NAP: Training (2014)

Citation:

Wu CL, Williams C. A low glycemic index meal before exercise improves endurance running capacity in men. Int J Sport Nutr Exerc Metab. 2006; 16(5): 510-527.

PubMed ID: 17240783
 
Study Design:
Randomized Crossover Trial
Class:
A - Click here for explanation of classification scheme.
Quality Rating:
Positive POSITIVE: See Quality Criteria Checklist below.
Research Purpose:

To investigate the effects of low and high glycemic index (GI) meals three hours prior to exercise on endurance running capacity in men.

Inclusion Criteria:
  • Healthy
  • Male
  • Recreational runners.
Exclusion Criteria:

Not reported.

 

Description of Study Protocol:
Design
  • Subjects completed two experimental trials [low GI (LGI) and high GI (HGI)] in a counter-balanced design. The trials were randomized and separated by at least seven days.
  • After an overnight fast (12 hours), the subjects ingested either a LGI or HGI meal three hours prior to running at 70% VO2max until exhaustion. 

Intervention

  • HGI meal: Corn flakes (Kellogg's Ltd., Manchester, UK), skim milk, white bread, jam, a glucose drink (Lucozade original, GlaxoSmithKline, Brentford, UK) and water (GI=77)
  • LGI meal: All Bran (Kellogg's), skim milk, peaches, apples and apple juice (GI=37)
  • Each meal provided 2g per kg body mass of available CHO and had the same energy content (675kcal per 70kg or 9.6kcal per kg body mass) and macronutrient composition (84% CHO, 12% protein, 4% fat).

Statistical Analysis

A two-way (time x treatment) ANOVA with repeated measures was used to analyze the overall differences in the physiological and biochemical responses to the two main trials. When a significant difference was found, the Tukey post-hoc test was used to locate the differences in means at specific time points. For non-time-dependent variables, Student's T-test for paired observations was used.

Data Collection Summary:

Timing of Measurements

  •  Blood samples were obtained at baseline; at 15 minutes, 30 minutes, 60 minutes, 90 minutes, 120 minutes and 180 minutes during the post-prandial period; at 15 minutes, 30 minutes, 45 minutes, 60 minutes and 90 minutes during the exercise period; and at fatigue
  • Expired air samples were obtained at baseline; 15 minutes, 30 minutes, 60 minutes, 90 minutes, 120 minutes, 150 minutes and 180 minutes during the post-prandial period; at 15 minutes, 30 minutes, 45 minutes, 60 minutes and 90 minutes during the exercise period; and at fatigue
  • Heart rate (HR) and rate of perceived exertion (RPE) were obtained at 15 minutes, 30 minutes, 45 minutes, 60 minutes and 90 minutes during the exercise period and at fatigue
  • Gut fullness (GF) and thirst scale (TS) were obtained at 15 minutes, 30 minutes, 60 minutes, 90 minutes, 120 minutes, 150 minutes and 180 minutes during the post-prandial period; at 15 minutes, 30 minutes, 45 minutes, 60 minutes and 90 minutes during the exercise period; and at fatigue.

Dependent Variables

  • Endurance capacity: Length of time it took subjects to reach exhaustion during running protocol
  • Plasma glucose concentration: Analyzed from blood samples
  • Serum insulin concentrations: Analyzed from blood samples
  • Plasma free fatty acid (FFA) concentrations: Analyzed from blood samples
  • Plasma glycerol concentrations: Analyzed from blood samples
  • Blood lactate concentrations: Analyzed from blood samples
  • Substrate utilization: CHO oxidation rates and fat oxidation rates were estimated from VO2 and VCO2 using stoichiometric equations
  • HR
  • RPE: Measured using six to 20 scale
  • GF: Measured using six to 20 scale
  • TS: Measured using six to 20 scale.

 Independent Variables

LGI vs. HGI.

Control Variables

  • Energy content and macronutrient composition of test meals
  • Environmental conditions (temperature and humidity) during trials
  • Subjects were instructed to abstain from alcohol, caffeine and tobacco consumption for 24 hours before each main trial
  • Subjects were required to record their diet for two days before the first main trial using the food-weighing method, and were asked to repeat the same diet for the two days before the second main trial
  • Exercise protocol
  • Hydration protocol during exercise.
Description of Actual Data Sample:
  • Initial N: Eight males
  • Attrition (final N): Eight
  • Age: 28.9±1.5 years
  • Other relevant demographics: VO2max was 690.6±1.5ml per kg per minute
Anthropometrics
  • Mass: 70.5± 2.2kg
  • Height: 175.0±1.9cm
Location

Loughborough, United Kingdom.
Summary of Results:

Key Findings

  • Endurance capacity: Average running time of the LGI trial (108.8±4.1 minutes) was significantly longer than the HGI trial (101.4±5.2 minutes) (P=0.038). Six out of eight subjects ran longer in the LGI trial.
  • Plasma glucose concentration in the HGI trial was significantly higher at 30 minutes, 60 minutes and 90 minutes during the post-prandial period compared to the LGI trial (P<0.05). The LGI trial maintained higher concentrations of plasma glucose during the first 30 minutes of exercise (P<0.05); however, there were no differences in these values between the two trials during exercise after the first 30 minutes of each run (P>0.05).
  • Serum insulin concentrations: There were significantly higher serum insulin concentrations in the HGI trial at 30 minutes, 60 minutes, 90 minutes and 120 minutes during the post-prandial period compared to the LGI trial (P<0.05)
  • Plasma free fatty acid (FFA) concentrations: Plasma FFA concentrations increased gradually in both HGI and LGI trials; however, they were higher during the LGI trial than during the HGI trial throughout the exercise period (P<0.05)
  • Plasma glycerol concentrations: During exercise, plasma glycerol concentrations were higher during the LGI trial at 45 minutes, 60 minutes and 90 minutes and at fatigue than during the HGI trial (P<0.05)
  • Blood lactate concentrations: Blood lactate concentrations were significantly higher in the LGI trial than in the HGI trial at 15 minutes, 30 minutes, 60 minutes and 90 minutes during the post-prandial period (P<0.01)
  • Substrate utilization: Respiratory exchange ratios (RER) values were higher during the first 90 minutes of exercise in the HGI trial (P<0.05). During exercise, the rate of fat oxidation was significantly higher in the LGI trial at 15 minutes, 45 minutes, 60 minutes, 75 minutes and 90 minutes during exercise than during the HGI trial (P<0.05).
  • HR: There were no significant differences between the two trials
  • RPE: There were no significant differences between the two trials
  • GF: GF was rated higher in the LGI trial at 15 minutes, 30 minutes and 60 minutes during the post-prandial period than in the HGI trial (P<0.05). GF also tended to be rated higher during exercise in the LGI trial and was significantly higher at 75 minutes (P<0.05).
  • TS: There were no significant differences between the two trials.
Author Conclusion:

The ingestion of a mixed-LGH, high-CHO meal three hours before exercise resulted in a greater endurance capacity than following the ingestion of a mixed-HGI CHO meal. The differences in endurance capacity may be a consequence of the greater rate of fat oxidation in the presence of adequate blood glucose concentrations during exercise following the LGI CHO meal.

Funding Source:
Other: Not reported
Reviewer Comments:
  • Accuracy of food records and compliance to normal diet was not monitored
  • There was no blinding
  • A recent study reported that the ingestion of All Bran cereal resulted in a higher rather than lower rate of glucose disappearance; however, glycemic responses to the meals in the present study were consistent with the calculated GI values.

 

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? N/A
  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)? Yes
  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