DFA: Linoleic Acid (LA) and Intermediate Health Outcomes (2011)
To investigate the relationships of serum fatty acid (FA) composition and estimated desaturase activities with the liver fat marker alanine aminotransferase (ALT).
- Subjects were participants of the Uppsala Longitudinal Study of Adult Men (ULSAM)
- 1,221 (7%) of 1,681 men participated in the third investigation cycle that was carried out between 1991 and 1995, when participants were 71±0.6 years of age
- 1,181 men had a measure of ALT. Of these, 583 had data on serum FA composition.
- Participants with a previous diagnosis of liver disease were excluded
- Subjects consuming at least 20g of alcohol per day and subjects with ALT levels below detection levels (less than 0.10µkat per L) were excluded.
Recruitment
- Subjects were participants of the Uppsala Longitudinal Study of Adult Men, a population-based study starting in 1970 in Uppsala, Sweden
- 1,221 (7%) of 1,681 men participated in the third investigation cycle that was carried out between 1991 and 1995, when participants were 71±0.6 years of age
- 1,181 men had measures of ALT: Of these, 583 had data on serum FA composition.
Design
546 Swedish elderly men of a population-based cohort particpatted in this cross-sectional study.
Statistical Analysis- The distribution of variables was examined by Shapiro-Wilk W-test. To promote a normal distibution the following variables were logarithmically transformed before analysis: ALT, 16:1, 18:0, 18:3n-6, 18:3n-3, 10:5n-3, Δ6-desaturase, SCD-1, fasting insulin, NEFA, triglycerides and adiponectin.
- Alcohol intake was divided into quartiles. Linear regression analysis was used to associate FA composition (standardized to one SD) with ALT.
- The relationship between FA composition and ALT was adjusted for abdominal obesity (measured by waist circumference), lifestyle factors (alcohol intake, physical activity) and insulin sensitivity in a multivariate analysis
- To explore a potential independent link between FAs and liver fat, metabolic factors previously related to fatty liver, triglycerides, NEFA and adiponectin were added to the model
- To avoid colinearity in Model Two, FA variables with a P-value of <0.001 in Model One (linoleic acid and SCD-1 index) were included
- Potential selection bias was investigated by comparing participants with FA measurements (N=546) with those without FA measurements, in regard to ALT (N=634), WC, BMI, M and triglycerides.
Timing of Measurements
- All measurements were performed under standardized conditions which were previously described
- All blood samples were drawn after an overnight fast
- Insulin sensitivity was determined by euglycemic insulin clamp with infusion rate of 56mU.min per body surface area (m2), instead of 40
- Plasma glucose concentrations were maintained at 5.1mmol per L during the clamp by adjusting the rate of infusion of a 20% glucose solution
- Glucose disposal; M (mg per kg body weight per minute) was calculated as the amount of glucose taken up during the last 60 minutes of the clamp
- Serum adiponectin was analysed using a validated in-house time-resolved immunofluorometric assay (TR-IFMA), based on commercial reagents
- Physical activity was assessed by a self-administred questionnaire
- Alcohol intake was assessed by a seven-day dietary record.
Dependent Variables
Serum fatty acid composition
- Palmitic acid (16:0) percentage
- Palmitoleic acid (16:1) percentage
- Stearic acid (18:0) percentage
- Oleic acid (18:1) percentage
- Linoleic acid (18:2n-6) percentage
- y-Linolenic acid (18:3n-6) percentage
- Alpha ±-Linolenic acid (18:n-6) percentage
- Dihomo-y-linolenic acid (20:3n-6) percentage
- Arachidonic acid (20:4n-6) percentage
- Eicosapentaenoic acid (20:5n-3) percentage
- Docosahexaenoic acid (22:6m-3) percentage.
Desaturase activities
- Δ5-desaturase index
- Δ6-desaturase index
- Stearoyl coA desaturase;1 index.
Independent Variables
Alanine aminotransferase (μkat per L).
Control Variables
- Abdominal obesity
- Lifestyle factors
- Insulin sensitivity.
- Initial N: 1,221 of 1,681 men participated in the third investigation cycle of the Uppsala Longitudinal Study of Adult Men (ULSAM). 1,181 men had ALT measures: Of these, 583 had serum FA composition data.
- Attrition (final N): 583 elderly men
- Age: 71.3 years (71.0 to 71.50 years)
- Ethnicity: Swedish elderly men
- Other relevant demographics: BMI, 26.3±3.4kg/m2
- Anthropometrics: Participants with measurements of FA composition did not differ form the sub-sample without measurements of FA composition with regards to WC, BMI, ALT, M and triglycerides (P>0.28 for all)
- Location: Sweden.
Key Findings
- In linear regression analyses adjusting for lifestyle, abdominal obesity and insulin sensitivity, the dietary biomarker linoleic acid (n-6), but not n-3 FAs, was inversely related to ALT
- Desaturation products including palmitoleic, oleic, y-linolenic and dihomo-y-linolenic acids and Δ6-desaturase and SCD-1 indices were directly related to ALT (all P<0.05)
- After further adjustment for factors previously linked to fatty liver (serum lipids, adiponectin concentrations), SCD-1 index (P=0.004) and insulin resistance (P<0.0001) were independent determinants of ALT activity, whereas waist circumference, triglycerides, non-esterified FA and adiponectin were not.
