The purpose of this study was to explore the clinical, demographic, behavioral and genetic correlates of the omega-3 index in a well-defined cohort of subjects from the Framingham Heart Study. The cohort was comprised of 91% white participants (Offspring) and 9% minorities (Omni). The omega-3 index was lower in Offspring than Omni despite fish oil supplementation being more prevalent in the former; this effect remained after multivariable adjustment. Whether a higher dietary intake of n-3 fatty acids in Omni is responsible for this difference is unknown since no dietary information was available for this cohort. Higher levels of the omega-3 index in African Americans compared with Whites have been reported in acute myocardial infarction patients [40
Overall the omega-3 index was directly associated with several factors in fully adjusted models. A significant relationship with age has been frequently reported [41
] and could reflect increased fish intake in older individuals. However, we observed this association after controlling for n-3 fatty acid intake [as have some [42
] but not all [53
] other studies], and thus it may be a function of decreased n-3 fatty acid turnover in older subjects [55
]. In an earlier study of 298 Offspring participants with RBCs tested 6.7 years apart [56
], the mean omega-3 index did not change, suggesting that higher levels of the index in older people may not be due to aging per se, but perhaps to the “attrition of the susceptible.” Also female sex was associated with a higher omega-3 index. This too has been reported previously [48
] [but inconsistently [42
]], and could be due to enhanced synthesis of long chain n-3 fatty acids from alpha-linolenic acid mediated by estrogen [59
]. This explanation is tentative, however, since the vast majority of women in this cohort were post menopausal, and only 10% were on hormone therapy. The reduced DPA (intermediate between EPA and DHA) and increased DHA levels in post menopausal women vs. men observed here are consistent with increased conversion of DPA to DHA seen in premenopausal [61
] women. A higher omega-3 index was also directly associated with a greater use of aspirin and lipid lowering drugs, and with higher LDL-C levels. This confirms smaller crosssectional observations made in Canadian native Americans [50
] and Inuits [49
]. In the present study, the people taking aspirin and/or lipid drugs were more likely to be taking fish oil supplements (58% vs. 42%, p
< 0.0001). In addition, they may have been taking higher n-3 fatty acid doses than the supplementers not on pharmacotherapy (dosage amounts were not available) which could explain the association of higher omega-3 index levels with medication use even after adjustment.
As regards the direct association between plasma LDL-C and the omega-3 index, n-3 fatty acid supplementation has been reported to increase LDL-C, particularly in hypertriglyceridemic patients [62
], but this may be due more to a change in particle composition than in particle number [63
]. Whether nutritional variations in n-3 fatty acid intakes affect LDL-C is not known, but the relation between the omega-3 index and LDL-C became non-significant when dietary EPA + DHA was included in the model (Supplementary Table 5
There were several inverse associations with the omega-3 index in the fully adjusted model. That with smoking status, which has been noted before [40
], was driven by the DHA (not the EPA) component (Supplementary Table 3
). This relationship may be due to the oxidative destruction of most highly unsaturated fatty acids [65
], and/or from altered metabolism of EPA and DHA in smokers. The fact that smoking was associated with lower omega-3 levels independent of EPA + DHA intake and fish oil supplementation (Supplementary Table 5
) argues against intake differences.
Waist circumference was inversely associated with the omega-3 index (and both of its fatty acid components) in univariate and multivariable models. Studies in three Canadian cohorts reported contradictory findings in this regard: inverse associations between waist girth and RBC n-3 fatty acids in Cree Indians [50
] but direct relations in Inuits [49
] and Quebecers [48
]. Therewas no univariate relation with girth in a Spanish cohort [53
]. In a previous study, lower levels of EPA + DHA were associated with higher levels of pro-oxidants in RBCs from obese subjects [67
], and with higher levels of inflammatory markers [43
] suggesting that increased adiposity, oxidative stress, and inflammation and lower membrane n-3 fatty acids appear to coexist.
The triglyceride-lowering effects of supplemental n-3 fatty acids are well-known [13
]. However, relations between triglycerides and n-3 fatty acid biomarkers in (largely) non-supplementing populations have received less attention. Inverse associations were, however, observed in the Heart and Soul study [70
], Canadian Inuits [49
], Spanish patients at high CHD risk [53
] and outpatients in Kansas City [42
]. These are likely to be causal since n-3 fatty acids lower TG production in the liver [71
]. Finally, a higher omega-3 index was associated with a lower heart rate. This too has been reported in intervention studies [72
] and observed crosssectionally [10
]. N-3 fatty acids appear to improve autonomic tone as evidenced by their ability to increase heart rate variability in some but not all studies [75
Taking advantage of the pedigree information from the Framingham study, we were able to estimate that 24% of the variability in the omega-3 index was due to polygenic factors (i.e., heritability). This increased the total explained variability to 73%, which was greater than previously reported [42
]. The largest shift in amount of explained variance between heritability (15% decrease) and covariates (25% increase) was observed after dietary EPA + DHA intake was included in the model. That is to say, that related participants apparently maintained similar eating habits, and that the variance in the omega-3 index attributed to genes was overestimated when EPA + DHA consumption was not taken into account. Genome-wide association studies are needed to identify genetic loci that may determine the observed heritability [77
In the Offspring cohort (where dietary data were available), the correlation between EPA + DHA intake and the omega-3 index (0.59) was stronger than that observed previously using the same FFQ but with adipose tissue EPA + DHA as a biomarker (r
= 0.43) [30
]. As RBC membranes are more enriched in these long chain n-3 fatty acids than is adipose tissue [78
], this finding provides stronger confirmation of the validity of the FFQ for these specific fatty acids.
4.1. Strengths and limitations
Among the strengths of this study were the large, well-characterized, and racially-diverse (via the inclusion of Omni) cohort; and the use of validated analytical methods for clinical CVD risk markers (including the omega-3 index). Also this study included a formal analysis of heritability. There were also limitations. Causal relations cannot be deduced from a cross-sectional analysis, so the associations observed here – unless previously observed in n-3 fatty acid intervention studies – must be considered only hypothesis-generating. For those individuals reporting fish oil supplementation we had no information on the dosage, potency or intake frequency of the capsules. As noted earlier, we did not have data on the dietary habits nor pedigree information of the Omni participants.
In summary, a higher omega-3 index was independently associated with some factors known to be associated with reduced risk for cardiovascular disease (female sex, smaller waist girth, lower triglyceride levels, less smoking, lower heart rate, and a greater use of aspirin and lipid pharmacotherapy). On the other hand, a higher index was also associated with a slightly higher LDL-cholesterol level. After adjusting for n-3 fatty acid intake and other relevant covariates, 24% of the variability in the omega-3 index was explained by Mendelian inheritance. Overall, an increased omega-3 index appears to coexist with a healthier cardiovascular risk profile.