Patient receiving a probiotic supplement showed reduction in Beck Depression Inventory scores (p = 0.001). There were also lower serum insulin levels (p = 0.03), serum nightly CRP levels (p = 0.03), and homeostasis model assessment of insulin resistance (p = 0.03) in the probiotic group. There were no significant changes to fasting plasma glucose or lipid profiles. Here, INS is linked to major depressive disorder.