INS and Alzheimer disease: Decreased levels of insulin and IR are found in the cerebrospinal fluid (CSF) of AD patients due to long-term peripheral hyperinsulinemia and decreased insulin transport across the blood-brain barrier (BBB) (Craft et al., 1998; Rivera et al., 2005; Steen et al., 2005; Gil-Bea et al., 2010; Stanley et al., 2016).