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 effects of CLA supplementation during supervised resistance training on body composition and strength
  • Secondary analyses included measures of myofibrillar and bone degradation, RMR and resting substrate utilization
  • It was hypothesized that CLA, when combined with resistance training, would enhance increases in lean tissue mass and strength, reduce fat mass, increase RMR, increase fat oxidation and reduce urinary markers of bone and muscle protein degradation, compared with resistance training alone.
Inclusion Criteria:
  • Healthy
  • Non-smoking
  • Did not have any known pre-existing diseases as assessed by a physical activity readiness questionnaire and initial interviews.
Exclusion Criteria:

None listed.

Description of Study Protocol:


  • Initial study was a randomized, double-blind, placebo-controlled trial (method of randomization was not given)
  • Following the initial study subjects were asked if they wanted to continue training for an additional seven weeks, crossing over to the opposite supplement group after a two-week washout period. Thus, there was an additional crossover trial.

Blinding Used

  • Double-blind
  • An individual who was not involved in any other aspect of the study was responsible for randomizing the participants and coding the supplements to ensure that all participants and investigators remained blinded throughout the study
  • Investigators were blinded during the entry and analysis of data by coding the groups.


  • Placebo: 7g of sunflower oil per day
  • CLA: 5g of CLA per day in seven 1g capsules; c9,t11 isomer 36.1%, t10, c12 isomer 36.3%.

 Statistical Analysis

  • Baseline characteristics were assessed by a two-factor (supplement group x gender) ANOVA
  • For the first seven-week study, all dependent variables were assessed by a three-factor (supplement group x gender x time) ANOVA, with repeated measures on the last factor
  • To clarify presentation of the results, a two-factor ANOVA (supplement group x gender) was used to evaluate change scores for the dependent variables. Change scores were determined by subtracting baseline measurements from week seven measurements.
  • An LSD post hoc test was used to identify differences between means when interactions were found
  • For the crossover study, a three-factor (gender x supplement x time) ANOVA, with repeated measures on the last two factors, was used for assessment of dependent variables
  • A two-factor (gender x supplement) ANOVA, with repeated measures on the second factor, was used to compare the change scores between the seven-week phase during which CLA was consumed vs. the seven-week phase during which placebo was consumed
  • All results are expressed as mean ± SD
Data Collection Summary:

Timing of Measurements

  • An initial seven-week study, two-week washout period, followed by the second seven weeks
  • Cessation of any strength training for at least three days before testing was required of each participant
  • Two separate sessions of testing were necessary both before and after the seven-week training period
  • During the first session the following were measured, in order:
    • RMR and respiratory exchange ratio
    • Body composition
    • Muscle thickness of knee extensors and elbow flexors
    • Peak torque of knee extensors
  • In the second session maximal strength (bench press and leg press) was measured
  • There was a minimum of two days of rest between the first and second sessions to offset any fatigue or muscle soreness caused by peak torque measurements.

Dependent Variables

  • Body composition: Determined using air displacement plethysmography (Bod Pod SIL) for measurement of body volume. Body density was calculated by dividing the individual's mass by body volume, corrected for estimated lung volume. Percent body fat was estimated by the equation derived by Siri (ref 30). Body fat was determined as body mass multiplied by percent body fat, and lean tissue mass as the difference between body mass and body fat mass. Reproducibility for lean tissue and fat mass was determined on 29 subjects and measured on two occasions, one week apart. The validity of the investigator's Bod PoD was checked in a separate experiment by measuring 12 subjects on the Bod Pod and by dual-energy x-ray absorptiometry (DXA, Hologic ODR 2000).
  • Muscle thickness of elbow flexors and knee extensors of the right limbs: Assessed using B-Mode ultrasound (Aloka SSD-500)
  • Muscular strength: Bench press and leg press strength were assessed by the IRM using a standard testing protocol
  • Maximal torque: Concentric knee extension torque was assessed using an isokinetic dynamometer set at a velocity of 600.s-1 (Biodex System 3)
  • Resting metabolic rate and respiratory exchange ratio: Before measurement of RMR each participant was asked to refrain from all exercises for 48 hours, omit any caffeine intake for 24 hours and fast for 12 hours. Measurements were taken in the morning between 5:30 and 9:00 A.M. and the testing before and after the seven-week intervention was completed at the same time of day for each subject. Upon arrival the participants were weighed and then rested in the supine position on a bed for 30 minutes. RMR and respiratory exchange ratio (RER) were assessed by open circuit indirect calorimetry using a ventilated hood connected to a metabolic cart (Sensor Medics Vmax29 Series), with subjects supine. Measurements continued until the "steady state" had been reached.
  • Myofibrillar protein degradation and bone resorption: Myofibrillar protein degradation was assessed 3-methylhistidine, and bone resorption was assessed from cross-linked N-telopeptides of Type 1 collagen, from urine samples collected before and immediately following the seven weeks of training and supplementation. Urine collection was preceded by a three-day meat-free diet. The concentration of 3-methylhistidine was determined by high-performance liquid chromatography. The cross-linked N-telopeptides of Type 1 collagen was determined using a competitive-inhibition enzyme-linked immunosorbent assay.
  • All dependent variables measured at baseline and seven weeks were measured again after 14 weeks, the end of the crossover study.

