The Mediterranean diet is rich in fish, grains, nuts, fruits and vegetables. The diet is low in dairy products, red meat and processed foods.

In this study, funded mainly by the Spanish government, the researchers made sure people got regular training sessions in the particulars of each diet. They also checked people's actual consumption of olive oil and nuts with lab tests.

One thing the researchers didn't do was set any limits on calories or targets for exercise.

While lots of research has found benefits from the Mediterranean diet, many of the studies have observed what people have eaten and looked for associations. One of this study's strengths is that it randomly assigned people at high risk of developing cardiovascular disease to diets that stood to help then.

The study was stopped early (after a median follow-up of 4.8 years) because the benefits from the Mediterranean diet were already becoming apparent. Overall, the people consuming the diets rich in olive oil or nuts had about a 30 percent lower risk of having a heart attack, stroke or dying from a cardiovascular cause.

In absolute terms, there were about 8 of those problems for every 1,000 person-years in the Mediterranean diet groups compared with 11 per 1,000 person-years in the low-fat diet group.

How does the Mediterranean diet work? The prevailing theory is that it lowers bad cholesterol and triglycerides while increasing protective good cholesterol. It may also also help the body's ability to process sugar.

Copyright 2013 NPR.

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