To examine the relationship between fish intake and coronary heart disease mortality.

The authors searched the electronic MEDLINE database (1966 onwards) using the indexing terms: 'fish', 'coronary heart disease', and 'journal article'. The authors also used the references of retrieved articles and used informal sources to search for additional relevant studies.

Study designs of evaluations included in the review

Prospective cohort studies. Follow-up ranged from 5 to 30 years.

Specific interventions included in the review

Fish or n-3 polyunsaturated fatty acids (VLCPUFA).

Participants included in the review

Participants at high or low risk of coronary heart disease who consumed fish or n-3 polyunsaturated fatty acids. Only 3 of the studies contained females.

Outcomes assessed in the review

Coronary heart disease mortality.

How were decisions on the relevance of primary studies made?

The authors do not state how the papers were selected for the review, or how many of the authors performed the selection.

Each study was scored for quality on a scale from 0 to 6 points (high quality = 5-6 points; intermediate quality = 3-4 points; and low quality = 2 or fewer points). Points were scored for dietary assessment method (up to 3 points), CHD death ascertainment (up to 1 point), number of CHD deaths (up to 1 point), and statistical presentation (up to 1 point). All the studies were of high quality in the criteria for study duration and completeness of follow-up. The authors do not state how the papers were assessed for quality, or how many of the authors performed the quality assessment.

The authors do not state how the data were extracted for the review, or how many of the authors performed the data extraction. Data were extracted for the categories of number of participants, number of years of follow-up, CHD deaths, CHD death rate (per 1,000 years), mean fish intake (g/d), mean n-3 intake (g/d), and dietary assessment method.

The mean intake of fish was calculated for certain ranges of intake and plotted on graphs. Multivariate adjusted relative risk (RR) and 95% confidence intervals (CIs) were calculated for each individual study.

How were the studies combined?

The studies were discussed in a narrative synthesis grouped according to the level of quality of the studies (4 high-quality studies; 4 intermediate-quality studies; and 3 low-quality studies).

How were differences between studies investigated?

The authors do not state how differences between the studies were investigated.

Eleven cohort studies were included in the review with 116,764 participants (although counting the participants in the table gives a count of 106,364 participants.)

In the 4 high-quality studies, the two largest were performed in populations at low risk of coronary heart disease. They found no protective effect of fish consumption. The other two high-quality studies were relatively small and included individuals at higher risk. They both found an inverse relationship between fish consumption and coronary heart disease death, suggesting that 40-60 g fish per day is optimal and associated with a risk reduction of 40- 60 percent.

In the 4 intermediate-quality studies, the authors found that fish consumption is inversely associated with coronary heart disease mortality in high-risk populations only. The 3 low-quality studies were of insufficient quality to be used for drawing conclusions.

The authors state that fish consumption is not associated with reduced coronary heart disease mortality in low-risk populations. However, fish consumption at 40-60 grams per day is associated with markedly reduced coronary heart disease mortality in high-risk populations. The underlying biochemical mechanism is not known and therefore causal inference is premature.

The authors have clearly stated their research question and some inclusion and exclusion criteria. The literature search appears thorough, however the authors do not mention any language restrictions or the inclusion of unpublished data in the review. Although the quality of the included studies was assessed, the authors have not reported on how the articles were selected or how many of the reviewers were involved in the data extraction.

The data extraction is summarised by treatment groups in tables. Study details are reported in tables and discussed in the text. The statistical pooling summarised the data as point estimates and there were tests for heterogeneity. The authors' conclusions appear to follow from the results.

Practice: The authors state that it is justified to recommend increased fish consumption up to an optimum of 40-60 g/day for fatal CHD prevention in high-risk populations. There is not sufficient evidence to assume that corresponding amounts of n-3 VLCPUFA taken as a dietary supplement can be similarly protective.

Research: The authors state that additional prospecstive cohort studies, and in particular intervention studies, are needed to further clarify the role of fish consumption in the prevention of CHD death. Future studies should focus on high-risk individuals and preferably include a test of the underlying biochemicl hypotheses.

This is a critical abstract of a systematic review that meets the criteria for inclusion on DARE. Each critical abstract contains a brief summary of the review methods, results and conclusions followed by a detailed critical assessment on the reliability of the review and the conclusions drawn.