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Lipids, Lipoproteins, and Cardiovascular Risk Factors |
1 Department of Epidemiology, University of Alabama at Birmingham, School of Public Health, Birmingham, AL; 2 Department of Laboratory Medicine and Pathology, University of Minnesota, Minneapolis, MN; 3 Department of Internal Medicine, University of Utah, Salt Lake City, UT; 4 JM-USDA-HNRCA, Tufts University, Boston, MA; 5 Division of Epidemiology and Community Health, University of Minnesota, School of Public Health, Minneapolis, MN; 6 Division of Statistical Genomics, Department of Genetics, Washington University, School of Medicine, St. Louis, MO.
aAddress correspondence to this author at: Department of Epidemiology, University of Alabama at Birmingham, School of Public Health, Birmingham, AL 35294. Fax 1-205-934-8665; e-mail edmondk{at}uab.edu.
| Abstract |
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Methods: Included were men [n = 497; mean (SD) age, 49 (16) years] and women [n = 539; age, 48 (16) years] from 187 families in a National Heart, Lung, and Blood Institute (NHLBI) family study of the Genetics of Lipid-Lowering Drugs and Diet Network (GOLDN) conducted in Utah and Minnesota. We used gas chromatography to measure erythrocyte fatty acids and obtained data on potential confounding variables from interviewer-administered questionnaires.
Results: The prevalence of the MetS as defined by the updated Adult Treatment Panel III criteria was 36.8% in Utah and 39.6% in Minnesota (P >0.05). In a multivariate model that included 4 fatty acid classes, covariates, and pedigree as a random effect, the odds ratios (95% confidence interval) for the MetS in the 1st, 2nd, 3rd, and 4th quartile of polyunsaturated fatty acids were 1.00, 0.72 (0.47–1.10), 0.67 (0.43–1.05), and 0.39 (0.24–0.64), respectively (P for trend = 0.0002). For the corresponding quartiles of saturated fatty acids, the odds ratios were 1.00, 1.19 (0.77–1.84), 1.48 (0.94–2.34), and 1.63 (1.01–2.63), respectively (P for trend = 0.03). Unlike n6 fatty acids, which showed an inverse association (P <0.05) with MetS, n3, trans, and monounsaturated fatty acids were not associated with the MetS (P >0.05). We observed significant correlations (P <0.05) between fatty acid classes, insulin, and components of the MetS.
Conclusions: Polyunsaturated fats are inversely associated with the MetS, whereas saturated fatty acids are positively associated with the MetS, probably through their effect on lipids, adiposity, insulin, and blood pressure.
| Introduction |
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-linolenic acid, a long-chain n3 fatty acid, is associated with a lower risk for cardiovascular disease (14)(15), it does not appear to alter the concentrations of plasma lipoproteins or their particle size (16). Linoleic acid, an n6 fatty acid and the most abundant polyunsaturated fatty acid in the human diet, can lower plasma glucose and LDL cholesterol (1)(17). n3 fatty acids contribute to membrane phospholipids, have anti-inflammatory properties (18)(19)(20), and are hypothesized to reduce abdominal adiposity, increase insulin sensitivity, and reduce the risk of the MetS (21)(22). Indeed, some recent studies (23)(24) have found associations between serum fatty acids and the MetS. Few studies have investigated the effects of dietary fat on the MetS, however, and results vary with age (25), smoking (26), and measurement error in dietary assessment, among other variables. Compared with questionnaire-based assessments of fat intake, measurements of plasma or erythrocyte fatty acids may be a more stable biomarker for the medium-term intake (i.e., months) of dietary fat (27)(28)(29) and may be more suitable for comparing fatty acid classes with regard to their effect on diabetes, cardiovascular disease, and intermediate phenotypes. Indeed, a recent prospective study found a correlation between dietary and erythrocyte trans fatty acid contents (r = 0.44; P <0.01) (30). We are not aware of any published studies that have examined the composition of fatty acids in erythrocyte membranes and the MetS. We investigated whether the fatty acid composition in erythrocytes is related to the MetS or its components.
