BDNF and major depressive disorder: In human clinical studies, administration of prebiotics like FOS and GOS showed a reduced awakening cortisone reaction, a biomarker of anti-anxiety and anti-depression, rats on prebiotics also showed high levels of brain derived neurotrophic factor (BDNF), serotonin receptor 5-HT and low levels of cortisone and L-Tryptophan which suggests that prebiotics can relieve mood disorders (Liu and Zhu, 2018).