Fish consumption and major depression.

Sir—WHO estimates that major depression is the greatest single cause of disability worldwide. The annual prevalence of major depression shows nearly a 60-fold variation across countries, in a pattern similar to crossnational differences in mortality from coronary artery disease, which suggest that similar dietary risk factors could be important. Among healthy volunteers, low plasma concentrations of an essential fatty acid found in fish, docosahexaenoic acid, predict low concentrations of a marker of brain serotonin turnover, cerebrospinal fluid 5-hydroxyindolacetic acid (CSF 5HIAA). The finding that low concentrations of CSF 5-HIAA are strongly associated with depression and suicide have been widely replicated. Since docosahexaenoic acid is selectively concentrated in neural tissues and important for nervoussystem function, we tested the hypothesis that a high consumption of fish could be correlated with a lower annual prevalence of major depression (figure). The cross-national comparisons of the prevalence of major depression reported by Weissman and colleagues are among the most reliable crossnational data available. The rigorous methodologies used in these studies— large sample size (35 000), random prospective design, repeat sampling techniques, multiple community sampling, and use of a structured clinical interview with uniform internationally accepted diagnostic criteria—create confidence in the validity and comparability of these data. The structured interviews were independently verified as culturally appropriate for each community. The core biological symptoms that define major depression were the main factors used to determine the differences in prevalence of major depression across countries, rather than mood ratings which are prone to cultural bias. The economic data on apparent fish consumption was calculated by fish catch plus imports minus exports and are not as reliable as data from direct dietary surveys or tissue analyses, but do provide a comparable estimate across countries. The data on the annual prevalence of major depression reported by the Ministry of Welfare in Japan included 130 000 individuals, but did not use structured instruments for diagnosis or randomised population-sampling methods. However, exclusion of these Japanese data did not significantly affect the correlation analysis (r=0·77, p<0·03). The direction and power of the correlation between apparent fish consumption and major depression accords with recent clinical reports of individuals that higher concentrations of docosahexaenoic acid in red-bloodcell membranes (r= 0·80, p<0·01), as well as higher ratios of eicosapentaenoic acid to arachidonic acid in plasma (r= 0·73, p<0·01), predict less severe symptoms of depression. This correlation between apparent fish consumption and lower annual prevalence of major depression does not show that fish consumption can cause differences in the prevalence of major depression or that eating fish or fish oils are useful in treatment. Various cultural, economic, social, and other factors can confound this simple correlational relation.