Associations between diet quality and anxiety and depressive disorders: A systematic review

•First systematic review on diet quality in clinical anxiety and depression cohorts.•Mediterranean diet indices were strongly associated with reduced depression.•Findings for anxiety and other diet quality measures were sparse or inconsistent.•Dietary interventions show promise, but methodological limitations are highlighted.•Prospective studies support the Mediterranean diet as an adjunct therapy. Emerging evidence suggests that a healthier diet is associated with a reduced risk for depressive symptoms. However, the relationships between diet quality and clinical depression and anxiety have not been established. This systematic review is the first to examine whole-of-diet associations in cohorts with diagnosed depression and/or anxiety disorders. Literature searches captured 44 studies reporting on diet quality (22 cross-sectional, 12 prospective, seven combining cross-sectional/longitudinal data) OR using a dietary intervention in adults with depression and/or anxiety diagnoses (3 RCTs). A best-evidence synthesis of 25 observational studies of higher methodological quality was conducted. There was strong evidence that adherence to a Mediterranean diet was associated with lower depression incidence in prospective studies. Level of adherence to national dietary guidelines was not consistently associated with anxiety and depression. Studies which examined other dietary quality measures reported conflicting or limited evidence in relation to both anxiety and depression, with considerably fewer studies reported in anxiety. Some dietary interventions showed promising findings in relation to improvements in depression/anxiety outcomes, however, a range of methodological limitations warrant further consideration. Heterogeneity across studies was high which limited the ability to compare findings and precluded meta-analysis. High-quality prospective studies generally support evidence that increased adherence to a Mediterranean diet may be a cost-effective and safe adjunct to existing therapies for clinical depression, which is supported by the findings of several dietary interventions.