NAP: Competition (2007)

Citation:

Riddell MC, Partington SL, Stupka N, Armstrong D, Rennie C, Tarnopolsky MA. Substrate utilization during exercise performed with and without glucose ingestion in female and male endurance-trained athletes. Int J Sport Nutr Exerc Metab. 2003; 13: 407-421.

PubMed ID: 14967866
 
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 compare substrate utilization between males and females during endurance exercise performed both with and without exogenous carbohydrate ingestion.
Inclusion Criteria:
  • Male and female endurance-trained athletes, predominantly runners, with cycling comprising less than 50% of training volume
  • Females were eumenorrheic and were tested in the mid-follicular phase of their menstrual cycle
  • Males were selected based on training history of consistent participation in endurance-type physical activity for at least one year (minimum of five days per week and 45 minutes per session) and peak oxygen consumption of at least 50ml per kg per minute
  • Females were matched to males based on similar training histories and by having a VO2peak of at least 45ml per kg per minute.
Exclusion Criteria:
Excluded if not included above.
Description of Study Protocol:
  • Recruitment: Subjects volunteered for study
  • Design: Randomized crossover trial
  • Blinding used: Subjects blinded; lab tests.

Intervention (if applicable)

  • Subjects each completed two exercise trials (90-minute cycle ergometry at 60% VO2peak) one week apart
  • Subjects drank, intermittently, either 8% exogenous carbohydrate (one gram glucose per kg per hour) enriched with U-13C glucose or an artifically sweetened placebo during the trial, within 30 seconds, at 20 minutes prior to the start of exercise and at zero, 15, 30, 45, 60 and 75 minutes of exercise.

Statistical Analysis

  • For measurements made repeatedly in both trials and in both genders, a three-way mixed ANOVA design [one between-factor (gender), two within-factors (trial and time)] was performed
  • Tukey's HSD post-hoc test was used to identify the location of the significant differences when ANOVA yielded a significant F-ratio
  • Inter-group differences in physical and functional characteristics were compared by using a non-paired T-test.
Data Collection Summary:

Timing of Measurements

Whole blood sampling and respiratory measurements were made at rest before ingestion of the first exogenous beverage, prior to the start of exercise and every 15 minutes during the exercise period.

Dependent Variables

  • Whole-body substrate oxidation was determined from RER, urinary urea excretion and the ratio of 13C:12C in expired gas during the final 60 minutes of exercise
  • Blood was analyzed for glucose and lactate.

Independent Variables

  • Subjects consumed, intermittently, either 8% exogenous carbohydrate (one gram glucose per kg per hour), enriched with U-13C glucose or an artifically sweetened placebo
  • Four-day dietary records were collected from each subject
  • Subjects were asked to rest the day prior to testing
  • Subjects consumed a pre-test meal of 375kcal for males and 235kcal for females, 55% carbohydrate, 30% fat, 15% protein.

Control Variables

  • Subjects were asked to repeat the same nutrient intake and keep intensity and type of exercise consistent for two days preceding trials.
Description of Actual Data Sample:
  • Initial N: 14 subjects, seven male, seven female
  • Attrition (final N): 14
  • Mean age: Males, 25.7±4.6 years; females, 23.3±1.5 years
  • Ethnicity: Not mentioned
  • Anthropometrics: Males were taller, heavier, leaner and had higher absolute VO2peak values, as expected, with gender differences
  • Location: Canada.
Summary of Results:

 Substrate Oxidation During Carbohydrate Trial

  30 Minutes 45 Minutes 60 Minutes 75 Minutes

90 Minutes

Females: Exogenous CHO 0.50±0.03 0.66±0.05 0.77±0.06 0.94±0.08, P<0.05 0.97±0.08, P<0.05
Females: Endogenous CHO 3.56±0.36 3.50±0.28 3.25±0.22 3.01±0.18 2.94±0.26
Females: Total CHO 4.14±0.35 4.15±0.28 4.05±0.23 4.07±0.23 4.00±0.28
Females: Fat 0.40±0.09 0.39±0.07 0.43±0.06 0.46±0.07 0.45±0.06
Males: Exogenous CHO 0.49±0.05 0.55±0.05 0.67±0.05 0.70±0.07, P<0.05 0.77±0.08, P<0.05
Males: Endogenous CHO 4.39±0.31 3.95±0.23 3.82±0.26 3.88±0.29 3.74±0.28

Males: Total CHO

4.89±0.29

4.50±0.21 4.48±0.22

4.58±0.25

4.52±0.23

Males: Fat

0.25±0.09

0.40±0.11 0.44±0.10

0.40±0.08

0.45±0.10

Other Findings

  • During the placebo trial, fat oxidation was higher in females than in males (0.42±0.07 vs. 0.32±0.09g per minute per kg lean body mass) at 30 minutes of exercise (P<0.05)
  • When averaged over the final 60 minutes of exercise, the relative proportions of fat, total carbohydrates and protein were similar between groups
  • During exogenous carbohydrate ingestion, both the ratio of 13C:12C in expired gas (P<0.05) and the proportion of energy derived from exogenous carbohydrates relative to lean body mass (P<0.05) were higher in females, compared to males at 75 and 90 minutes exercise 
  • When averaged over the final 60 minutes of exercise, the percentage of exogenous carbohydrates to the total energy contribution tended to be higher in females (14.3±1.2%) than in males (11.2±1.2%, P=0.09)
  • The reduction in endogenous carbohydrate oxidation with exogenous carbohydrate intake was also greater in females (12.9±3.1%) than in males (5.1±2.0%, P=0.05).
Author Conclusion:
  • The main finding of this study is that, during the later stages of prolonged running, female athletes utilize more ingested exogenous carbohydrates than male athletes exercising at the same relative intensity (60% of VO2peak)
  • In addition, it appears that exogenous carbohydrate intake may spare more endogenous carbohydrate in females than in males during prolonged running. Compared to males, females may oxidize a greater relative proportion of exogenous carbohydrate during endurance exercise which, in turn, may spare more endogenous fuel. Based on these observations, ingested carbohydrates may be a particularly beneficial source of fuel during endurance exercise for females.
Funding Source:
Government: NSERC (Canada)
University/Hospital: McMaster University Medical Centre, York University (all Canada)
Reviewer Comments:
Authors note that the failure to observe gender differences at other time points in the placebo trial may be related to the pre-exercise snack.
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? 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? Yes
  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? Yes
  10.1. Were sources of funding and investigators' affiliations described? Yes
  10.2. Was the study free from apparent conflict of interest? Yes