Relationships of Serum Fatty Acid Composition and Desaturase Indices with Serum Alanine Aminotransferase
Fatty Acid Standardized to 1 SD | Univariable Model | Multivariable Model One |
||
β | P-Value | β | P-Value | |
Palmitic Acid (16:0) |
0.06
|
0.0009
|
0.03
|
0.08
|
Palmitoleic Acid (16:1) |
0.11
|
<0.0001
|
0.09
|
0.08
|
Stearic Acid (18:0) |
0.02
|
0.25
|
0.02
|
0.37
|
Oleic Acid (18:1) |
0.05
|
0.01
|
0.04
|
0.03
|
Linoleic Acid (18:2n-6) |
-0.09
|
<0.0001
|
-0.07
|
0.0002
|
y-Linolenic Acid (18:3n-6) |
0.08
|
<0.0001
|
0.05
|
0.02
|
α-Linolenic Acid (18:n-6) |
-0.03
|
0.15
|
-0.02
|
0.25
|
Dihomo-y-Linolenic Acid (20:3n-6) |
0.11
|
<0.0001
|
0.06
|
0.002
|
Arachidonic Acid (20:4n-6) |
0.04
|
0.01
|
0.02
|
0.25
|
Eicosapentaenoic Acid (20:5n-3) |
0.05
|
0.004 |
0.04 |
0.03
|
Docosahexaenoic Acid (22:6m-3) |
0.02
|
0.25
|
0.01
|
0.53
|
Δ5-Desaturase Index |
-0.05
|
0.01
|
-0.02
|
0.34
|
Δ6-Desaturase Index |
0.09
|
<0.0001
|
0.06
|
0.004
|
Stearoyl coA Desaturase; 1 Index |
0.10
|
<0.0001
|
0.09
|
<0.0001
|
β, regression coefficient
Δ5-desaturase=(20:4m-6/20:3n-6)
Δ6-desaturase=(18:3n-6/18:2n-6)
Stearyl coA desaturase-1=(16:1/16:0).
Model 1
N=460
Mutivariable model adjusting for waist circumference, alcohol intake, physical activity and insulin sensitivity.
Associations Between Serum Alanine Aminotransferase Levels and Metabolic Variables (N=460)
Variables Standarized to 1 SD | Univariable Regression | Multivariable Model Two | ||
β | P-Value | β | P-Value | |
Waist Circumference |
0.10
|
<0.0001
|
0.02
|
0.44
|
Insulin Sensitivity (M) |
-0.15
|
<0.0001
|
-0.13
|
<0.0001
|
Serum Triglycerides |
0.08
|
<0.0001
|
0.0001
|
0.996
|
Serum Non-Esterified Fatty Acids |
0.08
|
<0.0001
|
0.02
|
0.34
|
Linoleic Acid (18:2n-6) |
-0.11
|
<0.0001
|
-0.03
|
0.21
|
Stearoyl coA Desaturase-1 Index |
0.12
|
<0.0001
|
0.07
|
0.004
|
Serum Adiponectin |
-0.05
|
0.007
|
0.02
|
0.35
|
β, regression coefficient
M, glucose disposal measured by euglycemic insulin clamp
Stearoyl coA desaturase-1=(16:1/16:0)
Model 2 also included alcohol intake and physical activity. None of these variables were significantly related to ALT in the univariable or multivariable models and are therefore not shown.
- Low dietary intake of linoleic acid and elevated SCD-1 index may contribute to higher ALT activity in elderly men, even independent of obesity and insulin resistance
- Increased proportions of palmitoleic, oleic, y-linoleic and dihomo-y-linolenic acids and high Δ6-desaturase, SCD-1 and lipogenic indices were realted to ALT, independent of obesity, lifestyle factors and insulin resistance
- SCD-1 may play a role in the development of fatty liver.
Other: | Nordforsk (Nordic Centre of Excellent in Food, Nutrition and health (SYSDIET), Swedish Nutrition Foundation (SNF) and Swedish council for working life and social research (FAS)/ |
- As an observational cross-sectional study no conclusion regarding causality can be drawn
- ALT activity was used as a liver fat marker instead of histology or imaging techniques
- Serum ALT correlates with liver fat, but only partially explains the variation. This would weaken the observed associations rather than create false associations.
- Desaturase activities were estimated and not directly measured, but desaturation indices are established in epidemiological studies and are related to desaturase activities in vitro and in animals
- FA composition was measured in a sub-group, but the sub-group did not differ from those without FA composition measurement with regard to WC, BMI, ALT, M and triglycerides. Therefore, the generalizability was probably not affected.
- Only men of the same age participated without data on women, other ethnic groups or other ages. Therefore, the study is not generalizable.
- Alcohol intake was self-reported and thus possibly under-reported.
Quality Criteria Checklist: Primary Research
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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? | 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? | 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? | 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%.) | N/A | |
4.3. | Were all enrolled subjects/patients (in the original sample) accounted for? | N/A | |
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? | Yes | |
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.) | Yes | |
5.3. | In cohort study or cross-sectional study, were measurements of outcomes and risk factors blinded? | Yes | |
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? | Yes | |
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? | Yes | |
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? | N/A | |
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? | 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? | N/A | |
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 | |