The report is a product of the VA/HSR Evidence Synthesis Program.

Benefits and Harms of the Mediterranean Diet Compared to Other Diets

A large number of studies have investigated the association between diet and mortality and morbidity. Of particular interest is the Mediterranean diet, which is characterized by:

• High intake of olive oil, fruits and vegetables, whole grains and cereals, legumes, fish, and nuts;
• Low intake of red meat, dairy products, and sweets; and
• Moderate intake of red wine with meals.

Epidemiologic studies have shown that the incidence of cardiovascular disease in populations that consume such diets is lower than in populations that consume a more typical "Western" diet that is rich in red meat, dairy products, processed and artificially sweetened foods, and salt, with minimal intake of fruits, vegetables, fish, legumes, and whole grains. Based on these studies, several randomized controlled trials (RCTs) were conducted to test the hypothesis that adopting a Mediterranean diet in adulthood reduces chronic disease burden (e.g., cardiovascular disease, cancer, diabetes, hypertension, cognitive impairment, and kidney disease) and/or all-cause mortality. These trials included populations from a variety of geographical locations, and with a spectrum of demographic and clinical characteristics.

VA's Evidence-based Synthesis Program (ESP), in conjunction with the Office of Quality and Performance – and in response to a request from VA's National Center for Health Promotion and Disease Prevention (NCP) and Primary Care Services (PCS) – commissioned this evidence report to update prior reviews, and to specifically assess the implications for the treatment and prevention of common chronic conditions in the Veteran population. Conducted by the ESP Center located at the Minneapolis VA Medical Center, investigators reviewed the literature from 1990 through August 2015 and identified 55 published studies that met their inclusion criteria.

Findings

Primary Prevention: Cardiovascular Disease and Type 2 Diabetes
PREDIMED (Prevention with Mediterranean Diet) is the only large randomized controlled trial that tested the effects of an intensive Mediterranean diet (5 of 7 components) on clinical outcomes. Compared to a low-fat control diet, either the Mediterranean diet supplemented with extra virgin olive oil or the Mediterranean diet supplemented with nuts was associated with a significant 30% reduction in major cardiovascular events – the primary endpoint. This trial also reported significant reductions in incident diabetes, but only in the group randomized to the diet supplemented with extra virgin olive oil.

A second primary prevention study – the Women's Health Initiative Dietary Modification (WHI-DM) trial – found no difference in the incidence of major cardiovascular events or diabetes between its two diet groups. Although this study met the definition of a Mediterranean diet used for this evidence report, it only included 2 of 7 Mediterranean diet components (fruits/vegetables and grains), and its primary goal was to lower total fat intake. Therefore, report investigators did not consider this intervention a true Mediterranean diet.

Secondary Prevention: Cardiovascular Disease and Type 2 Diabetes
Data from three RCTs indicate that a Mediterranean diet is associated with a significant reduction in new myocardial infarction, but no reduction in cardiovascular mortality, stroke, or other cardiovascular events.

Primary Prevention: Cancer
PREDIMED documented a significant reduction in breast cancer incidence in women randomized to the Mediterranean diet supplemented with extra virgin olive oil. No other RCT reported a reduction in any cancer outcomes. Pooled results of cohort studies showed a significant reduction in total cancer incidence, total cancer mortality, and colorectal cancer incidence, but not in the incidence of breast cancer.

Secondary Prevention: Cancer
Investigators found no evidence that a Mediterranean diet reduces breast, prostate, or colon cancer recurrence, or mortality.

Conclusions

Investigators identified a single primary prevention trial which found that consumption of a Mediterranean diet was associated with a significant reduction in major cardiac events, new-onset type 2 diabetes, and breast cancer incidence. For secondary prevention, data from two trials indicate that assignment to a Mediterranean diet reduces incidence of myocardial infarction, but not other cardiovascular outcomes. Cohort studies indicate that conformity to a Mediterranean diet pattern is associated with significant reduction in total cancer incidence, total cancer mortality, and colorectal cancer incidence. However, these associations have not been confirmed in RCTs. The available data on other outcomes such as cognitive impairment and rheumatoid arthritis were limited. The available data on dietary adherence suggest that sustained increases in the consumption of fruits, vegetables, and grains can be achieved with labor-intensive behavioral interventions in select populations.

Future Research

A major gap in the research is the absence of large-scale clinical outcomes trials of a Mediterranean diet in North American populations, both in primary and secondary prevention populations. Such trials are important to confirm results of the few trials performed abroad and to determine the acceptability of the Mediterranean diet to the American public. In addition, the following areas represent important avenues for future research:

• Modeling studies to ascertain if specific components or combination of components of the Mediterranean diet are more protective than others;
• Barriers to adoption of a Mediterranean diet in people used to consuming a traditional Western diet and interventions to address those barriers; and
• Relative advantages of the Mediterranean diets compared to other healthy diets (e.g., the DASH – Dietary Approaches to Stop Hypertension – diet).

Implications for Policy and Practice

Though the Mediterranean Diet may show some promise as a dietary approach for preventing overall mortality, cardiovascular disease, and some cancers, the quality of available evidence for the Mediterranean diet's effects on these outcomes is low or insufficient, and is especially lacking in North American and U.S. Veteran populations. Moreover, adherence to a Mediterranean diet is challenging outside the context of an intervention trial that offers considerable behavioral intervention. Therefore, the results of this review will not change the core content of NCP's "Eat Wisely" Healthy Living message, which emphasizes eating a variety of foods, including vegetables, fruits, and whole grains and that already features nuts, fish, and foods low in saturated fats. These findings also will not significantly alter the dietary recommendations featured in the MOVE! Weight Management Program, which are aligned with the 2014 VA/DoD Clinical Practice Guideline (CPG) for Screening and Management of Obesity. This CPG recommends offering any diet that produces a caloric deficit and has evidence for weight loss efficacy and safety, and also stresses that the focus should be on adherence to a diet that is individualized for medical and metabolic conditions and ease of long-term maintenance and adherence. NCP also concurs with the Report's recommendations regarding the need for supporting research that addresses current gaps in our understanding.

Reference
Bloomfield H, Kane R, Koeller E, Greer N, MacDonald R, Wilt TJ. Benefits and Harms of the Mediterranean Diet Compared to Other Diets. VA ESP Project #09-009; 2015.

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