BDNF and Alzheimer disease: Xie et al. (2020) meta-analysis showed that peripheral blood BDNF decreased gradually with the aggravation of cognitive impairment. Ng et al. (2019) meta-analysis showed that peripheral BDNF levels decreased in patients with AD and could only be detected at the late stage of the dementia spectrum. Qin et al. (2017) suggested that AD or MCI is accompanied by reduced peripheral blood BDNF levels, supporting an association between the decreasing levels of BDNF and the progression of AD.