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

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

To study the interactions and the relation among insulin resistance, obesity, inflammatory activity [circulating interleukin (IL)-6] and dietary FAs in lean and overweight healthy subjects.

Inclusion Criteria:
  • Lean and overweight individuals [Overweight (BMI over 24.4kg/m2 in men and over 22.9kg/m2 in women) and lean subjects] without any diagnosis of metabolic syndrome
  • Males and females
  • All women had regular menstrual cycles
  • All women were pre-menopausal and studied in their follicular phase.
Exclusion Criteria:
  • Subjects taking any medication (including glucocorticoids or estrogens) or with any evidence of metabolic disease other than obesity
  • Subjects with impaired fasting glucose were specifically excluded
  • Liver disease and thyroid dysfunction were specifically excluded by biochemical workup.
Description of Study Protocol:

Recruitment

A total of 232 subjects were evaluated as part of an ongoing epidemiological study dealing with non-classical cardiovascular risk factors.

Design

  • This is an observational prospective study with 123 overweight subjects (BMI, 26.9±2.4kg/m2) and 109 lean subjects (BMI, 21.7±1.7kg/m2; P<0.000001)
  • Serum fatty acids, IL-6, serum cholesterol and triglycerides, blood pressure, glucose, insulin and C-reactive protein were estimated
  • BMI and WHR were measured in subjects.

Statistical Analysis

  • Descriptive results of continuous variables are expressed as the mean ±SD. Normal distribution and homogeneity of the variances were tested.
  • Parameters that did not fulfill these tests (individual FAs and ratios and IL-6) were log transformed
  • The relations between variables were analyzed by simple correlation analysis. Subjects were divided into quintiles of HOMA value.
  • Comparison of variables in subjects of the highest vs. the lowest insulin sensitivity quintiles was performed using Student’s T-test and by stepwise multivariate linear regression analysis
  • Smoking status was considered as a categorical variable (0=non-smoker, 1=smoker). Levels of statistical significance were set at P<0.05.
Data Collection Summary:
  • Timing of measurements: NA
  • Dependent variables: Serum fatty acids profile, inflammatory markers, BMI, cholesterol, triglycerides, glucose and insulin, HOMA IR, blood pressure
  • Independent variables: Age, BMI, waist-to-hip ratio (WHR), smoking status, relation of saturated to n-6 to n-3 FAs
  • Control variables: Overweight, lean weight.
Description of Actual Data Sample:

Initial N

232 subjects.

Attrition (Final N)

123 overweight and 109 lean weight

Mean Age

  • Overweight: 37.9±12 years
  • Lean weight: 43.7±10.1 years.

Ethnicity

Spanish.

Other Relevant Demographics

Blood pressure, age, sex, smoking.

Anthropometrics

BMI, WHR.

Location

Spain.

Summary of Results:

