A study has raised questions about a widely held belief involving cholesterol. The belief is that high levels of so-called good cholesterol in the blood can reduce the risk of a heart attack.

The medical name for "good" cholesterol is high-density lipoprotein, or HDL. Doctors commonly believe it reduces the risk of heart attack by removing fatty deposits that can block the flow of blood in the arteries of the heart.

But Dr. Sekar Kathiresan at Massachusetts General Hospital in Boston says the effects of high HDL have never been proven. He says the new study that he and other researchers did suggests that HDL cholesterol does not affect the risk of heart attack.

"This has been the major assumption over the last thirty years, that if you raise HDL cholesterol, the risk for heart disease will be lowered. And I think that we have now broken that assumption, broken that link using this gene variant."

The researchers say about four percent of people have a genetic change that gives them a naturally high level of HDL. The study found that those who were genetically "programmed" to have higher HDL levels were just as likely to suffer heart attacks as those who were not.

A second analysis looked at fourteen gene variants that increase good cholesterol. Dr. Kathiresan says people with the most variants had no more protection against heart attacks than anyone else.

But he says there is no dispute about the effects of low-density lipoprotein, or LDL. LDL, the so-called bad cholesterol, causes a buildup of fatty substances inside arteries and increases the risk of heart attack.

Dr. Kathiresan and his colleagues published their findings in the Lancet medical journal.