NAP: Training (2007)

Citation:

Goedecke JH, Christie C, Wilson G, Dennis SC, Noakes TD, Hopkins WG, Lambert EV. Metabolic adaptations to a high-fat diet in endurance cyclists. Metabolism, 1999; 48 (12): 1,509-1,517.

PubMed ID: 10599981
 
Study Design:
Randomized controlled trial
Class:
A - Click here for explanation of classification scheme.
Quality Rating:
Positive POSITIVE: See Quality Criteria Checklist below.
Research Purpose:
To examine the effects of a high-fat diet on glucose tolerance, certain muscle enzyme activities and substrate metabolism during constant-load exercise.
Inclusion Criteria:
  • Endurance-trained male cyclists
  • Preliminary testing completed to ensure that subjects were sufficiently well-trained.
Exclusion Criteria:
None specifically mentioned.
Description of Study Protocol:
  • Recruitment: Recruitment methods not defined
  • Design: Randomized controlled trial
  • Blinding used: Lab tests used for measurements.
  • Intervention: Cyclists randomly assigned to control group consuming habitual diet or a high-fat isocaloric diet group for 15 days.

Statistical Analysis

  • One-way ANOVA for repeated measures was used to determine changes in muscle enzyme activities, plasma glucose and ingested glucose oxidation rates and circulating metabolite concentrations in the high-fat diet group
  • Least-significant difference post hoc analysis was used
  • Two-way ANOVA for repeated measures was used to determine differences in total fat and carbohydrate oxidation rates and time-trial performance times between the high-fat diet and control groups
  • Differences between groups for the oral glucose tolerance test and circulating blood metabolites were determined using two-way ANOVA for repeated measures at Days Zero and 15.
Data Collection Summary:

Timing of Measurements

  • Dietary intervention lasted 15 days
  • At five-day intervals, subjects underwent an oral glucose tolerance test and on the following day performed a 2.5-hour constant-load ride, followed by a simulated 40-km cycling time trial, while ingesting glucose + MCT emulsion.

Dependent Variables

  • Oral glucose tolerance test administered before experimental ride after overnight fast
  • Experimental ride after overnight fast
  • Steady state VO2 and VCO2 values were measured over five-minute intervals every 30 minutes during the constant-load exercise
  • Venous blood samples were drawn at rest and at 30-minute intervals during constant-load exercise and analyzed for glucose, lactate, insulin, free fatty acids, glycerol, beta-hydroxybutyrate and 14C-glucose concentrations
  • Muscle biopsies were obtained from the vastus lateralis muscles of the high-fat diet group on Days Zero, 10 and 15 and analyzed for carnitine acyltransferase, citrate synthase and three-hydroxyacyl-coenzyme A dehydrogenase. 

Independent Variables

  • Assigned to control group of habitual diet or high-fat isocaloric diet for 15 days
  • Subjects had completed three-day dietary records so that isocaloric high-fat diets could be devised
  • To aid adherence, subjects indicated their food preferences and meals were provided together with a diary to record any deviations from the diet 
  • 90 minutes before and during exercise, subjects ingested a 10%-14C-glucose + 3.44% medium-chain triglyceride (MCT) emulsion at a rate of 600mL per hour.
Description of Actual Data Sample:
  • Initial N16 male cyclists
  • Attrition (final N): 16 male cyclists; eight in control group, eight in high-fat diet group.
Age
  • Control group: 30±9 years
  • High-fat diet group: 24±3 years. 

Ethnicity

Not mentioned.

Other Relevant Demographics

  • Control group body fat: 12.8±5%
  • High fat diet: 7.5±2%.

Anthropometrics

There were no significant differences between groups with respect to age, weight, height, body fat, peak oxygen consumption and peak sustained power output.

Location

University of Cape Town, South Africa.

Summary of Results:

 

 

Habitual Diet; Controls

Habitual Diet; High-Fat

Experimental Diet; High-Fat

kJ

13,210±5,974

14,334±3,382

16,442±4,686

CHO percentage

53±10

48±10

19±1

Fat percentage

30±8

32±8

69±1

Protein percentage

13±3

14±2

10±1

Alcohol percentage

0.98±2

3.2±3

0

PS ratio

0.58±0.2

0.72±0.2

0.82±0.2

 Other Findings

  • The high-fat diet did not cause any significant differences in the area under the plasma glucose and insulin concentration curves during the OGTT
  • In the OGTT, plasma glucose concentrations at 30 minutes increased significantly after five days of the high-fat diet and remained elevated at Days 10 and 15 vs. the levels measured prior to the high-fat diet (P<0.05)
  • In the control group, plasma glucose at 30 minutes following glucose ingestion was similar at Days Zero and 15
  • Furthermore, plasma glucose concentrations at 120 minutes during the OGTT increased significantly after five days of the high-fat diet (P<0.05)
  • There were no differences in plasma insulin concentrations between the two groups
  • The high-fat diet had no effect on plasma glucose, lactate and insulin concentrations during the 150 minutes of constant-load exercise on the day following OGTT
  • Exposure to the high-fat diet for as little as five days significantly increased serum glycerol concentrations throughout the constant-load exercise (P<0.001)
  • When serum glycerol concentrations in the high-fat diet and control groups were compared on Days Zero and 15, serum glycerol in the high-fat diet group was significantly higher at 30, 60 and 90 minutes of the constant-load exercise (P<0.05)
  • The activity of carnitine acyltransferase in biopsies of the vastus lateralis muscle also increased from 0.45 to 0.54micromol grams per minute over Days Zero to 10 of the high-fat diet (P<0.01), without any change in citrate synthase or three-hydroxyacyl-coenzyme A dehydrogenase activities
  • Changes in glucose tolerance and carnitine acyltransferase activity were associated with a shift from carbohydrate to fat oxidation during exercise (P<0.001), which occurred within five to 10 days of the high-fat diet
  • Increases in the rate of fat oxidation with the high-fat diet resulted in corresponding decreases in the rate of total carbohydrate oxidation from about 2.5 to 2.0g per minute during the constant-load exercise (P<0.05)
  • During the constant-load ride, the calculated oxidation of muscle glycogen was reduced from 1.5 to 1.0g per minute (P<0.001) after 15 days of the high-fat diet
  • Although exercise performance tended to improve to a greater extent in the high-fat diet group vs. the control group, there were no significant effects of diet on repeated 40-km time trial performances
  • The difference in exercise performance between the high-fat diet groups was 0.29 minutes (95% confidence interval: -2.4 to 3.0 minutes) over the 15-day trial.
Author Conclusion:
  • In conclusion, exposure to a high-fat diet for as little as five to 10 days resulted in an increase in fat oxidation and a concomitant sparing of muscle glycogen during exercise
  • This shift in substrate metabolism may be due to increases in muscle carnitine acyltransferase activity, low muscle glycogen contents and possibly altered glucose tolerance.
Funding Source:
Government: Medical Research Council of South Africa
Industry:
Bromar Foods, Potato Growers Association of South Africa
Food Company:
University/Hospital: University of Cape Town
Not-for-profit
0
Foundation associated with industry:
Reviewer Comments:
  • Recruitment not well defined
  • Inclusion criteria and exclusion criteria were not well-defined, but preliminary testing was completed to ensure training level
  • Meals provided with diary to aid compliance
  • Groups may have had similar performance due to ingestion of CHO and MCT before and during exercise.
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? ???
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) No
  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? N/A
  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%.) N/A
  4.3. Were all enrolled subjects/patients (in the original sample) accounted for? N/A
  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? 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)? 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? 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