A French research team analysed the relationship between blood levels of essential fatty acids and severity of depressive symptoms in 1390 non-institutionalised elderly people (average age 75 years). They found that blood levels of EPA (found in oily fish) were lower in the depressed elderly (especially those taking antidepressants) than in elderly people who were not depressed.
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Plasma eicosapentaenoic acid is inversely associated with severity of depressive symptomatology in the elderly: data from the Bordeaux sample of the Three-City Study, C Féart et al., American Journal of Clinical Nutrition, 2008, Vol. 87, No. 5, 1156-1162