Key Findings

  • The absolute FA concentration did not differ significantly between men and women or between overweight and lean subjects. Percentage of palmitic acid (19.8±2 vs. 18.8±3, P=0.004) and percentage of docosahexaenoic (DHA) acid (2.15±0.6 vs. 1.92±0.6, P=0.03) were significantly higher in men than in women.
  • By definition, overweight subjects showed increased BMI (26.9±2.4kg/m2 vs. 21.7±1.7kg/m2, P<0.000001) and WHR (0.94±0.07 vs. 0.88±0.6, P<0.03) and were significantly older (42.1±8.9 years vs. 37.4±10 years, P=0.006). Circulating IL-6 level was below the minimum detectable concentration of the assay (0.094pg per ml) in 58 subjects and they were thus excluded from further analysis. These subjects did not significantly differ regarding age, sex and BMI from the remaining subjects.
  • The percentage of saturated FAs (29.9±1.8 vs. 30.4±2.1) and ω-6 FAs (40.6±5.0 vs. 40.6±5.2) were not significantly different in overweight and lean subjects, whereas ω-3 FAs were significantly lower in the former (2.7±0.8 vs. 3.0±1.0, P=0.04). The same findings were observed separately in men and women, except in that overweight men also had a decreased percentage at 14:0 and 20:5 FA and women with overweight had decreased (22:6) ω-3. The proportions of saturated/ω-3 or saturated/ω-6 FAs were not significantly different between groups.
  • In overweight subjects, the percentage of saturated FAs (P =0.01) and ω-6 FAs (P=0.001) were significantly associated with circulating IL-6, whereas the percentage of ω-3 FAs correlated negatively with C-reactive protein (P=0.04)
  • Saturated-to-ω-3 and saturated-to-ω-6 FA ratios were significantly and positively associated with C-reactive protein (P=0.0001) and IL-6 (P=0.001), respectively
  • The relations were stronger in smokers. In contrast, none of these associations showed statistical significance inlean subjects. All of these results persisted after controlling for age.
  • In overweight men, saturated/ω-3 FAs (P=0.01), but not age, sex, BMI, WHR or smoking status, independently contributed to 17% of IL-6 variance
  • In lean men, smoking status (P=0.02), but not the remaining variables, contributed to 8% of IL-6 variance. In women, any of these variables contributed independently to IL-6 variance. Those subjects in the most insulin-sensitive quintile (HOMA value) (quintiles; mean±SD, 0.73±0.19, 1.13±0.08, 1.45±0.09, 1.89±0.16 and 3.41±1.6 in men; and 0.75±0.11, 1.02±0.06, 1.35±0.12, 1.76±0.13 and 3.12±0.8 in women) had a significantly higher percentage of linolenic acid (C18:3 ω-3) (0.48±0.18 vs. 0.42±0.15, P=0.032) and significantly lower araquidic (C20:0, P=0.04), behenic (C22:0, P=0.009), lignoceric (C24:0, P=0.02) and nervonic (C24:1 ω-9, P=0.001) FAs than the remaining subjects.
  • In parallel, the most insulin-sensitive subjects had significantly decreased C-reactive protein (0.35±0.12 vs. 0.44±0.36, P=0.03). Specifically, serum C-reactive protein was significantly associated with percentage linoleic acid (C18:2 ω-6) and eicosapentaenoic (EPA) acid (C20:5 ω-3) in non-smoking men (P=0.03 and P=0.04, respectively) and with DHA acid (C22:6 ω-3) in non-smoking women (P=0.0001).
  • Serum IL-6 concentration correlated positively with the proportion of the saturated myristic (C14:0) and palmitic acid (C16:0) in smoking men and in non-smoking women. The absolute concentrations of the saturated FA C18:0 (stearic) and C24:0 (lignoceric) were also positively associated with IL-6 among smoking men (P=0.031 and P=0.038, respectively).
  • A tendency toward a negative association with linoleic acid (C18:2 ω-6) was shown in smoking men and in non-smoking women.
Author Conclusion:
  • Dietary FAs (as inferred from plasma FA concentration) seem to be strongly linked to inflammatory activity. Interestingly, this association is especially remarkable in subjects with increased fat mass, defined as BMI of 24.4kg/m2 in men and 22.9kg/m2  in women.
  • The percentage of saturated FAs and ω-6 FAs were significantly associated with circulating IL-6, whereas the percentage of ω-3 FAs correlated negatively with C-reactive protein in overweight subjects
  • Saturated-to-ω-3 and saturated-to-ω-6 FA ratios were significantly and positively associated with C-reactive protein and IL-6, respectively
  • Dietary FAs (as inferred from plasma FA concentration) seem to be linked to inflammatory activity in overweight subjects and in subjects with insulin resistance. Being overweight modulates the relations of FAs to inflammatory markers.
Funding Source:
Other: Not available
Reviewer Comments:
  • This study shows the association of inflammatory markers in relation to obesity and also correlation of fatty acids, however it is difficult to conclude the cause and effect for the measurements and outcomes. Data collectors were not blinded for outcomes.
  • Further studies are required in a controlled population.
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? N/A
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) N/A
  3.2. Were distribution of disease status, prognostic factors, and other factors (e.g., demographics) similar across study groups at baseline? N/A
  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? No
  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? 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? No
  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.) No
  5.3. In cohort study or cross-sectional study, were measurements of outcomes and risk factors blinded? No
  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? N/A
  6.4. Was the amount of exposure and, if relevant, subject/patient compliance measured? N/A
  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? 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? 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? N/A
  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)? N/A
  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