SNCA and Parkinson disease: The gut–brain axis might be a potential mechanism related to PD that affects infection, neuroinflammation, and the spread of alpha‐synuclein.47 Gut microbiome profiles are changed in patients with PD compared with HCs.48, 49, 50 Likewise, the profiles of patients with PSP and MSA are similar to those of patients with PD.50, 51 Although the small intestinal microbiome fluctuated more easily in response to the latest diet trends compared with the gut microbiome,52 the small intestinal microbiome may have a common pathogenic tendency that affects chemical absorption in the 3 diseases.53