The Effect of the Traditional Mediterranean-Style Diet on Metabolic Risk Factors: A Meta-Analysis

Importance: A Mediterranean-style diet has been shown to be effective in improving a variety of disease outcomes, including metabolic risk factors. Such dietary patterns are complex and, thus it is currently unclear as to which components and intervention characteristics are more greatly associated with reducing metabolic syndrome risk. Objective: To obtain overall effect sizes for the metabolic risk factors (waist circumference, HDL cholesterol, triglycerides, fasting blood glucose, systolic blood pressure, and diastolic blood pressure) and explain the variability across the current literature based on study design, sample, and diet characteristics. Data Sources: Six electronic databases (PubMed, EMBASE, Web of Science, CINAHL, CAB Direct, and Agricola) were searched from inception until August 4, 2014 using a comprehensive Boolean search strategy. Study Selection: Studies were included if pre- and post- intervention measurements of waist circumference were reported and the traditional Mediterranean-style diet was used as a dietary intervention. Data from 32 studies (N = 3,550) were included. Data Extraction and Synthesis: Independent researchers identified studies that met the inclusion criteria and coded methodological, participant, and intervention characteristics. Main Outcomes and Measures: Weighted mean effect size under random-effects assumptions were obtained and modeled after pooling the individual standardized mean differences for each study on the six metabolic risk factors. Results: There were significant beneficial effects in favor of the traditional Mediterranean-style diet for waist circumference, triglycerides, fasting blood glucose, systolic blood pressure, and diastolic blood pressure (d+=-0.58, 95% CI -0.81 to -0.35; d+=-0.33, 95% CI -0.69 to -0.19; d+=-0.51, 95% CI -0.80 to -0.22; d+=-0.74, 95%CI -1.03 to -0.46; d+=-0.92, 95% CI -1.41 to -0.43, respectively). The Mediterranean-style diet was significantly beneficial when, in general the intervention period was longer in duration, the study was conducted in Europe, the study used a behavioral technique, and the study was conducted primarily using small groups. Conclusions and Relevance: The traditional Mediterranean-style diet had a significant beneficial effect on five of the six metabolic risk factors. This dietary pattern appears to be most successful in reducing metabolic risk when it is recommended for longer periods of time and is implemented using social support