Independent Variables

  • Placebo: 7g of sunflower oil per day
  • CLA: 5g of CLA per day in seven 1g capsules; c9, t11 isomer 36.1%, t10, c12 isomer 36.3%.

Control Variables

  • Dietary assessment: Three-day food record before and after the seven-week intervention on days separate from the three-day meat-free period required before urine collection
  • Physical Activity Questionnaire: Refer to reference 17 for information on this. The only other information on this is provided as a footnote to Table 2. Subjects were asked how many times over a week they perform activities that are classified as "strenuous," "moderate," or "mild." Frequency of "strenuous" activities was multiplied by nine, frequency of "moderate" activities was multiplied by five, and frequency of "mild" activities was multiplied by three. The total was summed for the physical activity score.
  • Compliance, Side Effects, and Blinding Questionnaire: Subjects were given a questionnaire at the end of the study to evaluate side effects and the success of blinding for the supplement. The supplements consumed were confirmed by asking the subjects to return supplement envelopes, whether empty or not. To evaluate the success of the blinding the subjects were asked to indicate whether they thought they were in the CLA group, the placebo group, or "did not know". This questionnaire was administered again at the end of 14 weeks and the crossover study.
Description of Actual Data Sample:

Initial N: 85 (43 female and 42 male).

Attrition (final N):

  • Initial study:
    • Nine subjects (two females and two males in the CLA group and one female and four males in the placebo group) withdrew during the course of the study, all because of lack of time, except for one male in the placebo group who withdrew because he started smoking and was concerned that it might affect the results and one male each in the placebo and CLA groups who withdrew due to personal reasons
    • From Table 2: CLA (males 19, females 19); placebo (males 17, females 21) for a total of 76
  • Crossover study:
    • 27 subjects volunteered to continue training for an additional seven weeks, crossing over to the opposite supplement group. Of these subjects, 10 withdrew from the study (two males and three females from each of the CLA and placebo groups). Eight withdrew because of lack of time. Two subjects (one from the CLA group and one from the placebo group) withdrew because of gastrointestinal distress that they felt was related to the supplement.
    • Of the 17 remaining subjects for the crossover study, eight received CLA for the first seven weeks and placebo for the final seven weeks (CLA placebo, four male and four females; age = 28.1 (2.3) years, mass = 83.7 (6.3) kg, height = 176 (4) cm and nine received placebo for the first seven weeks and CLA for the final seven weeks (placebo CLA, four males and five females; age = 32.4 (3.0) years, mass = 81.4 (6.0) kg, height 172 (2) cm.

Age: Mean ages ranged from 28 to 32 years of age.

Other Relevant Demographics

  • Most participants had strength training experience and the majority had more than two years of experience and had been training for at least two times per week for a minimum of three months before involvement in the study. The initial physical activity scores are given below.
  • Subjects were moderately physically active, as determined by questionnaire.
  • Healthy (no explicit inclusion or exclusion criteria were available for this term) other than no known pre-existing diseases as assessed by a physical activity readiness questionnaire and initial interview
  • Non-smoking. 

Anthropometrics (from initial study) 

  • Mass (kg):
    • Placebo: Males 80.0±16.0; females 66.3±12.4
    • CLA: Males 84.9±13.7; females 66.4±13.2
  • Height (cm):
    • Placebo: Males 178±8; females 165.5±5
    • CLA: Males 178±9; females 167±9
  • Physical activity scores (arbitrary)
    • Placebo: Males 73±41; females 58±32
    • CLA: Males 73±44; females 60±35.
Summary of Results:

Key Findings

Study 1: Seven-week Comparison Between CLA and Placebo. 

