EAL

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 investigate the associations of fatty acids in plasma and diet with diabetes incidence.

Inclusion Criteria:

Fatty acid intake (per kJ) and plasma phospholipid fatty acids (percentage) were measured at baseline and diabetes incidence was assessed by self-report four years later.

Exclusion Criteria:

Participants with diabetes at baseline (self-reported or elevated plasma glucose)
Who had had a heart attack or had angina before baseline
Who did not report diabetes at baseline, but later reported a date of diabetes diagnosis before baseline
Those with extreme self-reported energy intakes (first percentile and above the 99th percentile)
Those with missing values for relevant risk factors.

Description of Study Protocol:

Recruitment

The Melbourne Collaborative Cohort Study (MCCS) recruited 41,528 people (17,049 men) between 1990 and 1994
People aged 40 years to 69 years were invited.

Design

This was a prospective case-cohort study of 3,737 adults aged 36 years to 72 years, including 364 incident cases of type 2 diabetes. There is complete data for these analyses.
Plasma phospholipid fatty acids were measured for all incident cases of diabetes and a random sample of the cohort (the sub-cohort), which included some randomly-selected cases
Fatty acid intake (per kJ) and plasma phospholipid fatty acids (percentage) were measured at baseline and diabetes incidence was assessed by self-report four years later
The following fatty acids and classes were analyzed
- Total SFAs, 15:0, 16:0, 18:0
- Total MUFAs, 16:1n-7, 18:1 n-9
- Total PUFAs
- Total n-6 fatty acids, 18:2n-6, 20:3n-6, 20:4n-6
- Total n-3 fatty acids, 18:3n-3, 20:5n-3, 22:5n-3, 22:6n-3
- Ratio of n-6 to n-3 fatty acids
- Total trans fatty acids
- Total conjugated linoleic acid (plasma only).
Fatty acid ratios in plasma phospholipid reflecting product-precursor ratios of elongase and desaturase enzymes were also calculated and examined
Logistic regression excluding (Model One) and including (Model Two) body mass index and waist-to-hip ratio was used to calculate odds ratios (ORs) for plasma phospholipid and dietary fatty acids.

Statistical Analysis

Means and SDs for each fatty acid in plasma phospholipid and diet were calculated by diabetes status at follow-up and T-tests were used to evaluate differences between the two groups
Age, country of birth, sex, physical activity score, five-year weight change, education level, smoking, body mass index (BMI), waist-to-hip ratio (WHR) and family history of diabetes were considered as potential confounders
Weight change, education and smoking were not associated with diabetes in the sub-cohort and were not included in subsequent models
Logistic regression models were computed first with age, sex, country of birth, physical activity, family history of diabetes and alcohol intake (Model One) and then with all confounders plus BMI and WHR (Model Two) for quintiles (based on the distributions in the sub-cohort) of plasma phospholipid fatty acid proportions and dietary fatty acids expressed as energy density
The following fatty acids and classes were analyzed:
- Total SFAs, 15:0, 16:0, 18:0
- Total MUFAs, 16:1n-7, 18:1 n-9
- Total PUFAs
- Total n-6 fatty acids, 18:2n-6, 20:3n-6, 20:4n-6
- Total n-3 fatty acids, 18:3n-3, 20:5n-3, 22:5n-3, 22:6n-3\
- Ratio of n-6 to n-3 fatty acids
- Total trans fatty acids
- Total conjugated linoleic acid (plasma only).
Fatty acid ratios in plasma phospholipid reflecting product-precursor ratios of elongase and desaturase enzymes were also calculated and examined in the same way as the fatty acids. Additional analyses were performed with adjustment for insulin in subjects who were fasting at baseline.
An interaction term for dietary linoleic acid and insulin was tested in Model One. Interactions between dietary linoleic acid and both BMI and age were also tested.

Data Collection Summary:

Timing of measurements: 1990 to 1994
Dependent variables: Each fatty acid in plasma phospholipid and diet; insulin levels and incident diabetes
Independent variables: Age, country of birth, sex, physical activity score, five-year weight change, education level, alcohol intake, smoking, BMI, WHR and family history of diabetes.

Description of Actual Data Sample:

Initial N: 41,528 persons (17,049 men)
Attrition (final N): 3,737 adults; 364 subjects were included in the present study with complete data
Age: 36 years to 72 years
Ethnicity: Australians
Other relevant demographics: Age, sex, country of birth, physical activity, family history of diabetes and alcohol intake; education level, smoking and family history of diabetes
Anthropometrics: BMI, WHR, weight change
Location: Melbourne.

