LEP and diabetes mellitus: Because extensive data from epidemiological studies have consistently confirmed that obesity, diabetes, and metabolic syndrome increase the risk of developing cognitive impairment and dementia (Baker et al., 2011; McCrimmon et al., 2012; Lehtisalo et al., 2016; Espeland et al., 2017; Pal et al., 2018), it is not surprising that more than 10 years ago, leptin was proposed to be the biochemical “link” connecting all of these pathologies (Harvey, 2010).