The study, published by the New England Journal of Medicine, followed 4,479 people in Spain -- men and women age 55 to 80 -- over a five year period, finding dramatic confirmation of previous observations of such a diet's health benefits.
"We observed that an energy unrestricted Mediterranean diet, supplemented with extra virgin olive oil or nuts, resulted in a substantial reduction in the risk of major cardiovascular events among high-risk persons," the study concluded.
It put the risk reduction at "approximately 30 percent, among high risk persons who were initially free of cardiovascular disease. These results support the benefits of the Mediterranean diet for cardiovascular risk reduction."
Participants in the study, which was led by Ramon Estruch, a professor of medicine at the University of Barcelona, were divided into three groups including a control group on a low fat diet.
One group followed a traditional Mediterranean diet supplemented by four tablespoons of extra-virgin olive oil a day. A second group followed the same diet, but instead of the olive oil consumed about 30 grams a day of mixed nuts.
Participants in those two groups also ate at least three servings of fruits and two of legumes a day. They also ate fish three times a week and favored white meat like chicken instead of red meat.
They were also strongly encouraged not to eat commercially baked goods, pastries and sweets, and to limit their consumption of dairy products and processed meats.
For those who normally drank wine with their meals, their diet called for seven glasses of wine a week.
Researchers could tell whether the study participants were following the diets by measuring markers for olive oil in their urine or a blood marker for the mixed nuts.
They found that participants stuck to the Mediterranean diets, but that the low-fat control diet led to only small reductions in fat.
"The interventions were intended to improve the overall dietary pattern, but the major between-group differences involved the supplemental items," the study said.
"Thus, extra virgin olive oil and nuts were probably responsible for most of the observed benefits of the Mediterranean diets," it said.