We read with interest the study by Villumsen et al1 which reported that patients with IBD had a 22% increased risk of Parkinson’s disease (PD) compared with non-IBD individuals. The study supported the theory that intestinal environment could influence the function of central nervous system which was also called the gut–brain axis. Recently, a meta-analysis by Zhu et al2 suggested that the overall risk of PD in patients with IBD was significantly higher than controls (OR 1.41; 95% CI 1.19 to 1.66). However, only four studies were included in the meta-analysis. We also noticed a case–control study by Camacho-Soto et al3 reporting that the risk of PD was inversely associated with IBD (OR 0.85; 95% CI 0.80 to 0.91), which was not included in the meta-analysis by Zhu et al. 2 To better understand this issue, we searched the PubMed and Web of Science for cohort and case–control studies investigating associations between IBD and risk of PD published before October 2018. Finally, five studies1 3–6 with totally 9 174 766 participants were included, and we performed a meta-analysis with the software STATA V. 12.0. The overall results showed that compared with reference individuals, patients with IBD did not have a significantly increased risk of PD (OR 1.19; 95% CI 0.93 to 1.44; pheterogeneity< 0.001), while the results with four cohort studies suggested that patients with IBD had significantly higher risk of PD (OR 1.26; 95% CI 1.17 to 1.35; pheterogeneity= 0.84)(figure 1). Subgroup analysis for different classifications of IBD with all studies suggested that patients with UC or Crohn’s disease (CD) did not have higher risk of PD, while the results with cohort studies showed that patients with UC or CD were associated on August 11, 2023 at Google Indexer. Protected by copyright. http://gut. bmj. com/