BDNF and major depressive disorder: Various exercise-induced factors released into the bloodstream from muscle, liver, and bone, have been shown to cross the blood-brain barrier (BBB) enhancing brain-derived neurotrophic factor (BDNF) signaling, and alleviating the symptoms of depression (Sleiman et al., 2016; Karnib et al., 2019; Rentz et al., 2020).