| Materials and Methods |
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data collection
Habitual dietary intake was assessed with the National Cancer Institute Diet History Questionnaire (32), and data on physical activity and other lifestyle variables, such as smoking and alcohol intake, were collected by means of an interviewer-administered questionnaire. Preprandial glucose and insulin concentrations were measured with colorimetric assays. The lipid profile (e.g., triglycerides, HDL cholesterol, LDL cholesterol, and total cholesterol) following at least 8 hours of fasting was measured by nuclear magnetic resonance techniques (31). Fatty acids in erythrocyte membranes were extracted with a mixture of chloroform and methanol (2:1, by volume), collected in heptane, and injected onto a Varian CP7420 100-m capillary column with a Hewlett-Packard 5890 gas chromatograph equipped with an HP6890A autosampler. The initial temperature of 190 °C was increased to 240 °C over 50 min to separate fatty acids from 12:0 through 24:1n9 (33).
statistical analysis
SAS Software version 9.1.3 (SAS Institute) was used for statistical analyses. From the 1 328 men and women in the GOLDN study, we excluded all individuals with a self-reported history of kidney disease, individuals with Graves disease, those in the top and lowest 1 percentiles of total energy intake, and those with missing data on major exposure variables and potential confounders. The final data set consists of information from 1 036 individuals in 187 families. The MetS was defined according to the National Cholesterol Education Program Adult Treatment Panel III criteria as updated in 2004 (34)(35). We tested the statistical significance of differences in the distribution of categorical variables according to MetS status with the Cochran–Mantel–Haenszel test after stratifying by field center; for continuous variables, we used the Mixed procedure in SAS with a mixed model that adjusted for the field center and the pedigree as a random effect.
We used ANOVA to test for associations between quartiles of fatty acid classes in erythrocytes, components of the MetS, and risk factors of cardiovascular disease (e.g., insulin, homeostatic model assessment of insulin resistance, LDL cholesterol, and LDL cholesterol particle sizes). In these analyses, the 4 fatty acid classes (saturated, monounsaturated, polyunsaturated, and trans fatty acids), age, sex, body mass index, smoking, alcohol intake, field center, hours spent viewing a TV or a computer (as a proxy for physical activity), total energy intake, and pedigree (as a random effect) were concurrently entered in the model.
The MetS as a dichotomous variable was the main outcome variable in a mixed multivariate model that simultaneously included all 4 fatty acid classes, potential confounders, and pedigree as a random effect. The covariates included in the final models were age (as quartiles), sex (men vs women), smoking (never, past, and current smokers), alcohol-intake status (current drinker vs noncurrent drinker), and hours spent on the TV or computer (as quartiles). Income and education did not change the models appreciably and were excluded from the final analyses. We used Proc Glimmix in the SAS package to fit multivariate mixed models with erythrocyte fatty acid classes, potential confounders, and pedigree as a random effect to study the relation between individual fatty acid classes and the components of the MetS. We estimated the odds ratios [95% confidence interval (CI)] for the MetS in the 1st, 2nd, 3rd, and 4th quartile of saturated fat, monounsaturated fat, polyunsaturated fat, trans fat, total n3, and n6 fat in erythrocytes. All 4 main fatty acid classes (saturated fat, monounsaturated fat, polyunsaturated fat, and trans fat) were entered in the models simultaneously. If total n3 and n6 fat was simultaneously included in the model with saturated, monounsaturated, and trans fatty acid variables, the total polyunsaturated fatty acid variable was excluded. To calculate the P for trend, we assigned the quartile median value of a given fatty acid class to each individual in the quartile. The resulting semicontinuous variables for each fatty acid class were entered in the model simultaneously.
