The combination of olive oil and leafy salad or vegetables is what gives the Mediterranean diet its healthy edge, say scientists.
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.