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.
Rachel Johnson, a University of Vermont professor who heads the American Heart Association's nutrition committee, said the study is very strong because of the lab tests to verify oil and nut consumption and because researchers tracked actual heart attacks, strokes and deaths — not just changes in risk factors such as high cholesterol.
"At the end of the day, what we care about is whether or not disease develops," she said. "It's an important study."
Rena Wing, a weight-loss expert at Brown University, noted that researchers provided the oil and nuts, and said "it's not clear if people could get the same results from self-designed Mediterranean diets" — or if Americans would stick to them more than Europeans who are used to such foods.
Dr. George Bray of the Pennington Biomedical Research Center in Baton Rouge, La., said he would give the study "a positive — even glowing — comment" and called it "the best and certainly one of the largest prospective dietary trials ever done."
"The data are sufficiently strong to convince me to move my dietary pattern closer to the Mediterranean Diet that they outline," he added.
Another independent expert also praised the study as evidence diet can lower heart risks.
"The risk reduction is close to that achieved with statins," cholesterol-lowering drugs, said Dr. Robert Eckel, a diet and heart disease expert at the University of Colorado.
"But this study was not carried out or intended to compare diet to statins or blood pressure medicines," he warned. "I don't think people should think now they can quit taking their medicines."
Marilynn Marchione can be followed at http://twitter.com/MMarchioneAP