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 determine the separate effects of three doses of highly enriched isomers of CLA as c9t11 or t10c12 on body composition, blood lipid profile and markers of insulin resistance in healthy men.

Inclusion Criteria:
  • Male
  • 20 to 47 years of age
  • Healthy with BMI higher than 18 and lower than 34kg/m2.
Exclusion Criteria:
  • Diagnosed CVD, diabetes, liver or endocrine dysfunction, or chronic inflammatory disease
  • Taking medication
  • Vegetarians or vegans
  • Heavy smokers of more than 10 cigarettes per day
  • Heavy consumers of alcohol
  • Using supplements such as vitamins or oils
  • Screened for abnormal fasting plasma cholesterol, TGs, and glucose and for markers of liver disease.
Description of Study Protocol:


By advertisements at two universities in the UK. 


  • RCT crossover design, no placebo
  • Subjects randomized by age, BMI and fasting plasma TG

Blinding Used

Double blinding.


  • Subjects consumed capsules of either the t10, c12 CLA isomer or the c9t11 CLA isomer for three consecutive eight week periods in progressively increasing doses. The intervention with each isomer was separated by a six-week washout period after which the groups crossed over and consumed supplements of the opposite CLA isomer. 
  • Subjects consumed either one, two or four capsules per day, which provided 0.59g per day, 1.19g per day or 2.38g per day of the c9t11 isomer or 0.63g per day, 1.26g per day or 2.52g per day of the t10,c12 CLA isomer.

Statistical Analysis

  • P<0.05 was considered significant. Pre-analysis was conducted to determine effects of period or treatment period. There were none that allowed data to be treated as paired samples from the crossover study. 
  • Two-factor repeated-measures ANOVA was used for analysis followed by one-factor repeated-measures ANOVA with Tukey tests for significant effect of dose, and paired T-tests for effect of isomer. 


Data Collection Summary:

Timing of Measurements

Liver enzymes were analyzed at baseline and at the end of each six-month arm of the study. Body measurements were conducted at baseline and at each visit (two months, four months and six months) for both arms of the study, as were blood measurements. 

Dependent Variables

  • Body mass, BMI, percentage body fat, fat mass, fat-free mass expressed as percentage and as kg; body composition was initially measured by four skinfold sites and interpreted using the Siri equation as well as by bioelectrical impedance analysis (BIA). The two sets of measurements were closely correlated and BIA only was used for the second arm of the study.
  • Plasma lipids: TG, NEFA, cholesterol, HDL, LDL, LDL:HDL, Total C:HDL
  • Markers of insulin resistance: Plasma glucose, insulin, HOMA-IR, revised QUICKI (quantitative insulin sensitivity check index).

Independent Variables

  • c9,t11 CLA isomer at three doses
  • t10,c12 CLA isomer at three doses.

Control Variables

  • No supplement (placebo was not used).
Description of Actual Data Sample:
  • Initial N: 49 men
  • Attrition (final N): 49 with some time point measures ranging from 39 to 49 subjects
  • Age: Group one=30.7±1.7 years (mean±SEM); group two=31.2±1.7 years
  • Anthropometrics: Groups were the same at baseline for weight and BMI 
  • Location: UK.
Summary of Results:

Key Findings

  • No effect of either isomer on weight, BMI, fat mass and fat-free mass at baseline and following six months of supplementation with each of the three doses of each isomer
  • There was a dose effect for both fat mass and fat-free mass
  • Comparison of marginal means for fat mass indicated a higher mean fat mass during supplementation with the lowest dose of CLA than at baseline and with the two higher doses of CLA, P<0.001
  • Comparison of marginal means  for fat-free mass expressed as kg only was lower during supplementation with the lowest dose of CLA than at baseline or the two higher doses, P<0.001)
  • There was an increased mean fat mass and a decreased mean fat-free mass at the lowest dose of both CLA isomers that returned to normal values at the two higher doses of each isomer. No explanation was available.

Effects of c9t11 and t10c12 CLA Isomers on Body Composition



0g per day


0.59g per day


1.19g per day





2.38g per day




0g per day


0.63g per day


1.26g per day


2.52g per day

P for dose effect

Percentage of body fat










Fat mass (kg










Fat-free mass (percentage) 










Fat-free mass (kg)










Other Findings

  • No differences in plasma TG, NEFA, cholesterol, HDL-C or LDL-C between isomer groups at baseline. However, there was an effect of isomer on TG, but no effect of dose and no isomer-by-dose interaction. Plasma TG was higher during supplementation with the t10,c12 isomer than with the c9t11 isomer, P<0.05.
  • There was an isomer and dose effect of CLA on total cholesterol but no isomer-by-dose interaction
  • There was no effect of CLA on HDL-C
  • There was a an effect of isomer on the LDL:HDL cholesterol (P<0.01) but no dose and no isomer-by-dose interaction
  • The marginal means for LDL:HDL and total:HDL-C differed between isomers, P< 0.01. The isomers had opposite effects with the c9t11 isomer decreasing the ratio from baseline and the t10, c12 isomer increasing the ratio.

Author Conclusion:

Neither of the CLA isomers affected body weight, body fat or fat-free mass. There were divergent effects of c9t11 CLA and t10, c12 CLA on some fractions of the blood lipid profile suggesting a benefit of c9t11 CLA supplementation and a detrimental effect of the t10, c12 CLA isomer. There was no evidence that either isomer adversely influenced insulin resistance.

Funding Source:
Government: Biotechnology & Biological Sciences Res Council; Dept. of Environment, Food, and Rural Affairs, the Scottish Exec. Environmental and Rural Affairs Dept, and Milk Development Council
In-Kind support reported by Industry: Yes
Reviewer Comments:

Supplements were donated by Natural ASA.

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) 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? 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%.) Yes
  4.3. Were all enrolled subjects/patients (in the original sample) accounted for? Yes
  4.4. Were reasons for withdrawals similar across groups? ???
  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? 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? 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? No
  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? ???
  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? 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