A systematic review of clinical trials found that adults who adhere to a Mediterranean diet with no restriction on fat intake may have a reduction in cardiovascular events, breast cancer and type 2 diabetes compared with those who are on other diets.
However, the Mediterranean diet had no association with a reduction in total mortality, and the researchers mentioned more randomized trials were needed to determine the diet’s effectiveness at improving health outcomes.
Lead researcher Hanna E. Bloomfield, MD, MPH, of the Minneapolis VA Medical Center, and colleagues published their results online in the Annals of Internal Medicine on July 18.
They noted that the Mediterranean diet was first described more than 60 years ago and became popular in the past 10 years after a large prospective cohort study found a significant association between adherence to the diet and a reduction in total mortality.
For this analysis, they searched Ovid MEDLINE, CINAHL and the Cochrane Library for primary studies and systematic reviews published from 1990 through April 2016.
The researchers defined a Mediterranean diet as a diet that placed no restrictions on total fat intake and included two or more of the following components: a high monounsaturated-to-saturated fat ratio, high fruit and vegetable intake, high consumption of legumes, high grain and cereal intake, moderate red wine consumption, moderate consumption of dairy products and low consumption of meat and meat products with increased intake of fish. They included observational studies that assessed the association between two or more of the components and any of their outcomes of interest.
Of the more than 10,000 abstracts that the researchers reviewed, 90 papers representing 56 studies met all of their inclusion criteria, although three studies were excluded because they contained possibly fraudulent data.
For primary prevention, they identified 72 articles reporting results from 44 studies that evaluated the association between adherence to a Mediterranean diet and outcomes in people who did not have a history of the outcome. However, only two of the studies were randomized, while 42 were cohort studies.
They noted a few trials, including the PREDIMED study that randomly assigned 7,447 participants to a Mediterranean diet with supplemental extra-virgin olive oil, a Mediterranean diet supplemented with nuts or a low-fat control diet.
After a mean follow-up period of 4.8 years, participants in the two Mediterranean diet groups had a 29 percent reduction in MI, stroke or cardiovascular death compared with the control group. However, the all-cause mortality was 4.7 percent in all of the groups.
For secondary prevention, they identified 17 articles reporting results from 12 studies that evaluated the association between adherence to a Mediterranean diet and outcomes. Of the trials, four were randomized studies, seven were cohort studies and one was a controlled study.
The researchers cited three randomized trials that included participants with cardiovascular disease. The Heart Institute of Spokane Diet Intervention and Evaluation trial enrolled 202 patients and found no difference in outcomes between the groups. The Lyon Heart Study enrolled 605 participants and found the Mediterranean diet group had a reduction in risk for new-onset MI and cardiovascular death.
The third study, which was conducted in Wales, enrolled 3,114 men and found no differences between the groups with regards to MI or cardiovascular mortality. However, the researchers noted that the study was interrupted by funding problems and questioned the integrity of its outcomes reporting and analysis.
The researchers cited a few limitations of their review, including that they only included English-language publications. They also used DerSimonian and Laird random-effects models, which may have affected their conclusions.
In addition, the review might have been subject to selective reporting and publication bias. Further, the researchers only found a few randomized controlled trials, while approximately half of the cohort studies were medium risk of bias.
“Future investigation should include randomized trials in U.S. populations to assess adherence, efficacy and effect on a wider range of clinical outcomes, modeling studies to determine whether specific Mediterranean diet components or combinations of components are more protective than others, and randomized trials to assess the relative effectiveness of the Mediterranean diet compared with other healthy diets, such as the DASH (Dietary Approaches to Stop Hypertension) or prudent diet,” the researchers wrote.