BDNF and Alzheimer disease: Serum BDNF levels were significantly lower in AD (excluding MCI) group than in controls with significant heterogenicity caused by age and MMSE scores as significant moderators during meta-regression analysis. Change in peripheral BDNF is only detected at the late stage of the AD continuum. Further investigation is needed, including molecular mechanisms, interventional trials, and BDNF potential use as an AD biomarker.