Summary of Results:

Key Findings

Persons who developed diabetes tended to be older, more obese, less active and more likely to have a family history of diabetes and to originate from southern Europe. Persons who developed diabetes had higher proportions of 18:0, total saturated fat, 16:1n-7, 20:3n-6, 20:4n-6, total n-3 fatty acids, 20:5n-3, and 22:6n-3 and lower proportions of 15:0, total polyunsaturated fat, n-6 fatty acids, 18:2n-6, n-6:n-3, trans-fats and conjugated linoleic acid at baseline, than did persons who did not develop diabetes.
Persons who developed diabetes had higher intakes of total fat, total monounsaturated fats, 16:1n-7, 18:1n-9, total polyunsaturated fats, n-6 fats, 18:2n-6, 20:4n-6, n-3 fats, 18:3n-3 and trans-fats and a lower intake of 15:0 at baseline, than did persons who did not develop diabetes
Adjusting for age, sex, country of birth, physical activity, family history of diabetes and alcohol intake (Model One), inverse associations were seen for 15:0, trans-fatty acids and 18: 2n-6. Positive associations were observed for 18:0, total SFAs, 16:1n-7 and 20:3n-6. After further adjustment for body size, these associations were attenuated, but still highly significant.
Strong positive associations were observed for stearoyl-CoA desaturase (ratio of 16:1n-7 to 16:0) and elongase (ratio of 20:3n-6 to 18:2n-6), whereas inverse associations were seen for Δ⁵desaturase (ratio of 20:4n-6 to 20:3 n-6) and the ratio of 18:1n-9 to 18:0, which also reflects stearoyl-CoA desaturase-1
For Model One, the OR for the top quintile of dietary fat intake was elevated compared with the lowest. Both 16:0 and 18:0, but not total SFAs, were associated with higher risk in Model One. 16:1n-7 showed a weak positive association with diabetes.
Positive associations were seen for dietary 18:1n-9, MUFAs, 18:2n-6, total n-6 fatty acids, PUFAs and 18:3n-3 intakes in Model One; after adjustment for body size however, these were no longer significant. The ratio of n-6 to n-3 fatty acids showed a positive association of borderline significance that was not attenuated by adjustment for body size.
Plasma phospholipid linoleic acid was associated with diabetes risk in the opposite direction (negative) to the association observed for diet (positive), which is unlikely to be explained by measurement error in the dietary estimates
For persons who developed diabetes and for those who did not, there were similar linear associations between dietary and plasma phospholipid linoleic acid; the interaction term was not significant (P=0.6 when quintile medians were modeled). Persons who developed diabetes had lower mean plasma phospholipid linoleic acid proportions; the mean difference, estimated from the regression, was 1.8 (95% CI, 1.4, 2.1) percentage points.
Models were recomputed for the fasting sub-group (N=2,324; cases, 224). There was a weak interaction between plasma insulin and linoleic acid intake (P for interaction, 0.09) and the association between dietary linoleic acid and diabetes risk was most apparent in persons with plasma insulin concentrations at or above the median value (at least 5.3pmol per L).
The OR for Quintile Five vs. Quintile One was 1.81 (95% CI, 1.01, 3.23; P for trend, 0.02), compared with lower values (P for trend, 0.30). There was no significant difference in associations of linoleic acid with incident diabetes across strata of age (P for interaction, 0.292) or BMI (P for interaction, 0.252).
The 12-month reliability coefficients (within-subject variation) for plasma phospholipid fatty acids ranged from 0.23 for palmitoleic acid to 0.89 for palmitic acid and from 0.32 for dietary palmitoleic acid to 0.49 for dietary oleic acid.

Author Conclusion:

Dietary saturated fat intake is inversely associated with diabetes risk
Our data showed clearly that for any level of dietary linoleic acid, cases had lower plasma phospholipid linoleic acid proportions than did controls, which suggests that some metabolic difference may exist in persons with pre-diabetes that explains the conflicting results. The relatively low proportions of plasma phospholipid linoleic acid in persons who went on to develop diabetes appeared to be balanced by higher proportions of some longer, less saturated metabolites of linoleic acid.
More research is required to determine whether linoleic acid is an appropriate dietary substitute.

Funding Source:

Government:	National Health and Medical Research Council (209057, 126403)
University/Hospital:	VicHealth and The Cancer Council Victoria

Reviewer Comments:

This is a prospective cohort case study
There was an association of dietary fatty acids and diabetes incidence
Further controlled clinical trials are required in different population groups.

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?	Yes
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.)	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?	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.)	Yes
	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?	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%.)	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?		N/A
	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.)	N/A
	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?	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?	Yes
	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?	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