  • Body composition:
    • No differences in any of the dependent variables at baseline between the CLA and placebo groups
    • Gender x time interaction for body mass (P<0.05). Males had a significant increase in body mass over time (82.5±14.4 to 83.4±15.0kg; P<0.01), whereas females had no change (65.9±12.6 to 66.2±12.6kg).
    • No differences between CLA and placebo groups for body mass over time
    • There were supplement group x time interactions for lean tissue mass, fat mass and percent fat
    • When evaluated by changes from baseline, the CLA group had a greater increase in lean tissue mass and a greater decrease in fat mass and percent fat compared with the placebo group (all P<0.05). 

Changes in Body Composition for Study One

  CLA (pre-study) CLA (post-study) Placebo (pre-study) Placebo (post-study) P supplement x time

Lean tissue mass (kg)




52.7±11.0  0.033

Fat mass (kg)



19.4±9.9 19.8±10.3 0.028 

Fat percentage


24.7±9.0* 26.3±9.8 26.5±10.0 0.041

*Significant compared with pre-value (P<0.05).

**Significant compared with pre-value (P<0.01).

  • Muscle thickness and strength:
    • There were no differences between CLA and placebo groups over time for muscle thickness
    • There were no differences between supplement groups or genders over time for leg press strength
    • For bench press strength, there was a significant supplement group x gender x time interaction (P<0.05). All groups increased bench stress strength over time (P<0.01); however, the males in the CLA group had greater increases than all other groups (P<0.05)
  • Knee extension torque: No differences between supplement groups or genders over time
  • Resting metabolic rate and respiratory exchange ratio: There was a supplement group x time interaction for absolute RMR (P<0.05). The CLA had no change over time, whereas the placebo group had a significant decrease in RMR (P<0.05). When expressed relative to lean tissue mass, there was no longer a difference in RMR between groups over time (P=0.10). There were no changes in respiratory exchange ratio over time in any groups.
  • Compliance and adverse events:
    • Compliance was similar between groups
    • There were no differences between CLA and placebo groups for number of adverse events. No serious adverse events occurred.
      • Events occurring with CLA group included:
        • Upset stomach or indigestion (6)
        • Heartburn or reflux (2)
        • Nausea (2)
      • Events occurring with placebo group included:
        • Upset stomach (5)
        • Diarrhea (2)
        • Loss of appetite (2)
        • Bloating (2)
        • Constipation (1).

Study 2: Cross-over Study.

  • There were no differences in any measures at baseline for the subjects in the CLA-placebo group vs. the placebo-CLA group
  • There were significant supplement x time interactions for body mass, fat mass, percent fat, elbow flexors muscle thickness, 3-methylhistidine and cross-linked N-telopeptides of Type I collagen (P<0.05)
  • Subjects increased body mass, fat mass and percent fat while on placebo (P<0.05) but not while on CLA
  • Subjects increased elbow flexor muscle thickness while on CLA (P<0.05) but not while on placebo
  • There were no differences for changes in lean tissue mass, knee extensor muscle thickness, strength, torque, RMR or respiratory exchange ratio while on CLA vs. placebo. 

Changes in Body Composition for Study Two, Cross-over (N=17)

  CLA (pre-study) CLA (post-study) Placebo (pre-study) Placebo (post-study) P Supplement x time
Body mass (kg) 82.7±17.1 83.0±17.2 81.9±17.2 83.2±17.2* 0.033

Lean tissue mass (kg)




56.3±11.1 NS

Fat mass (kg)


26.4±11.9 25.4±12.3 26.8±9.0* 0.038

Fat percentage

31.3±9.8 31.0±9.8


31.4±9.4* 0.043 

*Significant compared with pre value (P<0.05). 

  • Compliance and Adverse Events:
    • Compliance was similar between groups
    • Four subjects reported adverse events considered related to the supplement while taking CLA, and two reported adverse events while taking placebo (reported multiple adverse events).
      • Adverse events while taking CLA included:
        • Upset stomach (3)
        • Diarrhea (1)
      • Adverse events while taking placebo included:
        •  Boating (1)
        • Constipation (1)
        • Upset stomach (1)
        • Diarrhea (1)
    • Two additional subjects withdrew from the study during the second seven-week period due to recurring upset stomach (one while taking CLA and one while taking placebo). Note: Subjects dropped out at the beginning of the study, and thus were not included in the data analysis.

