The Mediterranean diet, believed to be the healthiest in the world, may not be that good for certain people as it may raise their risk of heart attacks, a new study has claimed.

It has long been thought that a diet rich in olive oil, nuts and oily fish is good for health because it can reduce the levels of bad cholesterol, which is blamed for clogging arteries and increasing the risk of heart attacks.

But researchers at University of Rochester, New York, found that some heart attack patients may have genetic mutations that mean the diet increases their risk of suffering further cardiac problems, the Telegraph reported.

They found that those at most risk of suffering subsequent heart attacks had large amounts of the high-density lipoprotein (HDL), or “good cholesterol”, in their blood that destroys unhealthy trans fats in foods such as biscuits and cakes.

They also had more of a protein known as CRP which causes inflammation - suggesting this influences whether good cholesterol protects or endangers individuals.

For their study, the first to find supposedly good cholesterol can harm a subgroup of people, the researchers followed 767 heart patients for two years. They found that about 20 per cent of the patients at high risk of another heart attack also had high levels of HDL and CRP.

Lead researcher Prof James Corsetti said, “It seems counter-intuitive that increasing good cholesterol - which we’ve always thought of as protective - leads to negative consequences in some people.”

“We’ve confirmed high HDL cholesterol is in fact associated with risk in a certain group of patients.” James Corsetti added.

The findings, published in Arteriosclerosis, Thrombosis, and Vascular Biology, could also explain disappointing results from a trial of an experimental drug called “torcetrapib” designed to increase HDL cholesterol.

In 2006, pharma company Pfizer had to halt the trial due to a surprisingly excessive number of unexplained heart attacks and deaths that were linked with higher levels of good cholesterol.

Co-researcher Prof Charles Sparks said, “The ability to identify patients who will not benefit from efforts to increase HDL cholesterol is important because they can be excluded from trials testing medications that aim to raise HDL cholesterol.”

“With these patients excluded researchers may find raising HDL cholesterol in the remaining population is effective in reducing cardiovascular disease risk.” Charles Sparks added.

The researchers believe genetics and environmental factors, particularly inflammation, decide what effect good cholesterol has on patients.

Given an inflammatory environment a person’s unique set of genes determines whether HDL transforms from good to bad in the heart disease process, they said.

In the high-risk subgroup of patients they also identified two genes associated with recurrent heart attacks - CETP that moves cholesterol away from the vascular system and is liked with HDL and “p22phox”, which influences inflammation-related processes and is associated with CRP.

Prof Corsetti said, “Our research is oriented around the ability to better identify patients at high risk.”

“Identifying these patients and determining what puts them at high risk may be useful in choosing treatments tailored to the specific needs of particular patient subgroups. This gets us another step closer to achieving the goal of personalised medicine.” Corsetti added.