Effects on Health Outcomes of a Mediterranean Diet With No Restriction on Fat Intake: A Systematic Review and Meta-analysis.

BACKGROUND: Mediterranean diets may be healthier than typical Western diets.

PURPOSE: To summarize the literature comparing a Mediterranean diet with unrestricted fat intake with other diets regarding their effects on health outcomes in adults.

DATA SOURCES: Ovid MEDLINE, CINAHL, and the Cochrane Library from 1990 through April 2016.

STUDY SELECTION: Controlled trials of 100 or more persons followed for at least 1 year for mortality, cardiovascular, hypertension, diabetes, and adherence outcomes, as well as cohort studies for cancer outcomes.

DATA EXTRACTION: Data extracted by 1 investigator was verified by another. Two reviewers assessed risk of bias and strength of evidence.

DATA SYNTHESIS: Two primary prevention trials found no difference in all-cause mortality between diet groups. One large primary prevention trial found that a Mediterranean diet resulted in a lower incidence of major cardiovascular events (hazard ratio [HR], 0.71 [95% CI, 0.56 to 0.90]), breast cancer (HR, 0.43 [CI, 0.21 to 0.88]), and diabetes (HR, 0.70 [CI, 0.54 to 0.92]). Pooled analyses of primary prevention cohort studies showed that compared with the lowest quantile, the highest quantile of adherence to a Mediterranean diet was associated with a reduction in total cancer mortality (risk ratio [RR], 0.86 [CI, 0.82 to 0.91]; 13 studies) and in the incidence of total (RR, 0.96 [CI, 0.95 to 0.97]; 3 studies) and colorectal (RR, 0.91 [CI, 0.84 to 0.98; 9 studies]) cancer. Of 3 secondary prevention studies reporting cardiovascular outcomes, 1 found a lower risk for recurrent myocardial infarction and cardiovascular death with the Mediterranean diet. There was inconsistent, minimal, or no evidence pertaining to any other outcome, including adherence, hypertension, cognitive function, kidney disease, rheumatoid arthritis, and quality of life.

LIMITATIONS: Few trials; medium risk-of-bias ratings for many studies; low or insufficient strength of evidence for outcomes; heterogeneous diet definitions and components.

CONCLUSION: Limited evidence suggests that a Mediterranean diet with no restriction on fat intake may reduce the incidence of cardiovascular events, breast cancer, and type 2 diabetes mellitus but may not affect all-cause mortality.

PRIMARY FUNDING SOURCE: Department of Veterans Affairs, Veterans Health Administration, Office of Research and Development, Quality Enhancement Research Initiative. (PROSPERO: CRD42015020262).