A landmark study from Spain reports that a Mediterranean diet supplemented with extra-virgin olive oil or nuts reduced the incidence of major cardiovascular events (heart attack, stroke, cardiovascular death) among people with high cardiovascular risk.

Researchers working on the PREDIMED (PREvención con DIeta MEDiterranea) trial write about their findings in the 25 February online issue of the New England Journal of Medicine.

PREDIMED is a multicenter, randomized, primary prevention trial of cardiovascular disease funded by the Spanish Ministry of Health. It is one of the world’s largest and longest dietary intervention trials.

People who follow a traditional Mediterranean diet have high intakes of olive oil, fruit, nuts, vegetables, and cereals, moderate intakes of fish and poultry, and low intakes of red and processed meats, dairy foods and sweets. They also drink wine in moderation, and only with meals.

Previous studies have suggested a Mediterranean diet protects against cardiovascular events and heart disease, and small trials have proposed some underlying mechanisms to explain the effect. But none has managed to rule out the possibility that other health factors or lifestyle could be responsible.

To single out the effect of a particular diet on the outcomes, the PREDIMED team randomly assigned 7,447 participants with high cardiovascular risk to follow one of three diets: a Mediterranean diet supplemented with extra-virgin olive oil, a Mediterranean diet supplemented with nuts, or a standard low-fat diet (the control), for five years, starting in October 2003.

43% of the participants were men, who were aged between 55 and 80 and, 57% were women between 60 and 80. None had cardiovascular disease when they enrolled in the study, but they did have high cardiovascular risks, either in the form of type 2 diabetes or at least three from a list of well-known major risk factors such as smoking, high blood pressure, obesity, and family history of heart disease.

Over the five years of follow-up, 288 participants had a cardiovascular event (ie stroke, a heart attack, or died of a cardiovascular disease).

Statistical analysis showed that compared to those following the control (low-fat) diet, participants on the Mediterranean diet with extra-virgin olive oil were 30% less likely to experience a cardiovascular event, and those on the Mediterranean diet with nuts were 28% less likely.

Lead and co-corresponding author Ramon Estruch, of the Hospital Clinic of Barcelona and general coordinator for PREDIMED, says in a PR Newswire press release issued on behalf of the California Walnut Commission (who supplied the nuts for the trial) that:

“… the results of the PREDIMED trial are of utmost importance because they convincingly demonstrate that a high vegetable fat dietary pattern is superior to a low-fat diet for cardiovascular prevention.”

Co-corresponding autor Miguel Angel Martinez-Gonzalez, of the Universidad de Navarra in Pamplona, agrees. He told Reuters Health that the “quality of fat in the Mediterranean diet is very good”.

Good sources of calories are eaten in preference to bad sources, he adds. He also points to the wide variety of plant foods in the Mediterranean diet, “including legumes and fruits as desserts”.

Martinez-Gonzalez says people wanting to move toward such a diet should start with small changes, such as not eating meat on two days a week, move to using olive oil for cooking, and replacing hard alcohol with red wine, and drinking it only at mealtimes.

In the Mediterranean group whose diet was supplemented with nuts, half of the nuts were walnuts (the mix was 15 g of walnuts, 7.5 g of hazelnuts, and 7.5 g of almonds). Research suggest walnuts rank amongst the highest in terms of the main quality of antioxidants for heart health, and only a handful a day can make a difference.

Written by Catharine Paddock PhD