Greenness and diabetes: a systematic review and meta-analysis of observational studies

Since diabetes represents a serious Public Health problem and access to green spaces seems to influence some of its risk behaviors, we performed a systematic review to evaluate the association between greenness exposure and diabetes mellitus type 2 (TDM2), prediabetes and gestational diabetes. Following the PRISMA guidelines, we conducted an online literature search in PubMed, Embase and Web of Science databases by using terms related to «green space», «diabetes mellitus», «gestational diabetes» and «prediabetes». The identified studies were imported into the Rayyan web app where we performed a triple-blind screening of title, abstract and full-text, including only original studies with observational design that assessed the relation between exposure to greenness and diabetes incidence/mortality reporting effect measures (relative risk, hazard ration and odds ratio) with 95% confidence intervals (CI). Lastly, we performed a metaanalysis using both a restricted maximum likehood random effects model, comparing the highest versus lowest greenness categories or continuous increase of greenness, and a onestage dose-response using a 3-knots restricted cubic spline model with fixed percentiles (10th, 50th and 90th). We identified 212 potentially eligible articles, 34 of which were eventually included in the quantitative analyses. In the preliminary forest-plot analyses almost all studies reported a decreased TDM2 risk, regardless of the heterogeneity in the satellite-based methodologies to assess greenness. Furthermore, the dose-response curve showed a progressive reduction of the RR of TDM2 incidence, especially for exposure above 0.5 NDVI (risk ratio=0.93; 95% CI: 0.15, 5.67). We observed a growing interest concerning the topic in recent years, and, considering the increasing disease burden, the identification of a protective role of greenness could help implement community preventive measures.