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.

The researchers tested gill tissue from crabs harvested in six different years from the Chesapeake Bay and the gulf coast of Florida. Specimens from each region produced similar results, confirming that the presence of AEP in males and absence in females is the norm for blue crabs.

But, Kleps said, it was still not possible to rule out that the difference between the sexes was due to a difference in their diet. Fortunately, while writing the first draft of the paper, Kleps happened to read that a rare gynandromorphic blue crab -- one half male, one half female -- had been captured by Romuald Lipcius of the Virginia Institute of Marine Science at the College of William & Mary.

The rare gynandromorph is divided down the middle, with a characteristic blue male claw and a female red claw. The underside of the crab is also visibly divided into male and female halves. After the crab died, Lipcius sent Kleps gill tissue from each side for analysis. The measured levels of AEP from the male and female gills provided additional evidence that AEP is a sex-specific compound.

"Since both sides of this strange crab have, of necessity, shared a diet and environment, we had completely independent confirmation of the sex-specific nature of this metabolite," said Kleps.

"That blue crabs have this sex-specific compound may be a fluke, or it might represent a common but overlooked process in animal development," he said.

uic