Several case–control and large population based prospective studies have shown an association of lower circulating leptin levels with AD or general cognitive decline (Lieb et al. 2009; Baranowska-Bik et al. 2015), but some clinical studies found no differences in AD and controls (Theodoropoulou et al. 2012; Warren et al. 2012; Teunissen et al. 2015). Here, LEP is linked to Alzheimer disease.