Researchers also provided the nuts and olive oil, so it didn't cost participants anything to use these relatively pricey ingredients. The type of oil may have mattered — they used extra-virgin olive oil, which is minimally processed and richer than regular or light olive oil in the chemicals and nutrients that earlier studies have suggested are beneficial.
The study involved people ages 55 to 80, just over half of them women. All were free of heart disease at the start but were at high risk for it because of health problems — half had diabetes and most were overweight and had high cholesterol and blood pressure.
They were assigned to one of three groups: Two followed a Mediterranean diet supplemented with either extra-virgin olive oil (4 tablespoons a day) or with walnuts, hazelnuts and almonds (a fistful a day). The third group was urged to eat a low-fat diet heavy on bread, potatoes, pasta, rice, fruits, vegetables and fish and light on baked goods, nuts, oils and red meat.
Independent monitors stopped the study after nearly five years when they saw fewer problems in the two groups on Mediterranean diets.
Doctors tracked a composite of heart attacks, strokes or heart-related deaths. There were 96 of these in the Mediterranean-olive oil group, 83 in the Mediterranean-nut group and 109 in the low-fat group.
Looked at individually, stroke was the only problem where type of diet made a big difference. Diet had no effect on death rates overall.
The Mediterranean diet proved better even though its followers ate about 200 calories more per day than the low-fat group did. The study leaders now are analyzing how each of the diets affected weight gain or loss and body mass index.
The Spanish government's health research agency initiated and paid for the study, and foods were supplied by olive oil and nut producers in Spain and the California Walnut Commission. Many of the authors have extensive financial ties to food, wine and other industry groups but said the sponsors had no role in designing the study or analyzing and reporting its results.