differ between trials.

Author Conclusion:

In summary, our findings indicate that:

A high-GI meal results in greater muscle glycogen storage than a low-GI meal in overnight fasted subjects with normal glycogen concentration
The relative shift in substrate metabolism during 30 minutes of exercise from CHO to fat, after ingestion of a low-GI, compared with a high-GI meal was accompanied by a reduction in the rate of muscle glycogen utilization and not blood glucose oxidation
Compared with macroglycogen, concentrations of proglycogen appear to be more sensitive to feeding and exercise when total glycogen is not supercompensated or depleted.

However, the greater fat oxidation and lower muscle glycogen utilization after the low-GI meal may not always translate to a greater endurance capacity than after the high-GI meal, as indicated by the inconsistency in the literature regarding the ergogenic effects of low-GI or moderate-GI meals over high-GI meals.

Funding Source:

Government:

states of malaya

Not-for-profit

Reviewer Comments:

Inclusion criteria, exclusion criteria and recruitment methods were not well-defined
Athletic performance was not measured.

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?	No
	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?	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