The findings of a new review suggest that following a Mediterranean diet with no restriction on fat intake may reduce the incidence of cardiovascular (CV) events, cancer, and type 2 diabetes.

"Our primary conclusion is that there is limited evidence from randomized trials that a Mediterranean diet with no restriction on fat intake may be associated with a reduced incidence of cardiovascular events, all-cancers, breast cancer, and type 2 diabetes mellitus but does not affect all-cause mortality," write Hanna E. Bloomfield, MD, MPH, from the Minneapolis Veterans Affairs Medical Center, Minnesota, and colleagues.

The results of the systematic review and meta-analysis were published online July 18 in Annals of Internal Medicine.

Despite advances in diagnosis and treatment, chronic illnesses, such as CV disease, cancer, and diabetes, remain among the leading causes of morbidity and mortality in the United States. According to Dr Bloomfield and colleagues, global deaths due to CV disease and the prevalence of diabetes have increased by more than 40% in the past 2 decades.

"Typical Western diets, which are high in saturated fats, sugar, and refined grains, are causally associated with development of cardiovascular disease, [type 2] diabetes, and some types of cancer, including breast and colorectal cancer," the authors write.

The Mediterranean diet is a fruit- and vegetable-rich diet that is high in monounsaturated fats (30% to 40% of total daily calorie intake) — in particular, olive oil — as well as legumes and fish, with a low to moderate intake of dairy and meat products. Several studies have shown its benefit in improving various clinical outcomes, including a reduction in total mortality.

Dr Bloomfield and colleagues conducted their study to evaluate the health benefits of a Mediterranean diet and to determine whether North Americans are likely to adhere to such a diet.

"Healthy Diets Can Include a Lot of Fat"

The authors searched various electronic databases to identify studies for inclusion. They included controlled trials that involved 100 or more persons who were followed for at least 1 year for mortality, CV disease, hypertension, diabetes, and adherence outcomes. They also included cohort studies whose participants were followed for cancer outcomes.

They defined a Mediterranean diet as one without restriction on total fat intake and that included two or more of seven components: high monounsaturated–to–saturated fat ratio, high fruit and vegetable intake, high legume consumption, high grain and cereal intake, moderate red wine intake, moderate dairy product consumption, and low meat and meat product consumption, but with high fish consumption.

A total of 90 articles representing 56 unique studies met their inclusion criteria, although three of these studies were linked to possible research fraud and ultimately were not included in the analysis.

According to the authors, data from one large primary prevention trial showed that, compared with individuals who followed a control diet, participants who followed a Mediterranean diet had a lower incidence of major CV events (myocardial infarction, stroke, or CV death; hazard ratio [HR], 0.71; 95% confidence interval [CI], 0.56 - 0.90), breast cancer (HR, 0.43; CI, 0.21 - 0.88), and diabetes (HR, 0.70; CI, 0.54 - 0.92).

The researchers also performed pooled analyses of primary prevention cohort studies that reported cancer outcomes and that compared groups with the highest and lowest Mediterranean diet adherence.

These results showed that, compared with individuals with the lowest adherence, those with the highest adherence to a Mediterranean diet had a 14% reduction in total cancer mortality (risk ratio [RR], 0.86; CI, 0.82 - 0.91; 13 studies), a 4% reduction in the incidence of all-cancers (RR, 0.96; CI, 0.95 - 0.97; three studies), and a 9% reduction in the incidence of colorectal cancer (RR, 0.91; CI, 0.84 - 0.98; nine studies).

"[W]hat we found in our study is that healthy diets can include a lot of fat, especially if it's healthy fat; and the emphasis in the United States at least for the past thirty years has been [that] it's important to reduce fat — fat of all kind— fat's the bad thing," reports Dr Bloomfield in an American College of Physicians Soundbites video.

"It turns out that the obesity epidemic in this country is probably more due to our increased consumption of refined grains and added sugar and not so much from our fat consumption," she adds.

However, the study found inconsistent, minimal, or no evidence pertaining to other outcomes, including all-cause mortality, hypertension, cognitive function, kidney disease, rheumatoid arthritis, and quality of life. In addition, no studies met the inclusion criteria for dietary adherence outcomes, the authors note.

"Future investigation should include randomized trials in US 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," they conclude.

The study was supported by the Department of Veterans Affairs, the Veterans Health Administration, the Office of Research and Development, and the Quality Enhancement Research Initiative. The authors have disclosed no relevant financial relationships.

Ann Intern Med. Published online July 18, 2016. Abstract