The omega-3 index (the summed percentage content of eicosapentaenoic and docosahexaenoic acids in red blood cells) is associated with a lower risk of fatal coronary heart disease and sudden cardiac death. We aimed to determine which socio-demographic, behavioural or clinical factors are independently associated with the omega-3 index and the extent to which seafood consumption mediates the index’s association with socio-economic status (SES).
As part of the cross-sectional MONA LISA-NUT survey (2005–2007), gas chromatography was used to analyse the red blood cell fatty acid composition in 503 French subjects aged 35–64 years. Dietary data were collected by trained dieticians via a validated food frequency questionnaire and a prospective 3-day food record. Risk factors were estimated with standardised measurements and questionnaires. SES was assessed through the self-reported educational and income tax levels.
The mean±s.d. omega-3 index was 6.02±1.75%. In the best parsimonious predictive model (which explained 32% of the variability in the omega-3 index), age, educational level and seafood servings were significantly and positively associated with the index. In contrast, waist circumference and smoking were inversely associated with the index. In a mediation analysis that took account of all these factors, seafood servings explained about 40% of the association between educational level and the omega-3 index. Similar results were obtained for the income tax level.
The inverse association between SES and omega-3 index is largely explained (40%) by an insufficient seafood intake. It remains to be seen which other factors mediate this association.
High dietary and blood levels of long-chain n-3 polyunsaturated fatty acids are associated with a lower risk of coronary heart disease (CHD) and sudden cardiac death.1,2 The fatty acid composition of red blood cells (RBCs) is considered to be a suitable biomarker of long-term long-chain n-3 polyunsaturated fatty acid (PUFA) intake.3 It is particularly useful in epidemiological studies and has a low biological variability.4 Furthermore, the omega-3 index (defined as the summed content of eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) in RBC membranes as a percentage of total fatty acids) is known to be highly correlated with human myocardial n-3 fatty acid content.5 Hence, the omega-3 index is increasingly considered to be a novel indicator of risk for primary cardiac arrest and fatal CHD.6,7
The dietary intake of EPA and DHA increases the omega-3 index, which may also vary according to additional individual or environmental factors. However, only a few studies (including just one in Europe8) have produced sometimes-conflicting information on the potential influence of demographic, behavioural and biological factors (such as gender, age, socioeconomic status (SES), body mass index, alcohol consumption and diabetes), on this parameter.8, 9, 10, 11, 12, 13, 14 Despite the fact that social inequalities have a confirmed impact on the incidence of CHD, the role of dietary n-3 fatty acid intake in these associations is yet to be established. More specifically, it is not known what proportion of the association between SES and the omega-3 index can be explained by diet (such as omega-3 fatty acid intake or seafood consumption that contains a high level of EPA+DHA) or other factors.11 Yet, a better knowledge of these factors could help to identify and provide effective dietary advice or supplementation with n-3 fatty acids in subjects at the highest risk of CHD morbidity and mortality.
Therefore, the objective of the present study was to determine which factors are independently associated with the omega-3 index and the extent to which seafood consumption may mediate the index’s association with SES.
Materials and methods
The MONA LISA Study was a cross-sectional, population-based, epidemiological, multicentre study conducted in 2005–2007 in the three French centres that had previously participated in the WHO-MONICA population surveys of cardiovascular risk factors.15, 16, 17, 18 Briefly, inhabitants aged 35–74 years living in the Lille Urban Community (northern France), the Bas-Rhin county (eastern France) and the Haute-Garonne county (southern France) were randomly sampled from electoral rolls after stratification by town size, gender and 10-year age group. The protocol was approved by the appropriate independent ethics committees. All subjects gave their written, informed consent to participation. The participation rate was 50% for men and 51% for women. All the subjects aged between 35 and 64 were invited to participate in a dietary assessment; 88% accepted and thus constituted the subsample for the MONA LISA-NUT study.
Measurement of the RBC fatty acid composition was scheduled in 540 MONA LISA-NUT study participants (30 men and 30 women per 10-year age group and per centre). To ensure that this subsample was adequately distributed throughout the year, we randomly selected 10 participants per gender, 10-year age group and centre in three different seasons: summer 2006 (summer 2007 for the 35–44 age group), winter 2006-2007 and spring 2007.
