These included brain-derived neurotrophic factor (BDNF) as a marker able to stimulate the survival and differentiation of certain neurons and synapses belonging to the central nervous system and peripheral nervous system [70], global cognitive function, cognitive decline, depression, dementia, Alzheimer’s disease, Parkinson’s disease, and a number of neurological domains (verbal and visual memory, processing speed, working memory, language, and executive function. Here, BDNF is linked to major depressive disorder.