EAL

NAP: Training (2007)

Citation:

Hawley JA, Palmer GS, Noakes TD. Effects of three days of carbohydrate supplementation on muscle glycogen content and utilization during a 1-hour cycling performance. Eur J Appl Physiol. 1997; 75: 407-412. PubMed ID: 9189727

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 determine whether elevating pre-exercise muscle glycogen content by supplementing an athlete's normal diet with extra CHO results in improved cycling performance during a one-hour time trial.

Inclusion Criteria:

Well-trained male endurance cyclists
All had been involved in endurance training for at least three years and competed regularly in local cycle races.

Exclusion Criteria:

None specifically mentioned.

Description of Study Protocol:

Recruitment: Recruitment methods not specified
Design: Randomized crossover trial
Blinding used: Subjects were blinded to investigation; lab tests used
Intervention: Subjects consumed either their normal diet or additional carbohydrate as Carbo Bars for three days.

Statistical Analysis

Statistical significance of the effects of supplementary dietary CHO on pre-exercise muscle glycogen content assessed with Wilcoxon's rank order test
Differences in plasma glucose, lactate concentrations, heart rate, peak power output and the distance attained during the performance rides were assessed using a paired Student's T-test
Rates of total CHO and fat oxidation during the two experimental trials were compared using a one-way ANOVA for repeated measures.

Data Collection Summary:

Timing of Measurements

Two three-day experimental trials separated by a seven-day washout period
After a five-minute self-paced warmup, subjects commenced a one-hour time trial
Muscle biopsies collected and blood samples taken at rest and after 15, 30, 45 and 59 minutes of the time trial.

Dependent Variables

Muscle biopsies taken from vastus lateralis for muscle glycogen content
Blood samples analyzed for plasma glucose and lactate
Gas exchange measurements taken.

Independent Variables

Normal diet for three days (5.9±1.4g per kg per day) or additional carbohydrate (9.3±0.7g per kg per day)
Four-day dietary records including one weekend day were obtained from subjects who had been instructed on how to record all fluid and food consumed
Subjects given dietary sheets and instructions on diet to follow
Additional carbohydrate was added to diet through potato-based Carbo Bars (each 100-g bar contained 6.8% protein, 0.31% fat, 72.9% CHO, 16.0% water and 3.6% ash)
During the normal diet trial, subjects consumed a Placebo beverage, which did not contain any calories
Diets were not isocaloric.

Description of Actual Data Sample:

Initial N: Six trained cyclists, all male
Attrition (final N): Six
Age: Mean, 20.2±2.6 years
Ethnicity: Not mentioned
Other relevant demographics: Mean VO_2max, 4.5±0.36L per minute
Location: South Africa.

Summary of Results:

	Normal Diet	CHO-Supplemented Diet	P-Value
CHO (g)	426±137	661±76	<0.05
CHO (g/kg body wt)	5.9±1.4	9.3±0.7	<0.05
CHO %	46.9±6.9	61.4±5.8	<0.05
Protein (g)	130±33	123±27	NS
Protein (g/kg body wt)	1.8±0.3	1.7±0.3	NS
Protein %	15.4±1.6	12.0±1.7	<0.05
Fat (g)	139±64	110±29	NS
Fat (g/kg body wt)	1.9±0.7	1.5±0.3	NS
Fat %	34.5±6.0	23.3±4.1	<0.05
Total energy (kJ)	14.657±4.788	17.334±2.545	NS

Other Findings

The carbohydrate-supplemented diet increased pre-exercise muscle glycogen stores, but there was no difference in the distance cycled in the time trials (40.41±1.44 vs. 40.18±1.76km for normal diet and CHO-supplemented diet, respectively)
With a normal diet, muscle glycogen declined from 459±83 to 175±64mmol per kg whereas, with the CHO-supplemented diet, muscle glycogen declined from 565±62 to 292±113mmol per kg
Accordingly, both muscle glycogen utilization and total CHO oxidation were similar
There were no differences in plasma glucose or lactate concentrations between trials. Plasma glucose concentrations averaged 5.5±0.5 and 5.6±0.6mmol per L and plasma lactate concentrations averaged 4.4±1.9 and 4.4±2.3mmol per L for normal diets and CHO-supplemented diets, respectively.

Author Conclusion:

In conclusion, the results of the present investigation show that when well-trained subjects increase the CHO content of their normal diet for three days from six to nine grams per kg per day, there is a modest, but significant, increase in muscle glycogen content
However, the increase in muscle glycogen content after CHO supplementation did not result in any improvement in one-hour cycle time trial performance, nor did it affect the rates of muscle glycogen utilization or total CHO or fat oxidation
As substantial quantities of glycogen were present in the working muscles at the end of both time trials, it is clear that CHO-loading has no benefit for athletes who habitually consume a moderate-to-high (approximately six grams per kg per day) CHO diet and who participate in high-intensity (80% VO_2max) events lasting 60 minutes.

Funding Source:

Government:

Medical Research Council of South Africa

Industry:

South African Potato Producers Organization

Commodity Group:

University/Hospital:

University of Capetown

Reviewer Comments:

Diets were not isocaloric
Macronutrient percentages were significantly different for protein as well as CHO
Recruitment methods, inclusion criteria and exclusion criteria 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?	N/A
	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?	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?		N/A
	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?	N/A
	3.3.	Were concurrent controls or comparisons used? (Concurrent preferred over historical control or comparison groups.)	N/A
	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?	???
	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?		???
	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?	N/A
	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?	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