Other Findings

  • Study One:
    • Males had greater lean tissue mass, elbow flexors muscle thickness, bench press and leg press strength, knee extension torque, RMR and lower percent fat compared with females at baseline (all P <0.01)
    • There was a time main effect (P<0.01) for muscle thickness of the knee extensors and elbow flexors and a trend for gender x time interactions for both knee extensors (P=0.06) and elbow flexors (P=0.08) muscle thickness, with a trend for males to have greater increases than females
    • For leg press strength and knee extension torque, there was a main effect for time (P<0.01)
    • There was a supplement group x time interaction for 3-methylhistidine (P<0.05). The CLA group had no change over time, whereas the placebo group had a significant increase in 3-methylhistidine (P<0.05).
    • There were no changes in cross-linked N-telopeptides of Type 1 collagen over time in either group
    • There were no differences between supplement groups from baseline to seven weeks for intake of calories, carbohydrate, protein or fat. Males had significantly greater intakes for calories (P<0.05), protein (P<0.01) and a trend (P=0.055) for greater fat intake compared with females.
  • Study Two:
    • Physical activity levels outside the training program were similar between the two phases (physical activity score (arbitrary units) = 52±33 during CLA and 51±25 during placebo), as were dietary intakes
    • Subjects increased 3-methylhistidine (P<0.01) and cross-linked N-telopeptides of Type 1 collagen (P<0.01) while on placebo, and decreased cross-linked N-telopeptides of Type 1 collagen while on CLA (P<0.05)
  • Questionnaire on Success of Blinding (These results appear to be only after the cross-over study):
    • After the CLA phase, six subjects correctly indicated they were taking CLA, six incorrectly indicated they were taking placebo and five indicated they did not know which supplement they were taking
    • After the placebo phase, seven subjects correctly indicated they were taking placebo, five incorrectly indicated they were taking CLA and five indicated they did not know which supplement they were taking.


Author Conclusion:
  • The initial study indicated CLA has small effects for decreasing fat mass and increasing lean tissue mass. Although these effects were statistically significant, the clinical significance is most likely quite small.
  • The effect of CLA on fat and lean tissue mass was not supported in the crossover study
  • When deciding whether to use CLA as a dietary supplement, one would have to weight these relatively small beneficial effects against the relatively high monetary costs of CLA
  • Limitation as mentioned by authors: A longer duration study may be necessary to see clinically relevant changes in body composition.
Funding Source:
Government: Natural Sciences and Engineering Research Council of Canada
Biooriginal Food and Science Corp
Food Company:
Reviewer Comments:
  • The study appears to have been originally designed as a randomized, double-blind, placebo-controlled trial. The crossover study is talked about in a separate section of the methods. It was apparently approved separately from the original study and a new informed consent for participation was required. Thus it appears a crossover was not in the original design of the study, but was added after the start of the original study. It is not clear why this was done. In the results sections, they are also referred to as study one and study two.
  • No intent to treat statistical analysis was used
  • Because these are apparently two different studies and were/are treated as such, this reviewer does not think it is appropriate to analyze the second study as a cross-over study. It was not designed that way from the beginning, and the attrition of subjects is also a factor that needs to be considered. At the start of the initial study the overall N was 76 and at the beginning of the cross-over study it was 17.
  • The investigators did not appear to control for prior exercise or training regimes and no information is given about inclusion or exclusion criteria for this. A brief statement is made about the majority of participants. A physical activity score was generated but this appears to have quite a bit of variation and seems to be a very arbitrary method of measuring physical activity for a study that is evaluating the impact of a supplement on resistance training.
  • It also appears that from the results that were was a decrease in physical activity and increase in body mass between study one and two 
  • Given the poor design of the study, lack of intent to treat, apparent lax inclusion or exclusion criteria and subject attrition rates, this reviewer questions if there was a conflict of interest present and that the industry sponsors had an influence on the study.
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? 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? 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? Yes
  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? ???
  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? N/A
  6.7. Was the information for 6.4, 6.5, and 6.6 assessed the same way for all groups? N/A
  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? ???
  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? ???
  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? 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? No
  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? 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? No
  10.1. Were sources of funding and investigators' affiliations described? Yes
  10.2. Was the study free from apparent conflict of interest? No