Introduction Major depressive disorder is the leading cause of disability around the world. Because of the high rate of medication discontinuation by patients and the risk of recurrence, factors such as nutrition could be useful for the prevention or treatment of depression. The relationship between depression and dietary patterns has been reported in a few studies but with controversial results. Therefore, we have decided to study the possible effects of cultural, social, racial, geographic and environmental conditions on this relationship in an Iranian population.

Methods and analysis In our case control protocol, 110 cases and 220 controls will be individually matched based on age, sex and area of residence. New cases of depression, based on the criteria of the Diagnostic and Statistical Manual of Mental Disorders, fourth edition (DSM-IV), will be recruited from two psychiatric clinics in Tehran. Interviewers will then go to each patient's home and invite qualified individuals to participate in the study as controls. Food intakes of all participants will be obtained by semiquantitative food frequency questionnaires covering the past year; these will be transformed into actual food intake (g/day). Dietary patterns will be determined by the principal components method. Conditional logistic regression, as a multivariate analysis, will be used for assessing the relationship between dietary patterns and depression, taking into consideration the potential role of different variables. The results may help to identify differences in dietary patterns between depressed and healthy people.

Ethics and dissemination The study protocol has been approved by ethics committee of Tehran University of Medical Sciences. At the beginning of the study, a written informed consent form will be signed and dated by subjects and investigators. The results will be published in due time.