Too much 'good' cholesterol can raise the risk of a heart attack

Researchers found that in large amounts the blood fat turns from being healthy to harmful - increasing death rates.

They say it could explain why repeated trials of drugs that boost HDL (high-density lipoprotein) have failed to cut cardiovascular disease.

A study of nearly 6,000 older people found those with very high levels were as prone to a heart attack or death as those who did not have enough.

It adds to a growing body of evidence that you really can have too much of a good thing when it comes to cholesterol.

Author Dr Marc Allard-Ratick, of Emory University in the United States, said: "It may be time to change the way we view HDL cholesterol.

"Traditionally, physicians have told their patients that the higher your 'good' cholesterol, the better.

"However, the results from this study and others suggest that this may no longer be the case."

The study of 5,965 people followed for an average of four years found those in the middle range of HDL levels were least likely to have a heart attack or die from cardiovascular disease.

On the other hand, risk soared in both those with low levels (less than 41 mg/dl) and very high levels (greater than 60 mg/dl) - producing a "U-shaped curve" when plotted graphically.

Dr Allard-Ratick told a meeting of the European Society of Cardiology in Munich that the danger rose by "nearly 50 per cent" among the latter group.

It backs several previous large population studies, including one by Cambridge University, showing deaths from cardiovascular disease and all other illnesses rise when good cholesterol reaches very high levels.

The paradox has baffled doctors for years - and Dr Allard-Ratick called for more research to get to identify the mechanism behind it.

He said: "Our results are important because they contribute to a steadily growing body of evidence that very high HDL cholesterol levels may not be protective, and because unlike much of the other data available at this time, this study was conducted primarily in patients with established heart disease.

"While the answer remains unknown, one possible explanation is that extremely elevated HDL cholesterol may represent 'dysfunctional HDL' which may promote rather than protect against cardiovascular disease."

He added: "One thing is certain - the mantra of HDL cholesterol as the 'good' cholesterol may no longer be the case for everyone."

Most of the cholesterol circulating in our blood is made by the liver, mainly from saturated fats.

LDL transports cholesterol from the liver to cells where it is needed for such processes as strengthening cell walls and making hormones.

HDL does the opposite, taking surplus cholesterol from cells back to the liver, where it is recycled or removed from the body in bile.

In the UK, levels are measured in millimoles (mmol) of blood per litre (L). Generally speaking, when it comes to HDL higher numbers are better.

The NHS recommends that total cholesterol should be less than 5mmol/l, with LDL less than 3 and HDL more than 1. There is no recommendation for a maximum level of HDL, as the assumption is that it is 'good'.

But Dr Allard-Ratick said HDL's protective effects appear to reach their maximum when blood levels are roughly 1.1 to 1.5mmol/l.

His participants, most of whom had heart disease and had been recruited from the Emory Cardiovascular Biobank project, were divided into five groups according to their HDL - less than 0.78 mmol/L, 0.8 to 1, 1.1 to 1.3, 1.3 to 1.5 and greater than 1.5.

Only the two groups in the 1.1 to 1.5 range were not at increased risk of heart attack or death from cardiovascular disease.

The average age of the volunteers was 63, and around a third were women. They had been recruited from the Emory Cardiovascular Biobank project, and whom already had heart disease.

Millions of people take statins to lower LDL cholesterol. People who have naturally higher levels of HDL are at lower risk of heart attacks and stroke.

Exercise, giving up smoking and eating plenty of fruit, vegetables and oily fish can all help to raise it. Moderate use of alcohol has been linked with higher levels of HDL cholesterol.

But HDL levels are sometimes improved by drugs such as prescription niacin and certain statins - particularly simvastatin and rosuvastatin.

However, clinical trials for several drugs specifically designed to increase HDL levels were halted early - because they didn't reduce the risk of heart attacks.

Heart disease kills around 66,000 people a year in Britain making it the second biggest killer, behind dementia.