All data were collected by trained medical staff according to standardised protocols (based on the WHO-MONICA Project protocol).19 Heart rate and systolic and diastolic blood pressures were measured with an automatic sphygmomanometer (OMRON 705IT, OMRON Healthcare Europe, Hoofddorp, The Netherlands) and a tailored sized cuff with the arm at heart level, after the subject had been sitting for at least 10 min. The mean of two measurements made 30 min apart was used in the study analysis. Standardised anthropometric measurements (weight and height) were made on subjects in light clothing without shoes and body mass index was calculated. Waist circumference (WC) was recorded with a tape measure at midpoint between the upper border of the iliac crest and the lower border of the rib cage.
During a face-to-face interview, participants filled out a questionnaire on demographic variables, SES (educational level, income tax level), smoking, alcohol consumption, their personal medical history and medication use. Participants were considered to have diabetes if they were being treated with hypoglycaemic medications or if their fasting blood glucose levels were at least 7 mmol/l. Alcohol consumption (from wine, beer, cider, aperitifs and spirits) was scored day by day over a typical week. The total mean daily alcohol intake (in grams) was then calculated. Subjects were classified as being abstinent, low alcohol consumers or high alcohol consumers (⩾20 g/day for women and ⩾40 g/day for men).20 Subjects reporting at least one cigarette per day were classified as current smokers. Participants reporting at least 20 min of intense physical activity outside work once a week or more were classified as performing regular, intense physical activity. Educational level or income tax level was considered as a proxy for SES. Subjects were classified as having a low educational level (no formal education or primary or middle school education), a moderate educational level (secondary education) or a high educational level (further or higher education).
The nutritional survey was carried out according to the EURONUT protocol.21 Subjects had to record their food consumption for 3 days (two week-days and one weekend day) and specify the nature and the estimated quantity of the food and drink consumed at each intake. These data were specified with the assistance of a validated handbook of photographed portions22 during a meeting with a trained dietician 1–7 days following the food record. A short, validated, qualitative food frequency questionnaire23 was also filled out by the dietician, who recorded the participant’s responses for the 24 items (split in nine frequency terms). One of these items referred to the seafood intake. The frequency of seafood intake was categorised in four levels: 1 or no serving per month; 2 servings per month; 1 serving per week; at least 2 or 3 servings per week.
Fatty acid measurements in RBCs
After a 10-h fast and before the physical examination, a blood sample was drawn into EDTA tubes and centrifuged at 4 °C to separate the cells from the plasma. The RBCs were frozen at −80 °C until analyses were performed in the same laboratory (UMR1019, Clermont-Ferrand, France). A full description of the laboratory measurements is shown in the Supplementary material. Briefly, the RBC phospholipids were extracted as described by Folch et al.,24 and fatty acid methyl esters were prepared as previously described25 and then purified on florisil (Chromabond, Macherey-Nagel, Germany). The fatty acid composition of the RBC membranes was measured by gas chromatography (Trace, Thermo Electron Corp, Madison, WI, USA) with a CP-Sil88 capillary column. The coefficient of variation for EPA+DHA peak was 2.7% (around 0.1% in absolute percentage). The omega-3 index was calculated as a percentage: [(EPA+DHA)/sum of all fatty acids] × 100.
All samples were analysed in the same laboratory at the Pasteur Institute of Lille (Lille, France). Total cholesterol and triglycerides were measured with enzyme-based assays (Olympus Corporation, Tokyo, Japan). High-density lipoprotein cholesterol was measured after sodium phosphotungstate/magnesium chloride precipitation (Olympus Corp). Low-density lipoprotein cholesterol was calculated with the Friedewald equation when triglyceride levels were lower than 4.56 mmol/l. Glucose was measured using the standard glucose hexokinase method (DuPont Dimension, Brussels, Belgium).
Linear regression analyses were used to assess the relationship between the omega-3 index (as the dependent variable) and 20 potential determinants selected a priori on the basis of previous studies. Nutrient intakes were adjusted for energy intake according to the residual method.26 To account for the number of tests in the univariate analyses, Bonferroni correction was applied and threshold for statistical significance was adjusted to 0.05/20=0.0025.
