BDNF and Alzheimer disease: Several recent meta-analyses analyzed the association between BDNF and AD cognitive impairment and found that the results are inconsistent or even contradictory, and higher, lower, or similar levels of circulating BDNF have all been reported in AD or MCI patients compared to healthy controls (Qin et al., 2017; Ng et al., 2019; Xie et al., 2020; Ma et al., 2022).