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 18-97 years and the study sample size was in the range 526-27 111. 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.