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NAP: Training (2014)

Citation:

Moore LJS, Midgley AW, Thurlow S, Thomas G, and McNaughton LR. Effect of the glycaemic index of a pre-exercise meal on metabolism and cycling time trial performance. J Sci Med Sport. 2010; 13: 182–188.

 
Study Design:
Randomized Crossover Trial
Class:
A - Click here for explanation of classification scheme.
Quality Rating:
Neutral NEUTRAL: See Quality Criteria Checklist below.
Research Purpose:

To investigate the effects of low or high glycemic index foods consumed prior to a 40km time trial (TT) on metabolism and subsequent endurance performance.

Inclusion Criteria:

 Not stated.

Exclusion Criteria:

 Not stated.

Description of Study Protocol:
Design

Ten male cyclists participated in this randomized, single-blinded crossover study. The subjects completed a familiarization trial one week before the first TT. Each subject completed two 40km TT on an ergometer, separated by at least seven days. Subjects followed the same diet and training schedule during the two days prior to each TT, verified by food and training diary analysis, and refrained from strenuous exercise, caffeine and alcohol 24 hours before each trial. The day of each TT subjects reported to the lab six hours post-prandial and had baseline blood and expired air samples taken, then 45 minutes prior to the TT were provided with a standardized meal that was either HGI or LGI. Subjects rested for 45 minutes; during that time post-prandial blood samples were taken. After a five-minute warm-up, subjects completed a 40km TT. Throughout the trial blood samples and expired air samples were collected. Heart rate and ratings of perceived exertion (RPE) were also recorded throughout the TT.

Blinding Used

Single-blinded.

Intervention

  • Subjects received one of two test meals in the test lab 45 minutes prior to the TT. The meals were either high glycemic index (HGI) or low glycemic index (LGI) and provided 1g per kg-1 body mass of carbohydrate. The meals had similar energy, fat and protein content.
  • The HGI meal consisted of:
    • Cornflakes
    • Semi-skimmed milk.
  • The HGI meal had a GI of 72 and provided 386kcal, 70g CHO, 6g fat and 16g protein
  • The LGI meal consisted of:
    • Branflakes
    • Semi-skimmed milk.
  • The LGI meal had a GI of 30 and provided 422kcal, 70g CHO, 8g fat and 19g protein
  • Each subject also consumed a standardized volume of fluid (650ml of water and milk) with each meal.

Statistical Analysis

  • A paired T-test was used to assess any differences in baseline data on all the variables
  • A repeated measures ANOVA on trial and time was used to determine metabolic and performance differences between trials
  • Sidak-adjusted post hoc tests were used to locate significant paired differences.
Data Collection Summary:

Timing of Measurements

  • At baseline, 45 minutes prior to the TT, subjects had a blood sample and expired air sample collected. After the test meal subjects had blood samples collected at 30 minutes and 10 minutes before the start  of the TT. 
  • Throughout the TT, five-minute expired air samples and blood samples were collected at:
    • 20 minutes
    • 40 minutes
    • 60 minutes
    • Last minute of cycling (expired air sample)
    • Immediately following end of TT while subjects were still seated on ergometer (blood sample).
  • Heart rate and RPE were recorded every 15 minutes throughout the TT.

Dependent Variables

  • Performance: Measured by 40km TT
  • Carbohydrate oxidation: Measured by expired air sample
  • Fat oxidation: Measured by expired air sample
  • VO2max: Measured by expired air sample
  • Triglyceride concentration (TGA): Measured by blood sample
  • Free fatty acid concentration (FFA): Measured by blood sample
  • Insulin: Measured by blood sample
  • Whole blood glucose concentration: Measured by blood sample
  • Whole blood lactate concentration: Measured by blood sample.

Independent Variables

  • HGI meal
  • LGI meal.

Control Variables

  • All TTs were done on the same ergometer at the same time of day
  • Two-day food and training diary prior to TT analyzed for all subjects
  • Temperature and humidity of lab consistent for all TTs
  • Subjects consumed a standardized volume of fluid (650ml) with each meal.
Description of Actual Data Sample:
  • Initial N: Ten males
  • Attrition (final N): Ten 
  • Age: 28±6 years
  • Other relevant demographics: Subjects cycled approximately 150km per week, and mean VO2max was 58.2±10.1ml per kg-1 per minute-1.

