Association between diabetes mellitus and risk of Parkinson's disease: A prisma-compliant meta-analysis

Background Previous studies showed inconsistent results regarding associations between diabetes mellitus (DM) and risk of Parkinson's disease (PD). The study aimed to make a meta-analysis to clarify whether DM is a risk factor for PD. Methods We searched for articles regarding the effect of DM on risk of PD and published before July 2020 with search terms as follows: ("diabetes mellitus" OR "diabetes") AND ("Parkinson's disease" OR "PD") in the following databases: PubMed, Web of Science, MEDLINE, EMBASE, and Google Scholar. We used STATA 12.0 software to compute multivariate odds ratio (OR) or relative risk (RR) and 95% confidence intervals (CI) regarding the association between DM and risk of PD. Results The present study finally included 7 case-control studies (including 26,654 PD patients) and 9 cohort studies (including 3,819,006 DM patients) exploring the association between DM and risk of PD. The meta-analysis indicated that DM was related to elevated risk of PD (OR/RR = 1.15, 95% CI 1.03-1.28, I-2 = 92.4%, p < .001). Subgroup study showed that DM was associated with higher risk of PD in cohort studies (RR = 1.29, 95% CI 1.15-1.45, I-2 = 93.9%, p < .001), whereas no significant association was indicated between DM and risk of PD in case-control studies (OR = 0.74, 95% CI 0.51-1.09, I-2 = 82.3%, p < .001). Sensitivity analysis showed no changes in the direction of effect when any one study was excluded from all meta-analyses. In addition, Begg's test, Egger's test, and funnel plot showed no significant risks of publication bias. Conclusion In conclusion, we have tried to determine whether prior onset of DM may contribute to the risk of developing PD. More and more large-scale prospective studies should be conducted to evaluate the relationship between DM and PD.