Association between serum bilirubin concentration and Parkinson's disease: a meta-analysis

Background: The antioxidant effects of bilirubin in Parkinson's disease (PD) have recently gained much attention from the research community. However, results from these studies have been conflicting. This meta-analysis is conducted to assess the relationship between the serum bilirubin concentration and the risk of PD. Methods: Two reviewers performed a systematic literature search across five databases (MEDLINE, PubMed, EMBASE, Web of Science, and Cochrane Central Register of Controlled Trials). The case-control studies regarding bilirubin levels in PD patients published up to April 2020 were included. These studies were subjected to rigorous scrutiny and data extraction to determine the standard mean difference (SMD) and the 95% confidence interval (CI), which were analyzed using the Stata V.12.0 statistical software. Results: A total of eight studies which included 1463 PD cases and 1490 controls were incorporated into our meta-analysis. SMD analysis showed that there was a higher total bilirubin (TBIL) and direct bilirubin (DBIL) levels in PD patients compared with controls (for TBIL, SMD: 0.300, 95% CI: 0.050-0.549, P = 0.018; for DBIL, SMD: 0.395, 95% CI: 0.102-0.688, P = 0.008). However, no significant relationship was found between the serum indirect bilirubin and PD patients (SMD: -0.223, 95% CI: -0.952-0.505, P = 0.548). A subgroup analysis based on ethnicity indicated that the serum TBIL was higher in PD patients of Caucasian descent in contrast to matched healthy controls (SMD: 0.511, 95% CI: 0.324-0.698, P = 0.000, I-2 = 58.0%). Conclusion: Higher serum bilirubin levels in PD patients suggest that bilirubin might play a role in the pathogenesis of PD and have the potential to be utilized as a biochemical marker for PD diagnosis and treatment.