G6PD and Parkinson disease: The Relationship between Antioxidant markers and Risk of PD: A forest plot revealed that there were a significantly decreased level of CAT activity (1.286, 95% CI: 2.123-0.445, P = 0.003), SOD activity (0.981, 95% CI: 1.751-0.225, P = 0.010), and GPx activity (2.027, 95% CI: 0.719-3.334, P < 0.001); whereas there were no statistically significant `different levels of TAS (0.334, 95% CI: 1.171-0.503, P = 0.400), thiols (0.18, 95% CI: 1.475-1.113, P = 0.700), and G6PD (1.415, 95% CI: 2.774-0.056, P = 0.040) were found between PD and control groups.