Past research suggested people who eat a Mediterranean-like diet have healthier hearts, but those studies couldn't rule out that other health or lifestyle differences had made the difference.
For the new trial, researchers randomly assigned study volunteers at risk of heart disease to a Mediterranean or standard low-fat diet for five years, allowing the team to single out the effect of diet, in particular.
"This is good news, because we know how to prevent the main cause of deaths - that is cardiovascular disease - with a good diet," said Dr. Miguel Angel Martinez-Gonzalez, who worked on the study at the Universidad de Navarra in Pamplona.
He and colleagues from across Spain assigned almost 7,500 older adults with diabetes or other heart risks to one of three groups.
Two groups were instructed to eat a Mediterranean diet - one supplemented with extra-virgin olive oil and the other with nuts, both donated for the study - with help from personalized advice and group meetings. The third study group ate a "control" diet, which emphasized low-fat dairy products, grains and fruits and vegetables.
Over the next five years, 288 study participants had a heart attack or stroke or died of any type of cardiovascular disease.
People on both Mediterranean diets were 28 to 30 percent less likely to develop cardiovascular disease than those on the general low-fat diet, the researchers reported Monday in the New England Journal of Medicine.
The new study is the first randomized trial of any diet pattern to show benefit among people initially without heart disease, said Dr. Dariush Mozaffarian, who studies nutrition and cardiovascular disease at the Harvard School of Public Health in Boston.
NOT DUE TO SINGLE INGREDIENT
It's the blend of Mediterranean diet components - not one particular ingredient - that promotes heart health, according to Martinez-Gonzalez.
"The quality of fat in the Mediterranean diet is very good," he told Reuters Health. "This good source of calories is replacing other bad sources of calories. In addition, there is a wide variety of plant foods in the Mediterranean diet," including legumes and fruits as desserts, Martinez-Gonzalez added.
"I think it's a combination of what's eaten and what's not eaten," agreed Mozaffarian, who wasn't involved in the new research.
"Things that are discouraged are refined breads and sweets, sodas and red meats and processed meats," he told Reuters Health. "The combination of more of the good things and less of the bad things is important."
Martinez-Gonzalez suggested people seeking to improve their diet start with small changes, such as forgoing meat one or two days per week, cooking with olive oil and drinking red wine with meals rather than hard alcohol.
Replacing a high-carbohydrate or high-saturated fat snack with a handful of nuts is also a helpful change, said Teresa Fung, a nutrition researcher at Simmons College in Boston who also wasn't on the study team.
"All of these steps are making, at the end of the day, a big difference," Martinez-Gonzalez said.
Fung pointed out many people in the new trial were already on medications, such as statins and diabetes drugs.
"The way I see it is, even if people are on medication already, diet has substantial additional benefit," she told Reuters Health.
That's likely the case for people without heart risks - including high blood pressure or cholesterol - as well, Fung added.
"This is a high-risk group, but I don't think people should wait until they become high-risk in order to change," she said.
SOURCE: http://bit.ly/YuyV7v New England Journal of Medicine, online February 25, 2013.