It has long been thought that a high level of "good" cholesterol in the blood, also known as HDL, is better for health. However, new European research is challenging this belief, suggesting that those with extremely high levels of good cholesterol have a higher mortality rate than people with normal levels.

Carried out by researchers from the University of Copenhagen, the study looked at data for 116,000 subjects from the Copenhagen City Heart Study and the Copenhagen General Population Study, in combination with mortality data from the Danish Civil Registration System.

Participants were followed for an average of 6 years, and the study was based on just over 10,500 deaths.

From the mortality rate based on these deaths and medical information on the participants, the team found that for men with extremely high levels of HDL in the blood, the mortality rate was 106 percent higher when compared to those who had normal levels.

In addition, men in the next group, with very high levels, also had a 36 percent higher mortality rate.

For women with extremely high levels, the mortality rate was 68 percent higher.

However, the team also found excessive mortality for people with extremely low levels of HDL in the blood. Those with medium levels of HDL in the blood had the lowest mortality. For men, this level was 1.9 mmol/L. For women, it was 2.4 mmol/L.

Previous US studies have also shown similar correlations between levels of HDL cholesterol and excessive mortality but in specific population groups. This is the first time excessive mortality has been shown in a more general population.

However, because the study only looked at a statistical correlation between mortality and HDL levels, it doesn't give any reasons why those with extremely high or low HDL levels have higher mortality.

Though it may prompt more questions than answers, study author Børge Nordestgaard noted that "These results radically change the way we understand 'good' cholesterol. Doctors like myself have been used to congratulating patients who had a very high level of HDL in their blood. But we should no longer do so, as this study shows a dramatically higher mortality rate."

Professor Nordestgaard now believes that we need to adjust our thinking about levels of good cholesterol, commenting that, "It appears that we need to remove the focus from HDL as an important health indicator in research, at hospitals and at the general practitioner. These are the smallest lipoproteins in the blood, and perhaps we ought to examine some of the larger ones instead. For example, looking at blood levels of triglyceride and LDL, the 'bad' cholesterol, are probably better health indicators." 

The results can be found in the European Heart Journal.