ABCG2 and Parkinson disease: Compared with the PD-nRBD group, KEGG pathway analysis of the altered metabolites revealed significant enrichment (p < 0.05) of the following seven pathways in the PD-RBD group: PPAR signaling pathway; D-amino acid metabolism; neuroactive ligand-receptor interaction; protein digestion and absorption; linoleic acid metabolism; ABC transporters; and arginine and proline metabolism (Figure 5B).