DFA: Conjugated Linoleic Acid (CLA) Supplementation and Intermediate Health Outcomes (2011)

Study Design:
- Click here for explanation of classification scheme.
Quality Rating:
Research Purpose:

To study the effects of CLA supplementation alone or along with aerobic endurance training on endurance performance, body composition, serum lipids, leptin, insulin and glucose levels in physically active, young females.

Inclusion Criteria:
  • Healthy
  • Other inclusion criteria not given.
Exclusion Criteria:
  • None apparent before study
  • Four subjects from the CLA supplemented group were excluded from the study because of gastrointestinal symptoms of discomfort. [reviewer: it is not clear when this took place, but appears to be after randomization to and start of study]
Description of Study Protocol:


Announcement in the School of Physical Education and Sports.


  • Randomized, single-blind, controlled
  • Method of randomization not given.

Blinding used



  • Control (C)
  • CLA-supplementation (CLA)
  • Exercise (Ex)
  • Exercise and CLA supplementation (Ex-CLA).

Statistical Analysis

  • All P values were two-tailed
  • Descriptive statistics were completed initially
  • Differences between baseline and post-training values for all parameters of each group were analyzed by using Wilcoxon test 
  • Kruskal-Wallis and posthoc Bonferroni tests were used to analyze differences between groups
  • Percentages of differences between baseline and post-training values were calculated as: {(after value)/(before value)-1}x100 for all variables
  • P-values <0.05 were regarded as significant. 
Data Collection Summary:

Timing of Measurements

Data collected at baseline and six weeks after exercise or CLA supplementation.

Dependent Variables

  • Daily Energy Expenditure: Basal metabolism rates (BMR) were estimated by Katch and Katch equation. BMR results were multiplied by 1.2 as an activity factor to find daily energy expenditure except exercise training. Energy consumption amounts in exercise training added to daily energy expenditure.
  • Blood samples: After 12-hour fasting; triglyceride, free fatty acids, total cholesterol, HDL, LDL, leptin, apo-AI, apo-B100, insulin and glucose parameters. Serum insulin and leptin determined by enzyme immunoassay. Others determined using enzymatic methods on an Integra 800 Analyzer (Roche Diagnostics).
  • Anthropometrics: Stature, waist and hip girth; waist to hip and waist to height ratios calculated; body fat ratio, body weight, fat mass and fat-free mass, measurements and BMI calculations were done using a Tanita 300 MA Bioelectrical Impedance Analyze System. All anthropometrics and BIA measurements were done at least one week prior to menses, between 15:00 to 16:30.
  • Exercise tests: Incremental treadmill test was applied to determine the running velocities corresponding to 65% and 80% of heart rate reserve and 30-minute running performance were tested on motor-driven treadmills (Cosmed T 150E). Telemetric heart rate monitors were used to monitor heart rates attained during the test.

Independent Variables

  • Control [please see review comment]
  • CLA: CLA supplemented at 3.6g per day of mixed CLA isomers (Skip, Sweden) [no information on isomers was given]
  • Exercise: Subjects in Ex and ExCLA participated in exercise training three times a week for six weeks. Aim was to sustain a treadmill running until exhaustion at 30-minute running speeds with pre-determined exercise intensity in each session. Running speeds were enhanced after two successive sessions that a subject could sustain the exercise over 33 minutes. CLA group and controls did not change their routine daily activities.
  • ExCLA: Exercise and CLA supplementation.

Control Variables

None given

Description of Actual Data Sample:
  • Initial N: 48 females
  • Attrition (final N):
    • 44 subjects had complete follow-up data
    • Four subjects from the CLA supplemented group were excluded from the study because of gastrointestinal symptoms of discomfort
  • Age: Mean ages ranged from 20.4 to 21.9 years
  • Ethnicity: Caucasian
  • Other relevant demographics: None given
  • Anthropometrics:
    • Body weight (kg): Means ranged from 57.5 to 64.7
    • Body fat ratio (percent): Means ranged from 23.1 to 27.4
    • Fat-free mass (kg): Means ranged from 44 to 47.3
    • Body fat mass (kg): Means ranged from 13.5 to 17.4
    • BMI (kg/m2): ExCLA 22.5±1.7; CLA 23.3±1.2; Ex 21.6±1.6; Controls 20.8±1.6 (CLA greater than control, P<0.01)
    • Waist girth (cm): Means ranged from 68.7 to 74
    • Hip girth (cm): Means ranged from 96.7 to 100.7
    • Waist to hip (cm/cm): Means ranged from 0.71 to 0.75
    • Waist to height (cm/m): Means ranged from 41.5 to 44.5  
    • Glucose (mg/dL): ExCLA 81.6±9.9***; 79.9±5.6**; 72.2±11.7; 60±17 (Different from control: ** P<0.01; ***P<0.001)
  • Location: Not given.
Summary of Results:

