Moreover, VD might modulate different physiological processes in the brain, such as the regulation of brain-derived neurotrophic factor (BDNF) and other neurotrophic factors, neurogenesis, neuroplasticity, neuroprotection, and neuroimmunomodulation [19,20], thereby involving the pathophysiological mechanisms of depressive disorders [21,22,23,24,25]. Here, BDNF is linked to depressive disorder.