The cutoff point for inclusion in the multivariable model was a P-value ⩽0.25 in a univariate analysis. Given that body mass index and WC were highly correlated, only WC was included in the candidate variables. Similarly, we chose to use educational level as a proxy for SES in our initial analyses and income tax in subsequent analyses. We selected parsimonious models with the GLMSELECT procedure. In line with a previous study,12 the best model was chosen using the Schwartz Bayesian criterion with stepwise variable selection. The ‘centre’ variable was forced into all models.
To test for a mediating effect of seafood consumption on the relationship between educational level and the omega-3 index, we used the three-step Baron and Kenny approach.27 Firstly, we regressed the omega-3 index against the educational level, in order to establish a significant association (estimate c). Secondly, we searched for a significant relationship between educational level and seafood consumption (estimate a). Thirdly, we regressed the omega-3 index against both educational level and seafood consumption, in order to obtain (i) an estimate of the relationship between educational level and the omega-3 index while controlling for seafood consumption (estimate c’) and (ii) an estimate of the association between seafood consumption and the omega-3 index (estimate b). The amount of mediation was quantified as 1-(c’/c).
To assess the significance of a mediating effect of seafood consumption on the relationship between educational level and the omega-3 index, we tested the product of the coefficients (a × b) with the percentile bootstrap method and thus derived its confidence interval.28,29
All statistical analyses were conducted with SAS software (version 9.2, 2010, SAS Institute Inc., Cary, NC, USA).
The RBC fatty acid composition was obtained for 519 participants with a dietary assessment. The final sample for the multivariable analyses comprised 503 participants with a complete set of covariate data. The participants’ characteristics by geographical area are summarised in Supplementary Table 1.
Omega-3 index levels
The individual omega-3 index values were normally distributed, with a mean±s.d. of 6.02±1.75% and range of 0.65–11.22%. On average, the index was significantly higher in women (P<0.05) and increased with age (P<10−4), with no evidence of an interaction between these factors (Figure 1). In both genders, the age-adjusted mean omega-3 index was lower in eastern France than in south-western France. An intermediate value was observed in northern France (Table 1).
Independent correlates of the omega-3 index
In univariate analyses, the following factors were positively associated with the omega-3 index: age, seafood consumption, educational level, income tax, energy-adjusted intake of EPA and DHA, residence in south-western France. In contrast, current smoking was inversely associated with the index (Tables 2 and 3). The best parsimonious predictive model of the omega-3 index is described in Table 4. With the exception of the forced ‘centre’ variable, the model includes 5 of the 14 candidate variables. The factors significantly, positively and independently associated with the omega-3 index were age, educational level and the frequency of seafood consumption. Having a high WC and (to a lesser extent) being a current smoker was associated with a low omega-3 index. Gender, the physical activity level, statin use, diastolic blood pressure, low-density lipoprotein cholesterol, heart rate, energy-adjusted omega-6 intake and α-linolenic acid intake were not independently associated with the omega-3 index. It is noteworthy that educational level and seafood consumption were present in all models, regardless of the method of selection (the Schwartz Bayesian criterion, the Bayesian information criterion or Mallow’s CP) and the demographic variables (gender, age and centre) forced into the model (data not shown).
The results were similar when we applied a fully adjusted model that included all the candidate variables and explained 34% of the variability in the omega-3 index (Supplementary Table 2). None of the variables interacted with gender (data not shown).
Seafood consumption as a mediator of the SES-omega-3 index association
All the estimates obtained with the three-step Baron and Kenny approach were statistically significant (Supplementary Table 3). Seafood consumption attenuated the association between SES and the omega-3 index (Table 5). Thus, the coefficient for highly educated participants fell from 1.020 to 0.624 after adjustment for seafood consumption, suggesting that seafood consumption mediates 39% of the educational level–omega-3 index relationship. According to the percentile bootstrap, this is a significant mediating effect. A 40% mediating effect of seafood consumption was also observed when educational level was assessed continuously (i.e., as years of fulltime education). Likewise, similar results were obtained when the income tax level was analysed as a proxy for SES. Adjustment for the complete set of candidate variables only slightly reduced these mediating effects (Supplementary Table 4).
