A high level of exposure to welding, alpha-synuclein in cerebrospinal fluid (CSF), nigral volume, serum urate, retinal nerve fiber layer thickness, ibuprofen use, and calcium channel blockers, was associated with a lower risk of developing PD than did rural living, dairy product intake, constipation, head injury, hydrocarbon exposure, well water drinking, energy intake, carbohydrate intake, beta-blockers, and having anxiety or depression, all of which were associated with a higher risk of developing the disease (Additional file 2: Table S1). Here, SNCA is linked to major depressive disorder.