EAL

NAP: Training (2007)

Citation:

Erlenbusch M, Haub M, Munoz K, MacConnie S, Stillwell B. Effect of high-fat or high-carbohydrate diets on endurance exercise: a meta-analysis. Int J Sport Nutr Exerc Metab. 2005; 15 (1): 1-14.

PubMed ID: 15902985

Study Design:

Meta-analysis or Systematic Review

Class:

M - Click here for explanation of classification scheme.

Quality Rating:

POSITIVE: See Quality Criteria Checklist below.

Research Purpose:

To clarify whether the literature favors a high-fat or high-carbohydrate diet to yield superior endurance exercise performance, as measured by exercise time to exhaustion or time trial performance.

Inclusion Criteria:

Online computer searches (SportDiscus and Medline) were conducted to locate research, published 1930 to present, concerning effects of high-carbohydrate and high-fat diets on endurance exercise performance
Key words noted
On completion of computer search, resulting studies were hand-screened and additional reference lists generated, until no new studies were found
A high-fat diet was defined as consisting of more than 30% dietary fat and a high-carbohydrate diet was defined as having over 50% carbohydrate
Endurance exercise was defined as 60% to 90% VO_2max
Acceptable dietary adaptation periods defined to be no less than three days
Only cycling and running modalities were accepted.

Exclusion Criteria:

Trials using medium-chain triglycerides as a substitute for high dietary fat intake were not included because endurance athletes do not typically use such interventions
Trials with variables incompatible with measures needed for the analysis were extracted.

Description of Study Protocol:

Recruitment: 20 published trials were analyzed to compare exercise performance under different diets
Design: Meta-analysis
Blinding used: Not applicable
Intervention: High-carbohydrate or high-fat diets.

Statistical Analysis

For each study, an effect size was calculated to quantify the effect of various diets on endurance performance using a modified version of Glass' effect size equation
A pooled standard deviation was used as a control group
To correct for small sample size bias, each effect size was multiplied by a correction factor prior to further analysis
To determine the influence of moderator variables (i.e., training status, experiment duration, gender), each variable was correlated to effect size via regression analysis
The test for homogeneity was also completed to determine if all studies are estimating the same measures.

Data Collection Summary:

Timing of Measurements

Only studies with dietary adaptation periods not less than three days.

Dependent Variables

Endurance exercise performance, as measured by exercise time to exhaustion or time trial performance
Exercise intensity
Mode of exercise.

Independent Variables

High-fat or high-carbohydrate diets
Duration of dietary modification.

Control Variables

Training status
Gender.

Description of Actual Data Sample:

Initial N: 38 studies originally identified for inclusion

Attrition (final N)

20 analyzed, 18 not appropriate

Six rejected due to insufficient or missing data

Three rejected for exercise intensities above acceptable limits

Five rejected for insufficient dietary adaptation periods

Two rejected for use of abnormal exercise temperatures

Three excluded for using ice skating, rowing and manual labor rather than cycling or running

Total of 218 subjects in all studies: 163 male, 55 female.

Age

Not mentioned

Ethnicity

Not mentioned.

Other relevant demographics

Four trials used untrained subjects, 40 untrained subjects.

Location

South Africa.

Summary of Results:

Other Findings

Mean dietary modification period was 13.0±12.0 days (range, three to 49 days)

High fat diet was 32%±20% carbohydrate, 48%±16% fat

High carbohydrate diet was 70%±8% carbohydrate, 17%±7% fat

Mean time to exhaustion for the high-carbohydrate groups was 68.6±55.2 min (range 7.85-168.8 minutes), compared to 56.8±43.0 minutes (range 4.1-153.1 minutes) for high-fat groups

Mean time trial performance was 27.4±1.3km and 42.4±2.8km for those trials measuring distance and 70.1±50.1 minutes and 69.6±52.7 minutes for trials measuring time, respectively, for high-carbohydrate and high-fat diets

Mean quality score across all studies was 3.9±0.7

Eight did not demonstrate strict regulation and administration of diets

Five were not stated as randomized, three had three-day dietary modification periods

Six failed to provide a transition period longer than one day between diets

Six utilized a sample size of five or fewer subjects

In seven trials, subjects were not ensured to be in a physically rested state prior to performance tests.

The average effect size of -0.60 indicated a moderate effect and favoring subjects following a high-carbohydrate diet exercised longer until exhaustion

The training status of the subjects (trained or untrained) was significantly related to effect size (R=-0.576, P<0.01) and effect sizes separated between trained and untrained subjects were -0.05 and -2.84, respectively.

