GDF15 and Parkinson disease: For patients with PD and MSA, the serum levels of GDF15 were all higher than healthy people (SMD = 0.88, 95% CI: 0.29–1.47, Z = 2.92, p =0 .03), (SMD = 1.11, 95% CI: 0.68–1.53, Z = 5.13, p < 0.001) (Figure 2).