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The subjects were people at high risk of heart disease, and the diet was a Mediterranean one, high in olive oil or nuts.

The study, published last week in The New England Journal of Medicine, is now shaking up the field of cardiovascular medicine, infusing it with optimism. Scientists are calling for similarly rigorous studies of other popular diets that are routinely recommended by cardiologists even though there is little solid evidence that they work.

"We don't know what the best diet is," said Dr. Michael Lauer, the director of the division of cardiovascular sciences at the National Heart, Lung and Blood Institute. "This is a great opportunity to come together and use power of the scientific method to get closer to the right answer."

Other leading experts agreed.

"This is the start of where we need to go with nutritional clinical trials," said Dr. Neil J. Stone, a professor of preventive cardiology at Northwestern University's Feinberg School of Medicine and a former chairman of the American Heart Association's nutrition committee.

The National Heart, Lung and Blood Institute is supporting a large clinical trial to see if fish oil and vitamin D can prevent heart attacks and cardiovascular deaths. But while that is important, heart experts say it is time to also look at diets as a whole.

"We definitively need to test plausible diets, within the context of what is available and consumed in the U.S., that could lower the risk of heart disease," said Alice H. Lichtenstein, a professor of nutrition science and policy at Tufts University and the lead author of the American Heart Association's dietary guidelines.

Dr. Lawrence Appel, a professor of medicine at Johns Hopkins who was a member of the 2005 and 2010 United States dietary guidelines committees, said he was inspired by the Mediterranean diet study.

"Can we do a trial of fruits and vegetables?" he asked. "I think we can."

The situation in nutrition today reminds Dr. Lauer of a previous era in the 1950s when doctors were struggling to figure out how best to treat heart attack patients.

"Nutrition research is where heart attack research was," Dr. Lauer said. Patients in those days were advised to stay in bed for four to five weeks and take lidocaine to normalize their hearts' rhythms and nitroglycerin to open blood vessels.

But it turned out that treatment actually hastened death. It took years to find the answers, but eventually, Dr. Lauer said, dozens if not hundreds of large clinical trials radically transformed heart attack treatment.

But when it comes to diet and heart disease, doctors — and patients — have been going on hunches.

The new study could be a start in changing all that, heart researchers said.

It involved 7,447 people in Spain, half of whom were randomly assigned to follow a Mediterranean diet and the rest to follow the sort of standard low-fat diet that cardiologists often prescribe. It was ended early after less than five years because those on the Mediterranean diet had 30 percent fewer heart attacks, strokes and deaths from heart disease compared with people in the control group, who ate more or less the same way that they always had. They had been instructed to follow a low-fat diet but had not been able to comply.

Dr. Ramón Estruch of the University of Barcelona, the lead author of the study, said that although some had thought people would never allow their diets to be decided by a figurative toss of a coin, it was not hard to get people to switch to a Mediterranean diet.

"They wanted to eat the way their grandfathers ate," he said. Most study participants actually thought they were already eating that way. They learned they were not after answering questions about, for example, how often they ate red meat versus fish.

Dr. Estruch said he and his colleagues were so buoyed by the success of their study that they were planning another one. They intend to randomly assign people to consume the Mediterranean diet or to exercise while following a similar diet that is lower in calories. The hope is that adding weight loss and exercise will prevent even more heart disease.

But for now, chaos reigns. The public is bombarded with diet advice, often contradictory and often lacking a rigorous scientific grounding, medical experts said.

"Diets are an extreme case of accepting evidence we want to believe," said Dr. J. Sanford Schwartz, a professor of medicine at the University of Pennsylvania.

That includes doctors, he added, who overlook that the evidence for the low-fat diets they often recommend is the sort "we would never accept in the practice of medicine."

Those low-fat diets sound sensible — eat fruits and vegetables, fish and lean meats. Cut back on salt- and sugar-laden sodas and potato chips. Cut or sharply limit most fats, including olive oil and nuts. But such diets have not been tested in the way the Mediterranean diet was tested.

Doctors are in a bind, said Dr. Daniel J. Rader, a heart disease specialist at the University of Pennsylvania. When patients ask what to eat, he said, "you have to give them something."

"Given the importance of diets and given the decades of dietary recommendations we have given to people, you would think we would have had more dietary studies with hard endpoints to get at these questions," Dr. Rader said. But the best they have are studies that look at intermediate markers of risk, like cholesterol levels. In the end, he said, "most doctors just give dietary platitudes."

Kevin McNamara, 44, faced a decision about diet after he went into cardiac arrest in June while jogging along the Jersey Shore. Mr. McNamara, an investment manager, had no idea he had heart disease. He ended up in the nearest hospital and had bypass surgery to open two blocked arteries. When he returned to his home near Valley Forge, Pa., he sought advice on what to eat and was persuaded by a book that said a vegan diet would prevent and even reverse heart disease.

"I talked to some of my doctors, and they said it wouldn't hurt if I went on a vegan diet," Mr. McNamara said.

Now he wants to discuss the Mediterranean diet with his doctors. He worries because he wants to keep his cholesterol levels very low. "I am paranoid that any sort of fat will build up in my coronary arteries," Mr. McNamara said. But he added, "I am not sure if I want to do a vegan diet for the next 30 or 40 years of my life."

Vegan diets have never been rigorously tested, and probably never will be, Dr. Lichtenstein said. "Given the high cost of conducting a long-term diet study we have to think about what is feasible for the majority of the U.S. population," she said. "From past experience, we know it is highly unlikely people are going to make radical changes in their diet and stick with those changes long term."

But other diet studies are definitely in the offing, researchers said.

"As we go on, we realize we know less and less," Dr. Lauer said. "We can despair and just make things up, or we can celebrate and say we have a real opportunity to grow here."