This review assessed the effects of the Mediterranean diet on disease prevention and reported that there is limited evidence from which to draw conclusions. Although there are some concerns about the review methodology, the authors' cautious conclusions and recommendations for further research seem appropriate.

To assess the effects of the Mediterranean diet on disease prevention.

MEDLINE was searched from October 2004 to January 2005; the search terms were reported. The bibliographies of included studies were also checked. Only English language studies were eligible for inclusion in the review.

Study designs of evaluations included in the review

Clinical trials were eligible for inclusion in the review. Parallel and crossover randomised controlled trials (RCTs) and non-randomised clinical trials were included.

Specific interventions included in the review

Studies assessing the prolonged use of a whole Mediterranean diet, compared with usual diet or alternative types of diet (e.g. Swedish, Western, low fat, monounsaturated, high carbohydrate), were eligible for inclusion. Studies assessing individual components of the diet or an isolated intake of the diet were excluded from the review.

Participants included in the review

The authors did not specify which types of participants were eligible for inclusion. The participants included in the review were healthy, or had hypercholesterolaemia, cardiovascular risk factors/disease, rheumatoid arthritis or cancer, or had survived a myocardial infarction.

Outcomes assessed in the review

Studies reporting indicators for the primary or secondary prevention of disease were eligible for inclusion. The included studies assessed a variety of outcomes including: lipoprotein levels, glycaemic control, endothelial resistance markers, inflammation markers, antioxidant capacity, cholesterol levels, insulin resistance, glucose levels, body weight, and arthritis functionality and pain.

How were decisions on the relevance of primary studies made?

The authors did not state how the papers were selected for the review, or how many reviewers performed the selection.

The authors did not state that they assessed validity.

The authors did not state how the data were extracted for the review, or how many reviewers performed the data extraction. Difference in blood chemistry tests, body mass index, dietary and quality-of-life scores, dietary habits, and risk of morbidity and mortality were extracted.

How were the studies combined?

The studies were combined in a narrative.

How were differences between studies investigated?

Some differences were discussed in the text. Study details were tabulated, organised into the following categories: diabetes/lipoproteins/endothelial resistance, cardiovascular disease, arthritis, cancer, body composition and psychological function.

Thirty-five studies were included in the review. The total number of participants was unclear (n at least 15,274), but study sample sizes ranged from 11 to 13,000; over half of the studies included less than 60 participants.

Diabetes/lipoproteins/endothelial resistance (25 studies): most studies reported reductions in total cholesterol, low-density lipoproteins, triglycerides, apoprotein B and very low-density lipoprotein cholesterol, an increase in high-density lipoprotein cholesterol and an improvement in endothelium function, with Mediterranean diets. Insulin resistance and metabolic syndrome were reduced with the Mediterranean diet in some studies.

Cardiovascular disease and secondary prevention (3 studies): all 3 studies reported odds ratios for myocardial infarction of between 0.25 and 0.70.

Arthritis (1 study): a Mediterranean diet was associated with benefits in terms of pain, the number of swollen joints and quality of life.

Cancer (1 study): a reduced risk of malignant/non-malignant cancer was reported for those individuals on a Mediterranean diet compared with controls.

Body composition (3 studies): Mediterranean diets were associated with weight loss and a reduction in percentage body fat compared with control groups.

Psychological function (2 studies): neither study showed any changes in mood.

The results of the review disclose the mechanisms of the Mediterranean diet in disease prevention, particularly in the secondary prevention of cardiovascular disease, but further research is required.

This review addressed a very wide research question in terms of the participants, interventions and outcome measures. Literature searches were limited to just one electronic database and reference lists; non-English language publications were excluded from the review and there were no specific attempts to locate unpublished material, which suggests that relevant data might have been missed. It was also difficult to assess the reliability of the review methods as few details were given as to how the studies were selected and the data extracted. The authors do not appear to have assessed the quality of the included studies, which makes it difficult to assess the validity of the results, and insufficient study details make it difficult for the reader to assess their quality. It would have been helpful to the reader if the reviewers had indicated which trial citations related to the same study, as this was not always clear. Given the degree of heterogeneity between the studies, the use of a narrative synthesis was appropriate. The authors' cautious findings, and the suggestion for further methodologically sound experimental studies, also seem appropriate.

Practice: The authors did not state any implication for practice.

Research: The authors stated that further experimental studies are required to assess the effects of the Mediterranean diet in the primary prevention of cardiovascular disease, hypertension, diabetes, obesity, infectious diseases, age-related cognitive impairment, cancer and other diseases. Future studies should use food scores or patterns to assess adherence to the intervention. Studies are also needed to corroborate the underlying mechanisms of the benefits of the diet.

This is a critical abstract of a systematic review that meets the criteria for inclusion on DARE. Each critical abstract contains a brief summary of the review methods, results and conclusions followed by a detailed critical assessment on the reliability of the review and the conclusions drawn.