We all know lowering 'bad cholesterol' - known as Low-Density Lipoprotein or LDL - helps the heart.But scientists say raising levels of 'good cholesterol' may not protect you from cardiac disease.
In a challenge to conventional wisdom, a team from Harvard Medical school found no direct link between raising good cholesterol levels - or HDL - with a lower risk of a heart attack.
The study published in the medical journal The Lancet compared heart-attack risk among people who inherited known genetic variants that gave them higher HDL levels.
This should mean they had a lower risk of coronary disease. However, the study of more than 50,000 people found no such link.
This implies that it is best to focus on lowering the levels of LDL in order to tackle heart disease.
'Ways of raising HDL cholesterol might not reduce risk of myocardial infarction,' Dr Sekar Kathiresan from Massachusetts General Hospital, who led the study, said.
'With drugs or lifestyle changes to raise HDL, we cannot automatically assume that risk of myocardial infarction will be reduced,' he added.
HDL is known as 'good' cholesterol because its higher concentrations have been associated with lower risk of heart attacks in some studies, but its exact mechanism has always been uncertain.
In the new research, scientists studied genes involved in raising HDL in about 170,000 individuals and found that 15 HDL-raising genetic variants they tested do not reduce the risk of heart attack.
It was found that there was no difference in heart attack risk of individuals who carried genes involved in elevated HDL than those without the genetic variant.
'It is an interesting study which goes against prevalent evidence about HDL. Increasing HDL, in any case, is difficult, whether by lifestyle or exercise. So our primary target is lowering LDL cholesterol,' Dr Anoop Misra, head of Centre of Internal Medicine at Fortis Hospital, said.
Dr D. Prabhakaran, executive director, Centre for Chronic Disease Control, said: 'Heart attack is multifactorial and not confined to one single risk factor like low HDL.
'While understanding genetics to develop new drugs may be useful, it would be wrong not to address other risk factors such as high blood pressure, high blood glucose, obesity and tobacco.'