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To the Editor: The finding of a lack of seasonal affective disorders in Iceland, from a study by Magnusson et al. (1), is striking, especially when compared with findings from other countries of similar latitude. One reason for this finding may be the high content of fish in the Icelandic diet (225 lb per person per year) (2). The authors noted a similar and unexpected previous finding of a low prevalence of seasonal affective disorders in Japan, which also has a high per capita intake of fish (147 lb per person per year) (2). Despite a greater exposure to light in winter, most other countries have higher rates of seasonal affective disorder. Per capita fish intake in pounds per person per year is as follows: Canada, 51; Finland, 72; Netherlands, 25; Sweden, 59; Switzerland, 30; United Kingdom, 41; and the United States, 48 (2). We suggest that the difference in the prevalence of seasonal affective disorders between Icelandic descendants and other citizens in Winnipeg may be due to a cultural tradition of fish consumption, rather than differences in genetic predisposition. Our proposition is consistent with the finding in a cross-national analysis that greater seafood consumption predicted lower prevalence rates of major depression (r=–0.84, p<0.005) (3).
Seafood is rich in the omega-3 essential fatty acids eicosapentaenoic and docosahexaenoic acids. Docosahexaenoic acid is selectively concentrated in synaptic membranes, where it has a crucial role in maintaining the biophysical properties determining receptor conformation (4). Mechanisms through which eicosapentaenoic and docosahexaenoic acids may diminish depressive symptoms have recently been reviewed (5) and include modulation of serotonin turnover, phosphoinositol-mediated signal transduction, and L-type calcium channel regulation. Depletions of docosahexaenoic acid in RBC phospholipid membranes have been reported in depression (6). A placebo-controlled study in bipolar disorder (7) showed that supplementation with docosahexaenoic and eicosapentaenoic acids had marked mood-stabilizing and antidepressant activity. Thus, high levels of fish consumption should be considered a potential etiology for the finding of a lack of seasonal affective disorder among the Icelandic population.
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