A low omega-3 index has been shown to be a strong predictor of sudden cardiac death.7 Thus, determining the demographic, behavioural and clinical characteristics associated with a low omega-3 index might help to identify subjects at high cardiovascular risk. After correcting for geographical area, we found that seafood consumption, age, educational level, WC and smoking were independent correlates of the omega-3 index in middle-aged subjects. We also found that seafood consumption partially mediated the relationship between SES and the omega-3 index.
We observed a mean omega-3 index of 6%. The index was slightly higher in south-western France than in northern or eastern France, in agreement with the country’s well-known north-south decrease in CHD (which may be due in part to higher seafood consumption in the south). However, geographical area remained a significant predictor of the omega-3 index after adjusting for seafood consumption. The greater effect of living in the north of France (relative to the east) requires further investigation. The mean value of 6% is greater than the values of 4.3–5.5% usually reported in the United States,10, 11, 12, 13 lower than the mean value of 7.1% reported in a Spanish population8 and even lower than the values of 8.5–9.0% observed in a Japanese population9 and Alaskan Eskimos.30 These differences probably reflect the great country-to-country variation in seafood consumption and well-known disparities in CHD morbidity and mortality.
In line with the literature data, our results showed that age was an independent predictor of the omega-3 index even after adjusting for seafood intake.9,10,13,14 There are several possible explanations for this finding, including (i) a slow-down in n-3 fatty acid turn-over with age, (ii) lower intakes of linoleic acid or higher intakes of α-linolenic acid by older subjects or even (iii) survivor bias.31 Indeed, the older subjects in our sample tended to consume more α-linolenic acid and less linoleic acid (Supplementary Table 5). However, adjusting for these parameters failed to mask the positive effect of age.
The omega-3 index was higher in women than in men but the statistical significance was weak and disappeared in multivariate models. This finding has been noted in some previous studies8,10,13,14,32 but not in others.9,12 Some researchers consider that this reflects higher consumption of seafood by women than by men; however, this was not the case in the present study. Furthermore, studies in humans have suggested that sex hormones influence the enzymatic synthesis of long-chain PUFAs and lead to gender-specific differences in long-chain PUFA status.33 These gender differences notably concern the ability to synthesise EPA and DHA from α-linolenic acid.34 In rats, the concentrations of long-chain n-3 PUFAs in plasma and tissues were found to be positively associated with circulating concentrations of oestradiol and progesterone and negatively associated with circulating concentrations of testosterone.34
Few studies have looked at whether alcohol consumption is significantly correlated with the omega-3 index. As previously reported,12,13 we did not find that alcohol consumption was a significant, independent predictor of the omega-3 index; this contrasts with a report of a positive association by di Giuseppe et al.35 In the latter study, the positive association was mainly driven by wine drinking (and not by beer or spirits), and was only observed in women. Furthermore, we did not find any gender differences in the influence of alcohol consumption. Future research must clarify whether alcohol consumption is associated with the omega-3 index and, if so, what kind of alcoholic beverages are responsible for the association.
WC was inversely associated with the omega-3 index, even after accounting for other possible determinants. Associations have been inconsistently reported in the literature and have ranged from the absence of a univariate association for WC or body mass index8,9,14,32 to an independent, negative association.10,12 It has been suggested that high levels of oxidative stress in obese subjects are associated with low RBC levels of EPA and DHA.36 However, animal studies have shown that impaired hepatic metabolism of long-chain n-3 PUFAs may also be a factor, because obesity and diabetes both alter the expression and activity of the key enzymes involved in the synthesis of EPA and DHA from α-linolenic acid.37
A low omega-3 index in smokers has been reported by some studies12, 13, 14 but not others.9,10 Given that cigarette smoking is known to induce oxidative stress, greater oxidation of long-chain PUFAs may explain this low value in current smokers. In a study of schizophrenic patients,38 low levels of EPA and DHA observed in smokers have been related to differences in dietary intake of n-3 fatty acids. However, this cannot be the main reason in our study because seafood intake was taken into account. Lastly, one can conjecture that a low omega-3 index in smokers might be related to increased consumption of long-chain n-3 PUFAs in the synthesis of anti-inflammatory mediators.39
We found a positive association with SES when assessed as either educational or income tax level. Surprisingly, only half the previous studies of determinants of the omega-3 index have examined SES as a candidate factor.11, 12, 13, 14 Two studies reported a positive, independent association between SES and the omega-3 index.11,12 In fact, it is well known that people with low SES tend to eat less seafood, partly because they are less aware of the health benefits of seafood intake and partly because seafood is usually more expensive.
