Data published by our group and others demonstrate that PD patients have an altered intestinal microbiome (i.e., microbial dysbiosis) associated with reduced SCFA levels, and this alteration could contribute to the observed blunted GLP-1 response (Keshavarzian et al., 2015; Unger et al., 2016; Sun and Shen, 2018). Here, GCG is linked to Parkinson disease.