A leaky gut barrier has been linked to PD pathogenesis involving local flux of bacterial toxins, microbes, and potentially molecules like α-synuclein across the gastrointestinal epithelium [29–31]; while systemically gut inflammation drives pro-inflammatory cytokine production, blood–brain barrier disruption, and migration of inflammatory cells into the brain causing neuronal dysfunction and cell death [32, 33]. This evidence concerns the gene SNCA and Parkinson disease.