In a study of patients with CHD, Cohen et al.11 observed lower levels of RBC n-3 fatty acids in individuals with lower SES (as assessed by three different indices). The lack of nutritional data prevented Cohen et al., from investigating the role of fish consumption in these associations. Our study demonstrates that the differences in RBC EPA and DHA levels in a population of subjects with a wide range of educational levels were partly explained by differences in seafood consumption. This finding emphasises the need to provide people with low SES with foods rich in n-3 PUFAs at an appropriate cost. Conversely, it shows that factors other than seafood consumption—or age, WC, geographical area—affect the omega-3 index’s relationship with SES. These factors may influence the hepatic metabolism of long-chain PUFAs and the activity of the elongases and desaturases involved in the synthesis of long-chain n-3 PUFAs. Several factors, such as an unbalanced diet and low physical activity, may interact and lead to metabolic alterations related to obesity or diabetes.37
The present study’s strengths include its population-based setting, the multicentre design (with three distant geographical areas in France), the relatively large sample size and the assessment of a long-term biomarker of n-3 fatty acid intake that is known to have a relatively low intra-individual variability.4
The study also has some limitations. Firstly, this was an observational study; even though we took account of many potential confounding factors, we cannot rule out the possibility of residual confounding. For example, genetic factors have been shown to account for a large proportion of the variability in the omega-3 index.12 Secondly, the study’s cross-sectional design prevents causal inferences from being made. However, it is legitimate to consider that a higher educational level can enhance seafood intake, and thus the intake of EPA and DHA, rather than the opposite. Thirdly, data on supplementation were not available in all three areas. Nevertheless, very few people in France consume dietary supplements (at least relative to the United States). For example, 11% of the 35–54-year-old subjects in the nationally representative French INCA2 dietary survey (2006-2007)40 reported consuming some type of supplement. This is far lower than the values of 50%, for regular supplement intake, and 66%, for at least occasional supplement intake, recorded in the United States from 2007 to 2011.41 In the MONA LISA-NUT study, fewer than 2% of the participants living in eastern France reported consuming some fatty acid supplements; we consider that this low proportion is unlikely to seriously bias our results.
In conclusion, we found that seafood consumption, age, SES, WC and smoking were independently associated with the omega-3 index in a middle-aged French population. We also found that seafood consumption mediated a large part of the association between SES and the omega-3 index, which could help target health prevention funds to subjects with a high risk of cardiovascular death. Other factors, such as the intake of various fatty acids, may also be involved in this mediation and so further analyses of this association are required.
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We would like to thank the nurses, physicians, dieticians, computer scientists and secretaries and the Centre de Médecine Préventive de Lille, the Laboratoire d’Analyses Génomiques and the Service de Biologie Spécialisée de l’Institut Pasteur de Lille, the Centre de Santé de la MGEN de Strasbourg, the Unité de Coordination de la Biologie des Essais Cliniques des Hôpitaux Universitaires de Strasbourg, the Departments of Cardiology of the Toulouse University Hospital and the Human Nutrition Unit of Clermont-Ferrand, whose technicians from the CHLEO team have carried out the GC analyses. The MONA LISA Study was made possible by an unrestricted grant of Pfizer and by a grant from ANR (ANR-05-PNRA-018). Pfizer and ANR had no role in the design, analysis or writing of this article.
AW, CS, JD, JBR, DC, JF and DA designed the study or were involved in the study management; BM and BL were involved in the assessment of the fatty acid composition in red blood cell membranes; AW drafted the manuscript and performed the analyses. All authors contributed to the interpretation of the data and critically reviewed the manuscript.
The authors declare no conflict of interest.
Supplementary Information accompanies this paper on European Journal of Clinical Nutrition website
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Wagner, A., Simon, C., Morio, B. et al. Omega-3 index levels and associated factors in a middle-aged French population: the MONA LISA-NUT Study. Eur J Clin Nutr 69, 436–441 (2015). https://doi.org/10.1038/ejcn.2014.219
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