EAL

NAP: Competition (2007)

Citation:

Carter JM, Jeukendrup AE, Mann CH, Jones DA. The effect of glucose infusion on glucose kinetics during a 1-h time trial. Med Sci Sports Exerc. 2004; 36 (9): 1,543-1,550.

PubMed ID: 15354036

Study Design:

Non-randomized crossover trial

Class:

C - Click here for explanation of classification scheme.

Quality Rating:

NEUTRAL: See Quality Criteria Checklist below.

Research Purpose:

The first objective was to determine the fate of glucose infused during exercise exceeding 75% VO_2max
The second objective was to determine whether infused glucose would improve time trial performance at this intensity.

Inclusion Criteria:

Endurance-trained cyclists: All had been involved in studies with cycle ergometry at this intensity and in similar conditions.

Exclusion Criteria:

None specifically mentioned.

Description of Study Protocol:

Recruitment: Methods not specified
Design: Non-randomized crossover trial; counter-balanced but not randomized
Blinding used: Double-blind.

Intervention

Subjects completed two performance trials in which they had to accomplish a set amount of work as quickly as possible (991±41 kJ)
Subjects were infused with either glucose (20% in saline, 1.0g CHO per minute) or saline placebo
Each visit was separated by seven days.

Statistical Analysis

All time and effect data for both intensities were analyzed using repeated-measures ANOVA
Specific differences were determined using paired Student's T-test with Bonferroni correction as post-hoc test.

Data Collection Summary:

Timing of Measurements

Heart rate was recorded continuously
Blood samples were obtained at every 12.5% including baseline.

Dependent Variables

Time-trial performance
Heart rate measured via telemetry
Respiratory gas measurements
Blood samples analyzed for glucose, lactate, insulin.

Independent Variables

Subjects were infused with either glucose (20% in saline, 1.0g CHO per minute) or saline placebo
Subjects followed same diet prior to trials.

Control Variables

Starting muscle glycogen concentrations.

Description of Actual Data Sample:

Initial N: Six males
Attrition (final N): Six
Age: Mean age, 31.2±7.2 years
Ethnicity: Not mentioned
Location: United Kingdom.

Summary of Results:

Other Findings

Plasma glucose concentration increased from 4.8±0.1mmol/L to 5.9±0.3mmol/L during the placebo trial and from 4.9±0.1mmol/L at rest to 12.4±1.1mmol/L during the carbohydrate trial. These values were significantly higher at all time points during the carbohydrate trial, compared with placebo (P<0.001).
In the final stages of the time trial, the rate of glucose disappearance in the placebo trial was 49±5mmol per kg per minute, compared with 88±7mmol per kg per minute in the carbohydrate trial (P<0.05)
Rates of total carbohydrate oxidation did not differ at any time point between trials
Despite these differences, there was no difference in performance time between placebo and carbohydrate (60.04±1.47 minutes for placebo vs. 59.90±1.49 minutes for carbohydrate). There were no differences in heart rate between trials.
Infused carbohydrate oxidation in the last 25% of the carbohydrate trial was at least 675±120mmol per kg and contributed 17±4% to total carbohydrate oxidation.

Author Conclusion:

In conclusion, improved performance of high-intensity one-hour cycle time trials is not related to an increased availability of glucose and maintained carbohydrate oxidation rates
It appears that during exercise of this nature, endogenous supplies of carbohydrate (muscle and liver glycogen and plasma glucose) are not limiting
The mechanisms responsible for the improvement in performance of one-hour time trials with exogenous carbohydrate warrants further investigation.

Funding Source:

University/Hospital:

University of Birmingham, Birmingham Women's Hospital

Reviewer Comments:

Inclusion and exclusion criteria and recruitment methods were not well-defined
Power calculations were not done
Authors note that the invasive nature of the exercise protocol may have influenced results.

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?	???
	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?	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?		???
	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?	???
	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?		???
	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?	No
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