BDNF and major depressive disorder: Alterations in the composition of the gut microbiota also change the permeability of the gut barrier (Martel et al., 2022), in which bacteria and bacterial products can be transferred from the gut to the bloodstream, lymph nodes and other organs, to activate a systemic inflammatory response (Madison and Bailey, 2024; Caso et al., 2021; Miller and Raison, 2016), and hypothalamo-pituitary-adrenal axis activity (Osadchiy et al., 2019), reduce the levels of brain-derived neurotrophic factor (BDNF) and finally inducing depression (Du et al., 2020).