EAL

NAP: Competition (2007)

Citation:

Clark VR, Hopkins WG, Hawley JA, Burke LM. Placebo effect of carbohydrate feedings during a 40-km cycling time trial. Med Sci Sports Exerc. 2000; 32 (9): 1,642-1,647.

PubMed ID: 10994918

Study Design:

Randomized crossover trial

Class:

A - Click here for explanation of classification scheme.

Quality Rating:

POSITIVE: See Quality Criteria Checklist below.

Research Purpose:

The purpose of this study is to examine the placebo effect of a carbohydrate supplement on endurance performance.

Inclusion Criteria:

Well-trained male and female endurance cyclists
Must have completed a local 105-km cycle tour in less than 3.5 hours
Training regularly for the past year by riding at least 200km per week.

Exclusion Criteria:

Those who did not finish in the top 43 during the first time trial.

Description of Study Protocol:

Recruitment: Studies on performance need to be as large as possible, and this fact was noted in the study. High-caliber male and female cyclists volunteered.
Design: Randomized crossover trial, balanced repeated measures
Blinding used: The subjects were told what they were drinking, however only one of the times it was true. This was necessary to evaluate the placebo effect.

Intervention (if applicable)

Subjects underwent a time trial during which they drank water
One week later, subjects performed another time trial, during which they consumed a flavored drink with or without carbohydrates
Subjects were randomized into groups based on what they were told: The first group was told their drink contained carbohydrates; the second group was told that their drink did not contain carbohydrates; cyclists in the third group were told they had a 50-50 chance of getting a drink with carbohydrates.
Subjects drank their beverage after 10, 20 and 30 km of the trial
Everyone was advised that, if given carbohydrates, they would probably show an improvement in performance
Overall, six treatments were arranged into random sequence. Subjects were asked to maintain their usual training and diets for the 24 hours before and kept a dietary and training diary for the two days before the time trial.

Statistical Analysis

Non-repeated measures, mixed modeling.

Data Collection Summary:

Timing of Measurements

Measurements were only taken once and were based on the entire ride.

Dependent Variables

Performance time: Amount of time taken to complete the trial
Mean power: Highest average power output maintained for 60 seconds (watts)
Peak oxygen uptake: Estimated from the peak power.

Independent Variables

Formulation of the drink (carbohydrate or placebo).

Control Variables

Amount of fluid ingested
Exercise and diet 24 hours prior to test
Testing environment
Feedback during ride.

Description of Actual Data Sample:

Initial N: Fifty-four (49 males; five females) recruited; 43 subjects had a performance time of less than 65 minutes in the first time trial
Attrition (final N): 43 (41 males; two females)
Age: Described per group; see below in anthropometrics
Ethnicity: Did not describe.

Anthropometrics: Subject Characteristics of the Six Treatment Groups


Given Carbohydrate		Not Given Carbohydrate
Measure	Told Carbohydrate	Told Placebo	Not Told	Told Carbohydrate	Told Placebo	Not Told
Number of cyclists	Seven	Seven	Seven	Eight	Seven	Seven
Age (years)	29±9	37±6	23±9	31±13	33±11	26±10
Height (cm)	183±6	178±8	177±9	173±7	178±8	177±5
Weight (kg)	78±14	76±14	67±11	67±7	79±12	77±9
Peak power (watts)	397±60	365±34	333±48	366±76	367±31	366±46
VO2peak (ml per kg per min)	64±8	62±9	64±8	59±8	69±10	61±7
First time trial (minutes)	56±4	57±4	59±4	60±3	58±5	59±4

Location: New Zealand.

Summary of Results:

Variables	Told Carbohydrate	Told Placebo	Not Told
Power output: Taking into account the effect of speed in the first time trial and variances in change in performance in the three groups	4.3±4.8%	0.5±5.8%	-1.1±8.5%

Other Findings

The full placebo effect (the change in power for those given carbohydrates minus the change in power for those given placebo) was 3.8% (P<0.06)
The real effect of carbohydrate on power output was a slight reduction in power at 0.3% (P<0.86)
The coefficient of variance among the subjects in the not-told group was 1.6 times larger (P<0.05) than the combined coefficients of variation of the subjects who were told what was in the drink.

Author Conclusion:

The main finding of the study is that the placebo effect of carbohydrates in a drink consumed during a time trial lasting about an hour is about 4% enhancement of mean power for competitive cyclists of overall moderate ability
This equates to an increase in speed of about 1.5%
Slower subjects are less reliable
Uncertainty of treatment can reduce the relability of an endurance performance test. Therefore, it is extremely important to have blinded, controlled studies.

Funding Source:

University/Hospital:

Sports Science Institute of South Africa, University of Otago (New Zealand), RMIT University, Australian Institute of Sport (Australia)

Reviewer Comments:

It is important to note that the researchers did not know when the last meal of the subjects was or of what it consisted
Research suggests that those who have a meal within three hours prior to exercise are less effected by carbohydrate ingestion during exercise that lasts 60 minutes or less.

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?	N/A
	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?	Yes
	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?	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?	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)?	Yes
	8.5.	Were adequate adjustments made for effects of confounding factors that might have affected the outcomes (e.g., multivariate analyses)?	No
	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