A heart surgeon at work. Although higher HDL cholesterol levels are correlated with better heart health, efforts to show that HDL cholesterol has a protective effect in humans have come up empty-handed. REUTERS

'Good cholesterol' or HDL may not be as 'good' as believed in its ability to protect against heart disease. A study looking at the correlation between levels of the cholesterol and risk for cardiovascular disease has turned up with a dubious link.

Comparing 852 people with high levels of HDL (high-density lipoprotein) cholesterol in their blood against a control group of 1,156 people with low HDL cholesterol, the research showed that a protein mutation in the genome caused some to have high HDL as also a higher risk for coronary heart disease.

"When I started medical school in 1992, I was taught that anything that raised HDL cholesterol must be good for you," says Sekar Kathiresan, a preventative cardiologist at the Massachusetts General Hospital in Boston, and a co-author of the study. "We can now safely disregard that notion."

The work showed mutations in a protein called SR-BI that binds to HDL cholesterol and triggers its movement from the blood into the liver. Kathiresan and his colleagues found 19 people with at least one copy of the mutation in SR-BI. Sixteen of those participants also had high HDL cholesterol levels. One woman was the first person found to have two copies of the mutation.

While studies have proved that low levels of LDL cholesterol can prevent heart attacks, the role of HDL has been hazy. While high levels have been correlated with good heart health, conclusive evidence of protection has been evasive, writes Nature.

Earlier work has shown that mice that lacked SR-BI had high levels of HDL cholesterol in their blood but the mice had high rates of plaque build-up in their arteries, a condition known as atherosclerosis.

Others point out that SR-BI could have other functions not yet characterized, and that many animal studies show a role for HDL cholesterol in protecting against heart disease. But what the present study asks is how higher levels of HDL cholesterol are associated with reduced heart risk. It could lie in other functions of HDL cholesterol like a better clean up of triglycerides from the blood, suspects Kathiresan.

The debate on cholesterols continues. Last year the US FDA removed dietary cholesterol from its list of health concerns, much to the joy of meat eaters and dairy product consumers. Dietary intake of cholesterol only moderately influences blood cholesterol levels, a study had shown.

Blood cholesterol levels are still a risk factor for cardiovascular diseases but only about 20 per cent of cholesterol levels in blood comes from diet, and the rest is produced by the liver and is needed by the body. Cholesterol makes cell membranes flexible and is needed to make hormones like testosterone and oestrogen.