Objective: Diabetes has been associated with chronic neurodegeneration. We performed a systematic review and meta-analysis to assess the relationship between pre-existing diabetes and Parkinson's disease (PD).

Research design and methods: Original articles in English published up to 10 May 2011 were searched for in electronic databases (PubMed, Embase, and Scopus) and by reviewing references of eligible articles. Prospective cohort and case-control studies providing risk and precision estimates relating to pre-existing diabetes and PD were considered eligible.

Results: Nine studies/1,947 citations (cohort, N = 4; case-control, N = 5) fulfilled inclusion criteria for meta-analysis. In prospective studies, the onset of diabetes before onset of PD was found to be a risk factor for future PD (relative risk [RR] = 1.37 [95%CI 1.21-1.55]; P < 0.0001). This association was confirmed by secondary analyses based on estimates derived after the exclusion of participants who had vascular disease at baseline and/or who developed vascular disease during follow-up (RR = 1.34 [1.14-1.58]; P < 0.001) and by sensitivity analyses addressing the association with diabetes at baseline or during follow-up. However, the association found for case-control studies was not significant (odds ratio [OR] 0.75 [95%CI 0.50-1.11]; P = 0.835). Sensitivity analysis based on estimates adjusted for BMI confirmed the lack of a relationship between PD and diabetes (OR 0.56 [0.28-1.15]; P = 0.089).

Conclusions: Although data from cohort studies suggest that diabetes is a risk factor for PD, there is no conclusive evidence on this association. Further prospective studies focused on putative pathogenic pathways and taking a broad range of confounders into account is required to clarify this relationship.