| Results |
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fatty acids in erythrocytes and components of METS
Trans and polyunsaturated fatty acids correlated well with the corresponding fatty acids in the diet, regardless of MetS status. Age and sex-adjusted Spearman correlation coefficients were 0.31 (P <0.0001) for trans fatty acids and 0.14 (P <0.0001) for polyunsaturated fatty acids. Saturated fat and monounsaturated fatty acids are synthesized in the body and, as expected, did not show good correlations with diet [Spearman r = –0.02 (P = 0.43) for saturated fat; r = –0.08 (P = 0.01) for monounsaturated fatty acids]. The fatty acid compositions of erythrocytes were similar in the 2 field centers (data not shown); thus, the data on the composition of fatty acid subtypes are shown according to MetS status but not by field center (Table 2
). Compared with individuals without the MetS, the concentrations of saturated and monounsaturated fatty acids were significantly higher (P <0.05) among individuals with the MetS, whereas the concentrations of polyunsaturated and trans fatty acids were significantly lower (P <0.05) among individuals with the MetS. The ratio of total polyunsaturated fatty acids to saturated fatty acids (PS ratio) was significantly higher (P <0.0001) among individuals without the MetS than those with the MetS (Table 2
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In age- and sex-adjusted analyses, fatty acid classes showed significant correlations with components of the MetS, body mass index, insulin, LDL cholesterol concentration, and LDL cholesterol particle size (Table 3
). Most associations between fatty acid classes in erythrocytes, components of the MetS, insulin, homeostatic model assessment of insulin resistance, LDL cholesterol concentration, and LDL cholesterol particle size remained significant, even after adjustment for several potential confounders (see Figs. 1 and 2 in the Data Supplement that accompanies the online version of this article at http://www.clinchem.org/content/vol?/issue? ]. The potential confounding variables we adjusted for were age, sex, body mass index, smoking, alcohol intake, field center, hours spent on the TV or computer, total energy intake, and pedigree as a random effect.
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relation between fatty acid classes in erythrocytes and the METS
Table 4
shows odds ratios for the quartiles of fatty acid classes and the MetS. The odds ratios (95% CI) for the MetS in the 1st, 2nd, 3rd, and 4th quartiles for the polyunsaturated fatty acid class were 1.00, 0.72 (0.47–1.10), 0.67 (0.43–1.05), and 0.39 (0.24–0.64), respectively (P for trend = 0.0002). For the corresponding quartiles for saturated fat, the odds ratios were 1.00, 1.19 (0.77–1.84), 1.48 (0.94–2.34), and 1.63 (1.01–2.63), respectively (P for trend = 0.03). Monounsaturated and trans fatty acid classes were not associated with the MetS (P >0.05; Table 4
). In a model with quartiles for the PS ratio, monounsaturated and trans fatty acid classes, and potential confounders (Table 4
), the PS ratio was significantly inversely associated with the MetS (data not shown). The odds ratios (95% CI) for the MetS in the 1st, 2nd, 3rd, and 4th quartiles of the PS ratio were 1.00, 0.65 (0.43–0.98), 0.44 (0.29–0.68), and 0.34 (0.22–0.53), respectively (P for trend <0.0001). The median PS ratio was 0.91, 0.96, 0.99, and 1.05 for the 1st, 2nd, 3rd, and 4th quartiles of the PS ratio, respectively.
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The odds ratios (95% CI) for the MetS in the 1st, 2nd, 3rd, and 4th quartiles of the n6 polyunsaturated fatty acid class were 1.00, 1.04 (0.68–1.59), 0.51 (0.32–0.81), and 0.35 (0.21–0.59), respectively (P for trend <0.0001). For n3 fatty acids, the odds ratios for the corresponding quartiles were 1.00, 1.45 (0.93–2.25), 1.10 (0.70–1.74), and 0.66 (0.39–1.10), respectively (P for trend = 0.03).
Because smoking can affect several of the components of the MetS, we investigated whether the above associations between fatty acid classes and the MetS would vary by smoking status (Table 5
). The low number of current smokers (n = 76) in this study population precluded detailed analyses among current smokers. Thus, subgroup analyses were performed only with people who had never smoked and past smokers. As in the main analyses, polyunsaturated fatty acid class among never smokers was significantly (P <0.05) inversely associated with the MetS. No other significant associations (P <0.05) were detected in these subgroup analyses.
