BDNF and major depressive disorder: These changes exacerbate neuroinflammation, the accumulation of neurotoxic metabolites, and imbalances in gut microbiota and BDNF, ultimately contributing to the development of depression (Correia and Vale, 2022; Katamanin et al., 2024; Myint and Halaris, 2022; Ryan et al., 2024) (Figure 3).