If you've spent any amount of time reading up on healthy foods that can offer significant boosts to your physiological and mental well-being, you're almost sure to have come across the Mediterranean diet.

This famous (and yummy) mix of vegetables, fruit, fish, and whole grains – served with a liberal splash of olive oil – is common in cultures around the Mediterranean Sea, and has long been recognised for its broad nutritional benefits.

But now, scientists have discovered a significant caveat to the health advantages that the Mediterranean diet provides – and it's one that seems to strictly limit just who can snare the benefits of the regimen.

A new study by Italian researchers analysed adherence to the Mediterranean diet in almost 19,000 people who took part in a health study called the Moli-sani Project, and found that the cardiovascular benefits of the diet basically only extended to people who were well off or highly educated.

"In other words, a person from low socioeconomic status who struggles to follow a Mediterranean model, is unlikely to get the same advantages of a person with higher income," explains Marialaura Bonaccio from the IRCCS Neuromed in Pozzilli, Italy, "despite the fact that they both similarly adhere to the same healthy diet."

The researchers tracked participants in the Moli-sani Project for an average of four years, and found that adherence to the Mediterranean diet was associated with an approximate 60 percent reduction in risk of cardiovascular disease – but only for those who had education beyond high school or a household income greater €40,000 (approximately US$47,300) annually.

People in the study with lower education or incomes demonstrated no cardiovascular benefits to following the Mediterranean diet – even if their adherence to Mediterranean staples was the same.

Digging deeper to explain the discrepancy, the team found that while participants overall consumed the same amount of 'Mediterranean diet' food – the breakdown of what they ate differed depending on their socioeconomic circumstances, as did the quality of individual ingredients.

High socioeconomic participants consumed more fish within the diet, along with more organic products and whole grain foods – meaning their diets were richer in terms of antioxidants and polyphenols, and also offered greater diversity in fruit and vegetable choice.

According to the researchers, the findings suggest that people of low socioeconomic status may end up buying 'Mediterranean' food with lower nutritional value than those who are better off – a result that places a big question mark over how much value we can ascribe to the concept of the Mediterranean diet generally.

"We cannot be keeping on [saying] that the Mediterranean diet is good for health if we are not able to guarantee an equal access to it," says one of the team, Giovanni de Gaetano.

But until we know more, other scientists are saying that we shouldn't write off the Mediterranean diet just yet – as there could be a whole host of variables to explain the outcome here.

"Although the authors of this study suggest that the Mediterranean diet may be less effective in reducing heart disease in less well-off people, this is likely to be due to other differences between low and high income groups, rather than the diet not being effective," explains cardiovascular medicine researcher Tim Chico from the University of Sheffield, who wasn't involved with the study.

"These findings should not put anyone off a Mediterranean diet; this is still the best option for reducing risk of heart disease."

"However, there is no quick-fix diet; a healthy diet has to be a part of an overall heart-healthy lifestyle that includes not smoking, taking regular exercise, and maintaining a healthy weight."

The findings are reported in International Journal of Epidemiology.