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| Discussion |
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Studies on the relation between fatty acids and the MetS are still scarce. Most studies (23)(24)(25)(36) have assessed fatty acids in the diet or the plasma, thus making it difficult to directly compare these studies with our results. Nonetheless, our data are similar to those from the few studies in adolescents (24) and adults (23)(25)(37), in that these studies found higher concentrations of tissue polyunsaturated fatty acids, particularly linoleic acid, to be inversely associated with the MetS or some of its components. As in another study (25), total tissue n6 fatty acids (which are mainly linoleic acid and arachidonic acid) showed a significant inverse dose-dependent association with the MetS (P for trend <0.0001) and its components. This result is not surprising given that polyunsaturated fatty acids such as linoleic acid contribute to the regulation of the expression of genes involved in lipogenesis, fat oxidation, and lipid production, possibly contributing thereby to the MetS (20)(38)(39).
Our data are different from those of other investigators, who showed a strong inverse association between n3 fatty acids in plasma lipid esters (aggregated by the use of factor analysis) and the development of the MetS 20 years later (25). In our multivariate adjusted analyses, we did not find significant associations between total or individual n3 fatty acids and the MetS or most of its components. We did find a weak but significant inverse correlation only for n3 fatty acids and waist circumference (r = –0.07; P <0.05). n3 fatty acids showed a statistically significant inverse trend with MetS (P for trend = 0.03), but none of the comparisons between each of the top 3 quartiles and the lowest quartile were significant. These results were a surprise given that several recent studies have found inverse associations between
-linolenic acid (as well as other n3 fatty acids) and cardiovascular disease (14)(15) or the MetS (25). The reasons for this lack of association is not clear but may be due to lower amounts of n3 fatty acids in erythrocytes in our study population compared with other study populations (29)(37)(40). For instance, in the n3-supplementation trial by Riccardi et al. (37), the mean (SD) baseline proportions of eicosapentaenoic acid and docosahexaenoic acid in erythrocytes were 0.7% (0.1%) and 5.0% (0.4%) of the total fatty acids, respectively, whereas the corresponding proportions were 0.5% (0.3%) and 3.0% (0.9%) of the total fatty acids among our study participants without the MetS.
Unlike other studies on tissue fatty acids and the MetS, our study examined the association between trans fat and the MetS. Although we did not find a significant association between the class of total trans fatty acids and the MetS, we did find a significant inverse association (P <0.05) between trans fatty acids and HDL cholesterol. Because humans are unable to synthesize trans fatty acids in vivo, the trans fat in erythrocytes must have come from the diet, and dietary trans fat correlated well with erythrocyte trans fat (r = 0.31; P <0.0001). The inverse association observed between trans fatty acids and HDL cholesterol in this study and their association with dyslipidemia and cardiovascular disease in other studies (5)(30) underscores the importance of reducing the intake of trans fatty acids in the diet.
Unlike trans fatty acids and short-chain essential cis-polyunsaturated fatty acids, saturated and monounsaturated fatty acids in erythrocytes can come from both diet and in vivo biosynthesis, thus making them unsuitable as biomarkers of intake (29). This consideration is especially true for short-term intervention studies (25). Because of the cross-sectional nature of our study, we are unable to determine whether the observed associations are causal. Despite these limitations, the associations observed in our study are in close agreement with those from longitudinal studies in which the fatty acid composition of plasma lipid esters predicted the development of the MetS 20 years after the initial assessment (23).
The results from this study support the current recommendations to lower the intake of saturated and trans fatty acids and to increase the intake of polyunsaturated fatty acids as a goal for improving diet quality and lowering the risk for diabetes and cardiovascular disease. Because the composition of the fatty acids in tissues does not depend on diet alone, future studies are needed to determine the nondietary factors that might be associated with higher tissue concentrations of polyunsaturated fat and lower concentrations of saturated and trans fatty acids.
| Acknowledgments |
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Financial Disclosures: None declared.
Acknowledgment: We are grateful to the staff of the GOLDN study for the assistance in data collection and management.
| Footnotes |
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| References |
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