The test for homogeneity revealed significant heterogeneity among effect sizes [chi-square (19)=43.30, P<0.05], indicating that much of the variance could be explained by one or more moderator variables and that all of the trials are not describing the same effect.

Author Conclusion:

In conclusion, we found that endurance performance is enhanced following a high-carbohydrate diet, compared to a high-fat diet in untrained individuals, while the performance response in trained individuals appears to be blunted

Training status was the only moderator variable significantly correlated to effect size reflecting the significance of aerobic fitness to dietary modification with regard to performance

Past literature has used dissimilar design protocols making the need for consistent and high-quality future study the primary conclusion of this analysis

As a result of this heterogeneity across trials, a decisive endorsement for a high-carbohydrate diet cannot be offered with confidence, especially among trained athletes.

Funding Source:

University/Hospital: Kansas State University (Manhatten), Humbolt State University, California State University (Northridge)

Reviewer Comments:

Inclusion and exclusion criteria well-defined

Analysis well done.

Quality Criteria Checklist: Review Articles

Relevance Questions

1. Will the answer if true, have a direct bearing on the health of patients? Yes

2. Is the outcome or topic something that patients/clients/population groups would care about? Yes

3. Is the problem addressed in the review one that is relevant to dietetics practice? Yes

4. Will the information, if true, require a change in practice? Yes

Validity Questions

1. Was the question for the review clearly focused and appropriate? Yes

2. Was the search strategy used to locate relevant studies comprehensive? Were the databases searched and the search termsused described? Yes

3. Were explicit methods used to select studies to include in the review? Were inclusion/exclusion criteria specified andappropriate? Wereselectionmethods unbiased? Yes

4. Was there an appraisal of the quality and validity of studies included in the review? Were appraisal methodsspecified,appropriate, andreproducible? Yes

5. Were specific treatments/interventions/exposures described? Were treatments similar enough to be combined? Yes

6. Was the outcome of interest clearly indicated? Were other potential harms and benefits considered? Yes

7. Were processes for data abstraction, synthesis, and analysis described? Were they applied consistently acrossstudies and groups? Was thereappropriate use of qualitative and/or quantitative synthesis? Was variation in findings among studies analyzed? Were heterogeneity issued considered? If data from studies were aggregated for meta-analysis, was the procedure described? Yes

8. Are the results clearly presented in narrative and/or quantitative terms? If summary statistics are used, are levels ofsignificance and/or confidence intervals included? Yes

9. Are conclusions supported by results with biases and limitations taken into consideration? Are limitations ofthe review identified anddiscussed? Yes

10. Was bias due to the review's funding or sponsorship unlikely? Yes

Quality Criteria Checklist: Review Articles
Relevance Questions
	1.	Will the answer if true, have a direct bearing on the health of patients?	Yes
	2.	Is the outcome or topic something that patients/clients/population groups would care about?	Yes
	3.	Is the problem addressed in the review one that is relevant to dietetics practice?	Yes
	4.	Will the information, if true, require a change in practice?	Yes

Validity Questions
	1.	Was the question for the review clearly focused and appropriate?	Yes
	2.	Was the search strategy used to locate relevant studies comprehensive? Were the databases searched and the search termsused described?	Yes
	3.	Were explicit methods used to select studies to include in the review? Were inclusion/exclusion criteria specified andappropriate? Wereselectionmethods unbiased?	Yes
	4.	Was there an appraisal of the quality and validity of studies included in the review? Were appraisal methodsspecified,appropriate, andreproducible?	Yes
	5.	Were specific treatments/interventions/exposures described? Were treatments similar enough to be combined?	Yes
	6.	Was the outcome of interest clearly indicated? Were other potential harms and benefits considered?	Yes
	7.	Were processes for data abstraction, synthesis, and analysis described? Were they applied consistently acrossstudies and groups? Was thereappropriate use of qualitative and/or quantitative synthesis? Was variation in findings among studies analyzed? Were heterogeneity issued considered? If data from studies were aggregated for meta-analysis, was the procedure described?	Yes
	8.	Are the results clearly presented in narrative and/or quantitative terms? If summary statistics are used, are levels ofsignificance and/or confidence intervals included?	Yes
	9.	Are conclusions supported by results with biases and limitations taken into consideration? Are limitations ofthe review identified anddiscussed?	Yes
	10.	Was bias due to the review's funding or sponsorship unlikely?	Yes