The aim of this study is to determine the effect of probiotic capsule supplementation on severity depression among patients with major depressive disorder under treatment with citalopram. Study design: Randomized double-blinded controlled clinical trial. Inclusion Criteria: Patients with major depressive according to DSM-IV-TR criteria, aged 20 to50 years, the least score 12 on BDI –II scoring system. Exclusion Criteria: Pregnancy, alcohol and drug abuse within 6 months prior to the start of the project, psychosis, mental retardation, depression caused by physical illness or drugs and substances, any type of severe, chronic physical illness, chronic physical illness and any extreme and severe mental disorders (suicidal ideation, psychotic symptoms, symptoms of manic phase, etc) will be excluded. Population and sample size: 70 patients with major depressive disorder of eligible and referred to Karganejad hospital to Kashan University of Medical Sciences, Kashan, Iran in the study will be selected. Intervention: Patients will be assigned to receive either the probiotic capsule (intervention group: n=35) or placebo capsule (control group: n=35). Fasting blood samples will be taken at baseline and after 8-wk intervention. Intervention: 8 weeks (July 12, 2014-september 12, 2014). Outcomes: Fasting plasma glucose, serum insulin, hs-CRP, insulin resistance, lipid profiles (total cholesterol, HDL-cholesterol and triglycerides) will be measured at the beginning and end of the intervention.