 Key Findings [please see review comments as going through this section]

  • Daily energy expenditures were similar between exercising groups (ExCLA and Ex) and also between non-exercising groups (CLA and controls) while differences existed amongst exercising and non-exercising groups. (ExCLA 42±3; CLA 38.4±2.1; Ex 41.4±2.2; Controls 37.9±0.7kcal.-1kg.-1d). (P-values shown in Table II of the article; ranged from 0.012 to 0.032) 
  • When compared with baseline values, all variables improved significantly after the study procedure within each experimental group and as compared with controls, except for body weight and BMI of the CLA group and waist-to-hip ratio of CLA and Ex groups. Integrated analyses of variation ratios (percent difference as compared to baseline) are shown in Table III of the article. [actual six week data for each variable was not given]
  • A statistically significant difference was observed in waist girth variation ratio between ExCLA and Ex groups. [please see review comment]
  • Serum glucose levels of ExCLA (approximate percent 15.5; P<0.05) and CLA groups (approximate percent 9; P<0.01) were decreased significantly within the group and as compared with controls (P=0.032). [actual values not given]
  • Serum insulin concentration was reduced only in the ExCLA group (P<0.05) but this reduction was not significantly different from controls. [actual values not given]

 Other Findings

  • Baseline endurance levels did not differ amongst groups (shown in Table V of the article)
  • Other measures of endurance performance increased significantly in ExCLA and Ex groups (P<0.01) but did not vary in CLA and control groups as compared with baselines. The ExCLA and EX groups were not different from each other.
  • Baseline endurance levels did not differ amongst groups. V-HRR65, V-HRR80 and V-30min increased significantly in ExCLA and Ex groups (P<0.01) but did not vary in CLA and control groups as compared to baselines. The ExCLA and Ex groups were similar. 
Author Conclusion:
  • It was shown that both CLA and exercise were effective in improvement of body composition and these effects were cumulated when they have been used together
  • CLA supplementation alone or with exercise seems effective on serum glucose and insulin concentrations but ineffective on endurance performance. [please see review comments]
Funding Source:
University/Hospital: Committee of Scientific Projects of Celal Bayar University
Reviewer Comments:
  • There is an obvious language barrier with this article and failures in translation. So, some parts may not make much sense as I did not want to take too much leeway in adding in what I thought might be appropriate words.
  • No description is given for the control group. So this raises the question regarding the meaning of the single-blind aspect of the design. Were the subjects or investigators blinded? This is not clear, though you could assume the controls were given an actual control supplement and thus the subjects were blinded, but there is no mention of this anywhere in the article.
  • No method of randomization was given and given the N for each group it does not appear to be a true randomization. No subjects from the control withdrew, thus if true randomization, there should be a greater N in the control group.
  • The statistical analysis and especially the presentation of results is confusing. Actual data for the post-intervention is not given, the results are presented as variation or alteration ratios (percent difference compared to baseline). Table III can be deciphered somewhat as they show the ratio and if it is positive or negative. Table IV is set up differently and only shows the P values, so it is unclear in that table which direction differences occur. Also in all of the tables it is not very clear if there were differences between ExCLA and Ex or ExCLA and CLA.
  • The conclusions made by the authors are somewhat difficult to critique due to the lack of actual data presented and use of variation ratios etc. In looking at the baseline values, it also appears that the control group tended to have lower values in key variables such body fat mass, BMI and glucose which could have influenced the results obtained.
  • No limitations are given by the authors.
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? No
  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? ???
  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? No
  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? ???
  5.1. In intervention study, were subjects, clinicians/practitioners, and investigators blinded to treatment group, as appropriate? ???
  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? No
  6.1. In RCT or other intervention trial, were protocols described for all regimens studied? No
  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? No
  6.5. Were co-interventions (e.g., ancillary treatments, other therapies) described? N/A
  6.6. Were extra or unplanned treatments described? N/A
  6.7. Was the information for 6.4, 6.5, and 6.6 assessed the same way for all groups? ???
  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? ???
  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? ???
  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? ???
  7.6. Were other factors accounted for (measured) that could affect outcomes? ???
  7.7. Were the measurements conducted consistently across groups? ???
8. Was the statistical analysis appropriate for the study design and type of outcome indicators? No
  8.1. Were statistical analyses adequately described and the results reported appropriately? No
  8.2. Were correct statistical tests used and assumptions of test not violated? ???
  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)? No
  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? No
  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? No
  9.1. Is there a discussion of findings? Yes
  9.2. Are biases and study limitations identified and discussed? No
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