Depression is an established risk factor for the development of coronary heart disease (CHD) in healthy patients and for adverse cardiovascular outcomes in patients with existing CHD. Dietary factors resulting in lower levels of omega–3 fatty acids not only increase CHD risk, but may also be involved in the pathophysiology of depression.
The investigators measured red blood cell levels of two omega–3 fatty acids, docosahexaenoic acid (DHA) and eicosapentaenoic acid (EPA), and assessed depressive symptoms in a cross-sectional study of 987 adults with CHD. Omega –3 fatty acids were blindly measured in fasting venous blood samples using capillary gas chromatography to measure the fatty acid composition of red blood cell membranes. Red blood cell levels of EPA and DHA are presented as a percentage composition of total fatty acid methyl esters.
The investigators assessed current depression using the 9-item Patient Health Questionnaire. They evaluated the association between omega –3 fatty acid levels and depressive symptoms as continuous variables using linear regression.
The investigators also examined the association of omega–3 fatty acid tertiles with depression as a dichotomous variable using X2 analysis and logistic regression. All statistical tests were two-sided, and p<0.05 was considered statistically significant. The prevalence of depression ranged from 23% in participants in the lowest tertile of omega –3 fatty acids (< 3.1% of total blood fatty acids) to 13% in participants in the highest tertile ( >4.3% of total blood fatty acids; p for trend = 0.004). Each unit decrease in EPA + DHA was inversely associated with depressive symptoms as a continuous variable, and these associations persisted after adjustment for age, sex and race. Similarly, each SD decrease in EPA + DHA was associated with significantly greater odds of depression as a dichotomous variable (Patient Health Questionnaire score >10).
However, in both analyses, omega–3 fatty acid levels were no longer associated with depression after adjustment for education and household income level. This study extends this existing literature by finding a strong association between low omega–3 fatty acids and depression in outpatients with stable CHD, a population distinct from sicker, hospitalized patients with acute coronary syndrome. In addition, the investigators examined the role of several important potential confounders and measured erythrocyte membrane levels of fatty acids rather than using less accurate serum measurements or dietary questionnaires. However, the cross-sectional nature of this study precluded the investigators from making any definitive comments on causality.
Additionally, the cohort participants were mostly older, urban men and thus are not entirely reflective of the general population. To better understand the potential efficacy of omega –3 fatty acid supplementation for improving depressive symptoms in patients with CHD, future studies should carefully consider the role of socioeconomic status (SES) in this association.
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