Does diabetes decrease the risk of glioma? A systematic review and meta-analysis of observational studies

Purpose: Increasing epidemiologic evidence suggests that diabetes mellitus (DM) may be associated with a decreased risk of glioma. This systematic review assessed whether DM was associated with glioma risk. Methods: Electronic searches were performed in PubMed, Web of Science, EMBASE, and Cochrane Library databases up to August 30, 2018. A random-effects model was performed to calculate summary effect size with corresponding 95% confidence intervals (CIs). Results: In total, 10 studies (eight case-control studies and two cohort studies) matched the inclusion criteria. Meta-analyses of case-control studies showed that DM decreased the risk of glioma by 23% (odds ratio: 0.77, 95% CI: 0.61-0.96; P = .02, I-2 = 82.0%). However, no such effect was observed in cohort studies (relative risk: 0.71, 95% CI: 0.10-4.80; P = .72, I-2 = 61.6%). In the subgroup analyses, DM was associated with a decreased risk of glioma in Caucasians but not in Asians; the inverse association was slightly higher in males than in females. Conclusions: Our results indicate that DM decreases the risk of glioma, but the inverse association may vary in subgroups. The present conclusions should be confirmed with further studies. (C) 2018 Elsevier Inc. All rights reserved.