The association between dietary omega-3 fatty acids and cardiovascular death: the Singapore Chinese Health Study

Background: Although studies suggest that omega 3 fatty acids intake may reduce cardiovascular disease (CVD) mortality risk, few studies have differentiated dietary eicosapentaenoic and docosahexaenoic acid (EPA and DHA) from alphalinolenic acid (ALA), and epidemiological research in Asian populations is limited. Methods: The Singapore Chinese Health Study is a population-based cohort that recruited 63,257 Chinese adults aged 45 to 74 years from 1993 to 1998. Usual diet was measured at recruitment using a validated semi quantitative food frequency questionnaire, and mortality information was identified via registry linkage up to 31 December 2011. Cox proportional hazard models were used to calculate hazard ratios (HRs) with adjustment for potential confounders. Results: We documented 4780 cardiovascular deaths (including 2697 coronary heart disease (CHD) deaths and 1298 stroke deaths) during 890,473 person years of follow up. Omega 3 fatty acids intake was monotonically associated with reduced risk of cardiovascular mortality. Compared to the lowest quartile, the HR was 0.88 (95 per cent confidence interval, CI, 0.81 to 0.96), 0.88 (95 per cent  CI 0.80 to 0.97), and 0.83 (95 per cent CI 0.74 to 0.92) for the second, third, and highest quartile, respectively. Both EPA DHA and ALA were independently associated with reduced risk of cardiovascular mortality: HR comparing extreme quartiles was 0.86 and 0.81, respectively. The associations were similar for deaths from CHD and stroke and persisted in participants who were free of CVD at baseline. Conclusions: Higher intakes of marine (EPA DHA) and plant (ALA) omega 3 fatty acids are both associated with reduced risk of cardiovascular mortality in a Chinese population. (Authors abstract)

Most studies on omega 3 have been conducted in North American and European countries, and researches in Asian populations are limited. Two cohort studies in Chinese adults and one cohort study in Japanese adults have consistently found an inverse association of fish and EPA DHA intake with cardiovascular death; however, the results were not consistent with coronary heart disease (CHD) or stroke mortality.  Nevertheless, ALA was not examined in those studies. The plant oils have been widely used in Asian cooking, and legumes, soy and grains are major food items in Asian diet. Thus, ALA is a major source of omega 3 fatty acids, but no study has specifically examined the relation between ALA intake and cardiovascular mortality in Asians.

Here, data was assessed from the Singapore Chinese Health Study (SCHS), a population based prospective cohort of approximately 63,000 Chinese men and women in Singapore.  In this large cohort study of Chinese men and women, intake of omega 3 fatty acids (both marine and nonmarine based sources) was independently associated with reduced risk of cardiovascular mortality, particularly CHD deaths. The associations were independent of available established CVD risk factors and other dietary and lifestyle confounders in our database, and the lowest risk was observed in the subjects with high intakes for both EPA DHA and ALA. The associations were slightly stronger in participants without baseline CVD; however, the interaction was not significant. This analysis is one of the few studies that investigated both EPA DHA and ALA in the same cohort. These n3 were associated with a lower risk, which is consistent with the Cardiovascular Health Study.  One study suggested that ALA intake may particularly reduce CHD risk when EPA DHA intake is low.   This study is the first to investigate the relation of ALA intake and stroke risk in Asian populations.  The observed inverse association is consistent with a cohort study in The Netherlands ,and a nested case control study in US adults where serum concentrations of ALA were measured in 96 cases and 96 controls.  

The strengths of this study include high response and follow up rate, detailed collection of data through face to face interviews, and nearly complete mortality assessment with objectively obtained records. The dietary intake was assessed by a FFQ that was specifically developed and validated in this population, and has been shown to be reasonably accurate. In conclusion, the results provide supportive evidence that high dietary intake of both marine and nonmarine based omega 3 fatty acids is associated with reduced risk of cardiovascular death in the Chinese population, particularly for deaths from coronary heart disease and in individuals without cardiovascular disease at baseline. Further studies are still needed to evaluate the association with incident nonfatal cardiovascular events and to confirm our results in other populations. (Editors comments)