BDNF and major depressive disorder: The improving effects of diets on depression may be concluded as the following three hypotheses: (1) Diets with anti-inflammatory properties may relieve the inflammatory cytokine secretion, attenuating the inflammation in the brain, thus contributing to depression relieve; (2) Diets with antioxidative properties may decrease the oxidative stress in specific brain zone, thus alleviating depression symptom; and (3) Diets with enhancing ability of BDNF may directly improve patient conditions through up-regulating of BDNF.