| Summary: | While much evidence suggests that type 2 diabetes mellitus increases the risk of Parkinson’s disease (PD), the relationship between type 1 diabetes mellitus (T1DM) and PD is unclear. To study their association, we performed a two-sample Mendelian randomization (MR) using the following statistical methods: inverse variance weighting (IVW), MR-Egger, weight median, and weighted mode. Independent datasets with no sample overlap were retrieved from the IEU GWAS platform. All the MR methods found a lower risk of PD in T1DM (IVW—OR 0.93, 95% CI 0.91–0.96, <i>p</i> = 3.12 × 10<sup>−5</sup>; MR-Egger—OR 0.93, 95% CI 0.88–0.98, <i>p</i> = 1.45 × 10<sup>−2</sup>; weighted median—OR 0.93, 95% CI 0.89–0.98, <i>p</i> = 2.76 × 10<sup>−3</sup>; and weighted mode—OR 0.94, 95% CI 0.9–0.98, <i>p</i> = 1.58 × 10<sup>−2</sup>). The findings were then replicated with another independent GWAS dataset on T1DM (IVW—OR 0.97, 95% CI 0.95–0.99, <i>p</i> = 3.10 × 10<sup>−3</sup>; MR-Egger—OR 0.96, 95% CI 0.93–0.99, <i>p</i> = 1.08 × 10<sup>−2</sup>; weighted median—OR 0.97, 95% CI 0.94–0.99, <i>p</i> = 1.88 × 10<sup>−2</sup>; weighted mode—OR 0.97, 95% CI 0.94–0.99, <i>p</i> = 1.43 × 10<sup>−2</sup>). Thus, our study provides evidence that T1DM may have a protective effect on PD risk, though further studies are needed to clarify the underlying mechanisms.
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