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

Abstract

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).

Keywords: Mediterranean diet; dose-response analysis; incidence; meta-analysis; review; type 2 diabetes.

FIGURE 1

Flowchart of study selection process.

FIGURE 2

Forest plot displaying the RRs and 95% CIs of type 2 diabetes for the highest compared with lowest adherence to the Mediterranean diet based on prospective cohort studies. The black squares represent the RRs, the size of which shows the study's weight in the analysis (weights come from random-effects analysis), and the horizontal lines represent the 95% CIs for each study. The diamond's center is the RR's summary estimate, and its width represents the summary estimate's 95% CIs. ALSWH, Australian Longitudinal Study on Women's Health; ARIC, Atherosclerosis Risk in Communities; CARDIA, Coronary Artery Risk Development in Young Adults; CDCdC, CDC de Canarias; EPIC, European Investigation into Cancer and Nutrition; ES, effect size; HPFS, Health Professionals Follow-Up Study; MCCS, Melbourne Collaborative Cohort Study; MESA, Multi-Ethnic Study of Atherosclerosis; MEC, Multi-Ethnic Cohort; SCHS, Singapore Chinese Health Study; SUN, Seguimiento Universidad de Navarra; TLGS, Tehran Lipid and Glucose Study; UKB, UK Biobank; WHI, Women's Health Initiative; WHS, Women’s Health Study.

FIGURE 3

Linear dose–response association between adherence to the Mediterranean diet and risk of diabetes. Adjusted RRs and 95% CIs (dashed lines) are reported. The horizontal axis represents the score of adherence to the Mediterranean diet and the vertical axis represents the risk ratio for diabetes. A nonlinear relation between Mediterranean diet score and risk of type 2 diabetes was observed, with a steeper inverse relation with greater scores.