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.