BDNF and Alzheimer disease: BDNF has a potential role as a biomarker during the AD course, shown by an upregulation of BDNF serum levels that significantly increased in MCI and AD compared to healthy groups. However, the BDNF levels among aMCI, MCI-MD, mild AD, and moderate-severe AD are insignificantly different. Upregulation of BDNF is an early compensatory against neurodegeneration.