In this regard, low plasma leptin levels in late-life have been consistently associated with an increased risk for cognitive decline and the development of AD (Holden et al., 2009; Zeki Al Hazzouri et al., 2013; McGregor et al., 2014; Littlejohns et al., 2015; Yin et al., 2018; Lloret et al., 2019). Here, LEP is linked to Alzheimer disease.