Association between Adherence to the Mediterranean Diet and Risk of Type 2 Diabetes: An Updated Systematic Review and Dose–Response Meta-Analysis of Prospective Cohort Studies

Despite earlier meta-analyses on the association between adherence to a Mediterranean diet (MD) and risk of diabetes, there is no comprehensive and updated study assessing this issue. Furthermore, no earlier study has examined the nonlinear dose–response relation between consumption of an MD and risk of diabetes. The current systematic review and meta-analysis was conducted to investigate the linear and nonlinear dose–response relation between MD and incidence of diabetes. Using relevant keywords, electronic searches for prospective studies were conducted in ISI Web of Science, PubMed, and Scopus until January 2022. The reported HRs or ORs in the primary studies were regarded as RRs. The overall effect was calculated using a random-effects model that accounts for between-study variability. The potential nonlinear dose–response associations were tested using a 2-stage hierarchical regression model. Based on 16 prospective studies (with 17 effect sizes), we found that the greatest adherence to the MD was significantly associated with a reduced risk of diabetes (pooled RR: 0.83; 95% CI: 0.77, 0.90; I2 = 79%, P ≤ 0.001). Based on linear dose–response analysis, each 1-score increase in the Mediterranean diet score was associated with a 3% decreased risk of diabetes (HR = 0.97; 95% CI: 0.96, 0.98; P < 0.001). A nonlinear relation (P-nonlinearity = 0.001) was also observed between MD score and risk of type 2 diabetes. Even modest adherence to the MD was linked to a decreased incidence of type 2 diabetes. The protocol is also registered in the International Prospective Register Of Systematic Reviews (PROSPERO) database (https://www.crd.york.ac.uk/PROSPERO/; registration ID: CRD 42021265332). Statement of Significance: Based on the literature, although previous meta-analyses have reviewed the association between Mediterranean diet and risk of type 2 diabetes, several restrictions may distort these results. Notably, this is the first study to assess whether there is a nonlinear dose–response relation between adherences to Mediterranean diet and the risk of type 2 diabetes.