1 Department of Epidemiology and Public Health, University of Navarra, E-31008 Pamplona, Spain
2 Navarra Primary Care Health Services, E-31008 Pamplona, Spain
Yuan et al. (1) have reported that middle-aged or older men in Shanghai, China, who consumed at least one serving of fish and shellfish weekly had an important reduction in the risk of fatal acute myocardial infarction compared with less-frequent consumers. However, previously published results regarding this issue are not fully consistent, ranging from nonprotection to a large reduction in risk. These differences could be due to variation in study design, outcome ascertainment, and dietary assessment. In the study by Yuan et al., only 45 food items were included in the questionnaire, which could be a source of important misclassification leading to insufficient control for confounding in the analyses.
In a systematic review of cohort studies, Marckmann and Gronbaek (2) suggested that the benefit of dietary fish (or n-3 fatty acid) would be seen in high-risk populations only. Nevertheless, the results of Yuan et al. (1), obtained in a low-risk population, are contrary to this statement, but for fatal myocardial infarction only. As these authors mentioned, this finding fits with the arrhythmia hypothesis: n-3 fatty acid may prevent not the occurrence of myocardial infarction but mortality from myocardial infarction, protecting against fatal arrhythmia.
We recently reported the results of a case-control study showing an important risk reduction in the incidence of nonfatal myocardial infarction in a Spanish population with a high intake of marine foods (3). In our results, the adjusted odds ratio for nonfatal myocardial infarction was 0.42 (95 percent confidence interval: 0.19, 0.93) for subjects in the upper third of fish/shellfish consumption compared with those in the lower third. Similar figures were obtained when we analyzed n-3 fatty acid intake (odds ratio = 0.47, 95 percent confidence interval: 0.22, 1.00 for the upper third of fish/shellfish consumption). Although the design we used was a case-control study, the high intake of fish in a Spanish population and better dietary assessment (our validated food frequency questionnaire contained 136 items (4)) provide further reliability regarding our results. Comparable results were also obtained in another case-control study conducted in Italy (5), a country similar to Spain in the distribution of cardiovascular risk factors.
Two conclusions can be drawn. First, special attention must be given to dietary assessment when comparing different epidemiologic studies. Second, the protective role of n-3 fatty acids in nonfatal coronary events, especially myocardial infarction, should be reevaluated.
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