DFA: Linoleic Acid (LA) and Intermediate Health Outcomes (2011)

Study Design:
- Click here for explanation of classification scheme.
Quality Rating:
Research Purpose:
  • To determine concentrations of individual triglycerides (TGs) and fatty acid compostion in serum and major lipoprotein particles
  • To analyze how changes in different TGs and fatty acid composition are rrelated to insluin resistance and abdominal obesity
  • To determine whether and in which lipoprotein particles bioactive lipids, such as ceramides, plasmalogens and lysophosphatidylcholines, are present.
Inclusion Criteria:
  • Non-diabetic
  • 18 to 60 years of age
  • No known acute or chronic disease based on history, physical exam and standard laboratory tests and ECG
  • Alcohol consumption of less than 20g per day.
Exclusion Criteria:

None specified apart from inclusion criteria.

Description of Study Protocol:


By newspaper advertisements based on inclusion criteria.


  • Subjects fasted overnight and blood samples were obtained for fasting plasma glucose, fasting serum insulin, HbA1c, serum lipids and lipoproteins
  • Waist circumference and BP were measured
  • Lipoprotein fractions were separated by sequential flotation in an ultracentrifuge
  • Lipomic analysis (ultra-performance liquid chromatography couples to MS abbreviated as UPLC/MS-based analysis) was used for lipid profiling of TGs and lipoproteins.

Statistical Analysis

  • Means ±SD are presented
  • Correlation analyses were performed using Spearman's non-parametric rank correlation cofefficient
  • Weighted correleation coefficient for each serum TG was calculated by multiplying the percentage of each serum TG by the Spearman's correlation coefficeint.
Data Collection Summary:

Timing of Measurements

  • After overnight fast
  • All at once.

 Dependent Variables

  • Fatty acids (14:0 through 22:6, n-3) in serum and by concentration in specific lipoproteins
  • Bioactive lipids (ceramide, lysophosphatidylcholines, ehtanolamine, plasmalogens and ether-linked phosphatidylcholines).

Independent Variables

  • HOMA-IR values of insulin resistance
  • Plasma glucose
  • HgbA1c
  • Waist circumference.
Description of Actual Data Sample:
  • Initial N: 16 (14 women)
  • Attrition (final N): 16
  • Age: 46±5 years (39 to 60)
  • Ethnicity: Not given
  • Anthropometrics: BMI, 30.2±5.3; waist, 99±14cm
  • Location: Finland.
Summary of Results:

 Key Findings

  • 45 different TGs were found in serum
  • Serum TGs with the greatest positive weighted correlation coefficient were significantly positively related to HOMA-IR, when measured in VLDL, IDL and LDL, but not in HDL
  • TGs containing satrurated and monounsaturated fatty acids, such as TG (16:0/16:0/18:1) and TG (16:0/18:1/18:0), correlated positively, whereas those containing essential fatty acids, such as TG (18:1/18:2/18:2), correlated negatively with HOMA-IR and waist circumference. These TGs correlated positively with insulin resistance only within VLDL, IDL and LDL, but not within HDL.
  • Total serum palmitic, palmitoleic and oleic acids were positively related and linoleic acid negatively related to HOMA-IR
  • Linoleic acid was negatively related to HOMA-IR in serum and all lipoproteins (P<0.05). P-values varied by liproprotein from <0.001 to <0.05. 

Other Findings

  • Amounts of bioactive lipids varied by lipoprotein with some being present only in specific lipoproteins
  • Saturated, monounsaturated and polyunsaturated fatty acids containing TGs within the VLDL-IDL-LDL axis and HDLs were differently related to insulin resistance.
Author Conclusion:
  • Serum concentrations of specific TGs may be more precises markers of insulin resistance than total serum TG concentrations
  • Analysis of fatty acid composition in lipoproteins rather than total serum may help to define the role of saturated, monounsaturated and polyunsaturated fatty acids in the pathogenesis of the insulin resistance syndrome.
Funding Source:
Government: Academy of Finland, European Commission under the 6th Framework Programme
Sigrid Juselius Foundation, Jalmari and Rauha Ahokas Foundation and other foundations
Foundation associated with industry:
Other non-profit:
Reviewer Comments:
  • Results of the TG concentration remained the same when men's data were removed from analysis
  • Authors suggest that the the relationship between serum TGs and insulin resistance is weak, due to varying affects of fatty acid composition on insulin resistance dependent upon the lipoprotein in which they exist. Thus, analysis of lipoprotein-specific rather than total serum faty acid composition may more accurately reflect changes of fatty acids characteristic to insulin resistance.
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) N/A
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? Yes
  2.2. Were criteria applied equally to all study groups? N/A
  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? N/A
  3.1. Was the method of assigning subjects/patients to groups described and unbiased? (Method of randomization identified if RCT) N/A
  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.) N/A
  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? Yes
  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? 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? Yes
  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? ???
  5.1. In intervention study, were subjects, clinicians/practitioners, and investigators blinded to treatment group, as appropriate? N/A
  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.) ???
  5.3. In cohort study or cross-sectional study, were measurements of outcomes and risk factors blinded? ???
  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? N/A
  6.2. In observational study, were interventions, study settings, and clinicians/provider described? Yes
  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? 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? N/A
  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? N/A
  7.3. Was the period of follow-up long enough for important outcome(s) to occur? N/A
  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? ???
  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? ???
  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)? 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? 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