BDNF and major depressive disorder: With accumulated studies on depression, other biochemical and physiological factors have also been implicated in the pathogenesis of depression, including brain-derived neurotrophic factor (BDNF)-related neurotrophic atrophy [4], inflammation [5], hypothalamic–pituitary–adrenal (HPA) axis dysfunction [6], etc. Among them, the role of nutrients in depression has attracted more and more attention [7].