The combination of olive oil and leafy salad or vegetables is what gives the Mediterranean diet its healthy edge, say scientists.
When these two food groups come together they form nitro fatty acids which lower blood pressure, they told PNAS journal.
The unsaturated fat in olive oil joins forces with the nitrite in the vegetables, the study of mice suggests.
Nuts and avocados along with vegetables should work too, they say.
Inspired by traditional cuisine of countries such as Greece, Spain and Italy, the Mediterranean diet has long been associated with good health and fit hearts.
Typically, it consists of an abundance of vegetables, fresh fruit, wholegrain cereals, olive oil and nuts, as well as poultry and fish, rather than lots of red meat and butter or animal fats.
While each component of the Mediterranean diet has obvious nutritional benefits, researchers have been puzzled about what precisely makes the diet as a whole so healthy.
Prof Philip Eaton, from King's College London, and colleagues from the University of California in the US believe it is the fusion of the diet's ingredients that make nitro fatty acids.
In their study, part-funded by the British Heart Foundation, the researchers used genetically engineered mice to see what impact nitro fatty acids had on the body.
Nitro fatty acids helped lower blood pressure by blocking an enzyme called epoxide hydrolase.
Prof Eaton said: "Humans have this same enzyme so we think the same happens in people."
This explains why a Mediterranean diet is healthy, even though it contains fat, he says.
"With the fats in the Med diet, if taken together with nitrates or nitrites, there's a chemical reaction and these combine to form nitro fatty acids.
"It's nature's protective mechanism. If we can tap into this we could make new drugs for treating high blood pressure and preventing heart disease," he said.
He said human trials were planned.
Dr Sanjay Thakrar of the British Heart Foundation said: "This interesting study goes some way to explain why a Mediterranean diet appears to be good for your heart health. The results showed a way in which a particular compound could combat high blood pressure, which is a major risk factor for heart disease.
"However, more work is necessary as these experiments were conducted in mice and this compound could also be having its effect through other pathways."