Hanna E Bloomfield (Minneapolis VA Medical Center, Minneapolis, USA) and others report in Annals of Internal Medicine that limited evidence, from randomised control trials, indicates that a Mediterranean diet without a restriction on fat intake is associated with a reduced incidence of cardiovascular events and other outcomes such as cancer. They add that further studies are needed to determine whether specific components of combinations of the Mediterranean diet are more protective than others.
Bloomfield et al write that “many observational studies and a few clinical studies” have examined the association between a Mediterranean diet and various clinical outcomes, but add that “the definition of the diet varied among the investigations”. The aim of this present systemic review, the authors note, was to: “Summarise the literature on the effect of the Mediterranean diet on health outcomes.”
To be included in the review, Bloomfield et al report, a clinical study had to define the Mediterranean diet as “one that placed no restriction on total fat intake and included two or more of the following seven components: a high monounsaturated-to-saturated 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 consumptions of meat and meat products.” Observational studies were included if they assessed the association between two or more of these components and any of the outcomes being evaluated in the present review.
Of 10,349 studies on Mediterranean diets identified, 53 met all inclusion criteria (originally 56 but three were discounted after evidence indicated that they might be fraudulent). Focusing on data from randomised controlled trials, Bloomfield et al found that one randomised control trial—after an average follow-up of 4.8 years—showed that two types of Mediterranean diet, together, were associated with a 29% reduction in first-time major cardiovascular events but not all-cause mortality compared with a control diet. However in a review of three randomised controlled secondary preventions trials, they found that a Mediterranean diet was not associated with a reduction in myocardial infarction or cardiovascular mortality. Data from the randomised control trials also indicated that a Mediterranean diet may associated with a reduced incidence of all cancers, breast cancer, and type 2 diabetes.
“The observational data reveal that total cancer incidence and mortality and colorectal and lung cancer were lower in persons the highest adherence to the Mediterranean diet compared with those with the lowest adherence but show no association between Mediterranean diet adherence and breast cancer risk,” Bloomfield et al comment.
According to the authors, the definition of a Mediterranean diet that was used in this review “more accurately reflect the current conceptualisation of a Mediterranean diet” than other definitions (such as those with a low-fat element). They note: “The lack of a clearly articulated and widely accepted definition of a Mediterranean diet is an ongoing problem that must be resolved for this field to move forward.”
Bloomfield et al comment that the 2016 guidelines from US Department of Health and Human Services and Department of Agriculture continue to emphasise the importance of limiting fat intake despite evidence that a “Mediterranean diet in which total fat comprises 40% of energy intake results in fewer cardiovascular events than a low-fat diet.” They add “experts have expressed disappointment” with the guidelines because “an opportunity was missed to focus the public’s attention on the dangers of refined gain products and added sugar, which likely are the major culprits behind the US obesity epidemic”.
The authors conclude that further studies should review “whether specific Mediterranean diet components or combinations of components are more protective than others” and add that there should also be randomised controls trials that compare the effectiveness of Mediterranean diet with other healthy diets.
Bloomfield told Cardiovascular News: “When counselling patients, I suggest that they exclusively use olive or canola oil for cooking and that they eat lots of fruits, vegetables, beans, nuts and whole grain products, while limiting the amount of sugary products like soda, refined grains, and red meat.”