BDNF and depressive symptom measurement: Probiotics can promote the production of brain-derived neurotrophic factor (BDNF), which can regulate brain function, reduce anxiety and depression, ameliorate cognitive dysfunction, maintain and promote the development, differentiation, growth, and regeneration of 5-hydroxytryptamine (5-HT) and dopaminergic neurons (Ruiz-Gonzalez et al., 2021).