GCG and Parkinson disease: Since propionate has been reported to exert its effects by stimulating the release of Glp-1 from enteroendocrine L cells [32] or activating the enteric nervous system (ENS) to transmit its signal to the brain [33], we first measured the serum level of Glp-1 on the basis of the neuroprotective effects of Glp-1 on PD [34–36].