Diet and the risk of unipolar depression in adults: systematic review of cohort studies

Background Nutrition may be a risk factor for unipolar depression. We aimed to review the association between dietary variables and the risk of depression. Methods Fifteen databases were searched up to May 2010. Only longitudinal studies for which outcomes were unipolar depression and/or depressive symptoms in adults were eligible for inclusion. Eleven studies were included and critically evaluated. Participants were in the age range 1897 years and the study sample size was in the range 52627 111. Follow-up ranged from 2 to 13 years. The diversity of dietary variables and nonlinear associations precluded formal meta-analysis and so a narrative analysis was undertaken. Results Variables inversely associated with depression risk were the consumption of nutrients such as folate, omega-3 fatty acids and monounsaturated fatty acids; foods such as olive oil and fish; and a diet rich in fruits, vegetables, nuts and legumes. Some of these associations varied by sex and some showed a nonlinear association. Conclusions At the study level, weaknesses in the assessment of exposure and outcome may have introduced bias. Most studies investigated a cohort subgroup that may have resulted in selection bias. At the review level, there is a risk of publication bias and, in addition, narrative analyses are more prone to subjectivities than meta-analyses. Diet may potentially influence the risk of depression, although the evidence is not yet conclusive. Strengthening healthy-eating patterns at the public health level may have a potential benefit. Robust prospective cohort studies specially designed to study the association between diet and depression risk are needed.