Too much & # 39; good & # 39; cholesterol can increase the risk of a heart attack by almost 50 percent, according to a new study.
Researchers discovered that the blood fat changes in large quantities from healthy to harmful – with higher mortality rates.
They say this could explain why repeated studies with drugs that stimulate HDL (high-density lipoprotein) have failed to reduce cardiovascular disease.
A study of nearly 6,000 elderly people found that people with a very high level were so susceptible to a heart attack or death as those who did not have enough.
It contributes to a growing number of evidence that you can really have too much of a good thing when it comes to cholesterol.
Author Dr. Marc Allard-Ratick, from Emory University in the United States, said: "It might be time to change the way we view HDL cholesterol.
"Traditionally, doctors have told their patients that the higher your" good "cholesterol, the better.
"However, the results of 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, and found that those in the middle range of HDL levels were least likely to have a heart attack or died of cardiovascular disease.
On the other hand, the risk increased in both those with low levels (less than 41 mg / dl) and very high levels (more than 60 mg / dl) – producing a "U-shaped curve" when shown graphically.
Dr. Allard-Ratick told a meeting of the European Society of Cardiology in Munich that the danger with "almost 50 percent" increased under the last group.
It supports several earlier major population studies, including a study by the University of Cambridge, which shows deaths from cardiovascular disease and all other diseases increase when good cholesterol reaches a very high level.
The paradox has baffled doctors for years – and Dr. Allard-Ratick called for more research to identify the mechanism behind it.
He said: "Our results are important because they contribute to a steadily growing number of evidence that very high HDL cholesterol levels may not be protective, and because this study, unlike many other available data at this time, was mainly performed in patients with established heart disease.
"Although the answer remains unknown, a possible explanation is that extremely elevated HDL cholesterol & # 39; dfunctional HDL & # 39; can be used instead of protection against cardiovascular disease. & # 39;
He added: "One thing is certain: the mantra of HDL cholesterol because the" good "cholesterol can no longer apply to everyone. & # 39;
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 processes such as strengthening cell walls and making hormones.
HDL does the opposite and takes excess cholesterol from cells back to the liver, where it is recycled in the bile or removed from the body.
In the UK, the levels are measured in millimoles (mmol) of blood per liter (L). In general, if it comes to HDL, higher numbers are better.
The NHS recommends that the total cholesterol should be less than 5 mmol / l, with LDL of less than 3 and HDL more than 1. There is no recommendation for a maximum level of HDL, because the assumption is that & # 39; well & # 39; is.
But Dr. Allard-Ratick said that the protective effects of HDL seem to reach their maximum when the blood level is about 1.1 to 1.5 mmol / l.
Its participants, most of whom had heart disease and who were recruited from the Emory Cardiovascular Biobank project, were divided into five groups based on 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 range of 1.1 to 1.5 had no increased risk of heart attack or death due to cardiovascular disease.
The average age of the volunteers was 63 and about a third was female. They had been recruited from the Emory Cardiovascular Biobank project and already had heart disease.
Millions of people take statins to lower LDL cholesterol. People with naturally higher levels of HDL have a lower risk of heart attacks and strokes.
Exercise, quitting smoking and eating a lot of fruits, vegetables and fatty fish can all help to raise it. Moderate alcohol use has been associated with higher levels of HDL cholesterol.
But HDL values are sometimes improved by drugs such as niacin on prescription and certain statins – especially simvastatin and rosuvastatin.
However, clinical trials for various drugs specifically designed to increase HDL levels were discontinued early because they did not reduce the risk of a heart attack.
Heart disease kills about 66,000 people a year in Britain making it the second largest killer, behind dementia.