Anthropometrics

  • Height: 182.1±7.1cm
  • Weight: 76.4 ±9.9kg.

Location

United Kingdom.

Summary of Results:

Key Findings

  • The average TT time in the LGI trial (93±8 minutes) was significantly shorter than the HGI trial (96±7 minutes) (T=-3.3; P=0.009)
  • There was a significant main effect for time (F= 465.6, P<0.001) but not trial (F=0.1, P=0.81) and no significant trial-by-time interaction for heart rate (F=1.6, P=0.17)
  • There was a significant main effect for both time (F=200.2, P<0.001) and trial (F=5.8, P=0.039) for RPE, where RPE was lower in the LGI trial. There was no significant trial-by-time interaction for RPE (F=1.6, P=0.17).
  • There was a significant main effect for both time (F=75.1, P<0.001) and trial (F=15.7, P=0.003) where carbohydrate oxidation was higher in the LGI trial, but no significant trial-by-time interaction for carbohydrate oxidation rates (F=1.6, P=0.2)
  • A significant main effect for both time (F=11.2, P<0.001) and trial (F=20.1, P=0.002) was observed for fat oxidation where the HGI trial had higher fat oxidation rates, but no significant trial-by-time interaction (F=0.4, P=0.83)
  • There was no significant difference in VO2 between the HGI (71±9% VO2max) and LGI trial (72±10% VO2max) (T= 4.0; P=0.88)
  • There was a significant main effect for both time (F=11.2, P=0.001) and trial (F=24.2, P<0.001) for respiratory exchange ratio (RER), where the RER values were significantly  higher in the LGI trial (0.94±0.03) compared with the HGI trial (0.90±0.03). No significant trial-by-time interaction was observed for RER (F=1.5, P=0.22).
  • There was a significant main effect for time (F=7.7, P=0.008) but not trial for whole blood glucose (F=2.2, P=0.17). There was a significant trial-by-time interaction effect for whole blood glucose concentration (F=6.9, P<0.001) where the post-prandial glucose concentration was significantly different between trials (P=0.018).
  • There was a significant main effect for time (F=9.0, P<0.001) but not trial (F=2.8, P=0.16) for FFA and no significant trial-by-time interaction (F=0.2, P=0.91)
  • There was no significant main effect for time (F=1.2, P=0.35) or trial (F=1.9, P=0.23) and no significant trial-by-time interaction for TGA (F=1.1, P=0.40)
  • Following the ingestion of the HGI meal the insulin concentration was significantly higher (60.3ng per ml) than after the LGI meal (48.4ng per ml) (P=0.008). There was a significant main effect for both time (F=33.7, P<0.001) and trial (F=17.4, P=0.014) and a significant trial-by-time interaction (F=11.3, P<0.001) for insulin. The post-prandial insulin concentration was significantly different between trials (P=0.008).
  • There was a significant main effect for time (F=26.4, P<0.001) but not trial (F=0.8, P=0.39) and no significant trial-by-time interaction for whole blood lactate concentration (F=1.5, P=0.21).

 

Author Conclusion:

The LGI meal appears to be associated with greater availability of carbohydrate throughout the exercise period, which may have sustained energy production towards the end of exercise. Consuming a LGI carbohydrate meal providing 1g per kg-1 body mass of carbohydrate 45 minutes prior to endurance performance may have a beneficial effect on TT performance.

Funding Source:
Other: The authors report no financial assistance was provided.
Reviewer Comments:

The authors did not report how subjects were recruited or report the inclusion or exclusion criteria used. The authors also stated that expired air and blood samples were collected throughout the time trial but did not describe the timing of these measurements (the timing was gleaned from Figures One, Two and Three and assumed to be the points indicated on the graphs).

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? 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) N/A
  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.) N/A
  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? 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? 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)? Yes
  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? Yes
  10.1. Were sources of funding and investigators' affiliations described? Yes
  10.2. Was the study free from apparent conflict of interest? Yes