BDNF and major depressive disorder: The gut–brain axis is generally considered to be involved in psychiatric diseases, and neurobiological aetiological factors underlying depression have been proposed, including deficits in other neurotransmitters and neurotrophic factors, such as brain-derived neurotrophic factor (BDNF), changes in hippocampal neurogenesis, HPA axis dysregulation, and circadian rhythm disruption [67, 68].