Cholesterol is carried in the blood by proteins and together are called lipoproteins.
There are two main forms, lowdensity lipoprotein (LDL) and highdensity lipoprotein (HDL).
LDL cholesterol is often referred to as “bad cholesterol” because too much can lead to disease of the arteries. On the other hand, HDL carries cholesterol away from the cells and back to the liver.
A doctor can measure your levels of both and the NHS currently has no recommendation for a maximum level of HDL, the assumption being that it is “good”.
But the 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.
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.”
The study followed 5,965 people for an average of four years and found those in the middle range of HDL levels were least likely to have a heart attack or die from cardiovascular disease.
However, risk soared in those with low levels and very high levels.
Dr Allard-Ratick told a meeting of the European Society of Cardiology in Munich the danger rose by “nearly 50 per cent” among the latter group.
He said: “One possible explanation is that extremely elevated HDL may represent ‘dysfunctional HDL’ which may promote rather than protect against cardiovascular disease.”
The findings could explain why repeated trials of drugs that boost HDL have failed to cut cardiovascular disease.
Heart disease kills around 66,000 people a year in Britain, making it the second biggest killer behind dementia.