INS and diabetes mellitus: Insulin is degraded into the brain by the insulin-degrading enzyme (IDE), originally known as the main enzyme involved in the cleavage of insulin as well as other amyloidogenic peptides, such as the β-amyloid (Aβ) peptide, and it eliminates Aβ’s neurotoxic effects—one of the hallmarks of Alzheimer’s disease (AD)—which shows the relationship between IDE, diabetes, and AD [171,172].