Substantial research has linked lowered peripheral BDNF to decline in brain health and plasticity, in natural aging and Alzheimer’s disease (7, 8, 9), and also in stress-related mood disorders that involve neuronal atrophy (10, 11, 12, 13), such as major depression and bipolar disorder (14, 15, 16, 17, 18). Here, BDNF is linked to major depressive disorder.