Consequently, environmental influences such as certain behaviors and diet that reduce brain dopaminergic activities regulating metabolism can feed forward to potentiate oxidative stress, inflammation, insulin resisitance, and hyperglycemia of type 2 diabetes that in turn feeds back to potentially damage central dopaminergic neurons including those of the basal ganglia-substantia nigra to contribute to PD progression in susceptible patients. Here, INS is linked to type 2 diabetes mellitus.