Good cholesterol may be a bad guy after all!

Scientists are warning that the good cholesterol (HDL) known for it's ability to protect against heart disease, can turn nasty.

Research from the U.S. has shown that that some of the many proteins within HDL are potentially destructive and if too many are present in the molecule they could outweigh the beneficial effects of other proteins.

They say defective forms of high density lipoprotein (HDL), or "good" cholesterol, might make it harmful.

High density lipoprotein (HDL) is the bad side of the low density lipoprotein (LDL), which is in the main one responsible for the build up of hard deposits or plaques on the walls of arteries that can lead to heart disease.

HDL helps prevent such inflammation by cleaning up the arteries by removing cholesterol from their walls, and it may also stop the formation of blood clots caused by plaques rupturing, which cause most heart attacks.

It is assumed that people with high levels of HDL in comparison with LDL have extra protection against heart disease.

People can elevate their levels of HDL by consuming omega-3 fatty acids, found in oily fish, olive oil and other sources of monounsaturated fat, and small amounts of alcohol.

Dr. Jay Heinecke, from the University of Washington School of Medicine in Seattle, the study leader, says simply boosting HDL levels may not be enough to prevent heart disease because the right proteins in HDL might have to be targeted.

Dr. Heinecke's team carried out a detailed analysis of the composition of HDL and found it contained 48 proteins of which 22 were involved in the cholesterol chemistry, and 13 which had not been seen in HDL before.

Dr. Heinecke says HDL is still a big mystery and there is much which is unknown about heart disease.

Heinecke says that heart attacks can occur in people whose cholesterol levels appear normal and that conventional diagnostic tests for cholesterol levels do not always give a clear picture of the disease.

Experts say important interventions for fighting heart disease include exercise, a well-balanced diet, and taking heart medications as prescribed.

The study was funded by the National Institutes of Health and was presented at the annual meeting of the American Chemical Society in Boston.