New research this week confirmed the heart-healthy benefits of a Mediterranean diet.

But there’s an expensive catch.

A team of Italian scientists matched up the popular food plan with income and level of education in nearly 19,000 men and women living in southern Italy from 2005-10. They reached the eye-raising conclusion that the diet significantly reduced the risk of cardiovascular disease, but only among the rich and well-educated.

“We found heart advantages were limited to high socioeconomic status groups, even if groups showed the same adherence to the Mediterranean diet,” Marialaura Bonaccio, the study’s lead author, said in a CNN report. “No benefits occurred for participants in the low income and low education group.”

She said the same problem may apply for other diets, saying diets focus on “quantity, rather than on quality” of the food.

Along with considering education, household income and marital status, researchers also took physical activity, body mass index (BMI), smoking status and health history into account, according to the study.

Bonaccio and her colleagues examined the variety of fruits and vegetables, meat and fish consumed by their subjects, along with cooking methods.

The study was published in the International Journal of Epidemiology.

Bonaccio told CNN that differences in the quality (and price) of what people ate and what they used to make it may have led to the result. She used olive oil as an example.

Assuming that a person with higher income would be more inclined to buy a bottle of extra virgin olive oil costing 10 euros as opposed to 2 or 3, she said, “our hypothesis is that differences in the price may yield differences in health components and future health outcomes.”