EAL

NAP: Competition (2007)

Citation:

McConell G, Kloot K, Hargreaves M. Effect of timing of carbohydrate ingestion on endurance exercise performance. Med Sci Sports Exerc. 1996; 28 (10): 1,300-1,304.

PubMed ID: 8897388

Study Design:

Randomized crossover trial

Class:

A - Click here for explanation of classification scheme.

Quality Rating:

NEUTRAL: See Quality Criteria Checklist below.

Research Purpose:

To compare the effects of carbohydrate ingestion throughout exercise with ingestion of an equal amount of carbohydrates late in exercise.

Inclusion Criteria:

Well-trained male cyclists and triathletes
Subjects completed a medical questionnaire.

Exclusion Criteria:

None specifically mentioned.

Description of Study Protocol:

Recruitment: Methods not specified
Design: Randomized crossover trial
Blinding used: Double-blind.

Intervention

Subjects cycled for two hours at 70±1% VO_2peak, followed immediately by a 15-minute performance ride, while ingesting either a 7% carbohydrate-electrolyte solution, an artificially sweetened placebo (both every 15 minutes) or the placebo for the first 90 minutes, then a 21% glucose solution at 90, 105 and 120 minutes
The amount of carbohydrates ingested was the same in both carbohydrate trials (157.5g)
Trials were separated by a minimum of five days.

Statistical Analysis

Data from three trials were compared using a two-factor ANOVA for repeated measures
Specific differences were located using the Student-Newman-Keuls post-hoc test.

Data Collection Summary:

Timing of Measurements

Blood samples obtained pre-exercise and at zero, 15, 30, 60, 90, 105, 120 and 135 minutes
Heart rate and expired air samples were collected every 30 minutes during exercise and at five-minute intervals during a 15-minute performance ride.

Dependent Variables

Blood samples were analyzed for glucose, hemoglobin, lactate, insulin
Expired air samples using indirect calorimetry through Douglas Bag
Heart rate.

Independent Variables

A 7% carbohydrate-electrolyte solution, an artificially sweetened placebo or the placebo for the first 90 minutes, then a 21% glucose solution
Subjects were provided with food
Instructed to refrain from caffeine, alcohol and tobacco prior to trials.

Control Variables

Diet and exercise controlled for 24 hours prior to testing.

Description of Actual Data Sample:

Initial N: Eight men
Attrition (final N): Eight
Age: Mean, 23±1 years
Ethnicity: Not mentioned
Location: Australia.

Summary of Results:

	Control	7% Carbohydrate	Placebo - 21% Carbohydrate
Total Work (kJ)	242±9	268±8, P<0.05	253±10
Heart Rate (beats per minute)	166±3	170±3	171±3
VO₂ (l per min)	3.96±0.14	4.37±0.14, P<0.05	4.21±0.15
Percentage VO_2peak	80.5±3.2	88.5±1.2, P<0.05	85.1±1.1
RER	0.93±0.02	0.97±0.01, P<0.05	0.96±0.02, P < 0.05
CHO oxidation (g per minute)	3.66±0.37	4.64±0.25, P<0.05	4.30±0.32, P < 0.05
Lactate (mmol/l)	5.5±1.1	9.0±1.3	7.0±1.2

Other Findings

Oxygen uptake and heart rate were similar among trials
At the start of the performance ride, plasma glucose averaged 4.2±0.2, 5.2±0.1 and 5.7±0.2mmol per liter in control, carbohydrate-electrolyte and placebo-carbohydrate, respectively (all P<0.05).
Plasma insulin levels were similar just prior to the performance ride in both carbohydrate trials, with both higher than control and a similar pattern was observed with RER
Work performed during the performance ride was significantly greater in the 7% carbohydrate group (268±8kJ), compared with controls (242±9kJ).
Performance in placebo-carbohydrate group (253±10kJ) however, was not improved compared with control, despite higher plasma glucose levels and plasma insulin levels similar to the 7% carbohydrate group.

Author Conclusion:

In conclusion, ingestion of carbohydrates throughout prolonged exercise improves the ability to produce work during a subsequent 15-minute performance ride, while ingestion of the same quantity of carbohydrates late in exercise does not significantly improve work output, compared with ingestion of a placebo
The inability to significantly improve work output when carbohydrates were ingested late in exercise occurred despite elevated plasma glucose and insulin levels
These results suggest that carbohydrate ingestion may improve performance through mechanisms other than or in addition to increased carbohydrate availability to contracting skeletal muscle.

Funding Source:

Industry:

Cadbury Schweppes Pty. Ltd

Food Company:

University/Hospital:

University of Melbourne

Reviewer Comments:

Inclusion criteria, exclusion criteria and recruitment methods were not well-defined.

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?	???
	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?	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?	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)?	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?	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