By and large, the medical community suggest that higher levels of the good kind of cholesterol are desirable, as it may protect against heart disease and stroke. By contrast, it is the "bad" cholesterol, or low-density lipoprotein (LDL), that blocks the arteries.
The new research challenges this belief - at least in part. As the authors note, this is the first time that a study has drawn a connection between high HDL cholesterol levels and excessive mortality in the general population.
The study was carried out by researchers from the University of Copenhagen in Denmark. Prof. Børge Nordestgaard, of the university's Department of Clinical Medicine, is the corresponding author of the study, and Christian M. Madsen, of the university's Department of Clinical Biochemistry, is the paper's first author.
'Good' cholesterol raises mortality risk
Madsen and colleagues combined data from the Copenhagen City Heart Study, the Copenhagen General Population Study, and the Danish Civil Registration System.
In total, they examined data on more than 116,000 people and clinically followed them for an average period of 6 years, during which time more than 10,500 people died.
Blood tests for both types of cholesterol levels were taken non-fasting, and statistically, the researchers adjusted for all known variables that are normally associated with all-cause mortality. Such factors included age, body mass index (BMI), smoking - both current and cumulative - alcohol consumption, physical activity, and diabetes.
Overall, 0.4 percent of the men and 0.3 percent of the women had extremely high levels of HDL in their blood. Extreme levels were defined as equal to or higher than 3.0 millimoles per liter for men, and equal to or higher than 3.5 millimoles per liter for women.
The study found that men with extreme levels of HDL in their blood had a 106 percent higher chance of dying prematurely than men with normal levels of this type of cholesterol. Women with extremely high levels of HDL cholesterol were 68 percent more likely to die prematurely than women with normal levels.
Additionally, the mortality rate in men with "very high" levels of the supposedly good kind of cholesterol also had a 36 percent higher mortality rate than men with normal levels.
The lowest mortality rate was found in those with medium levels of HDL. This was defined as 1.9 millimoles per liter for men and 2.4 millimoles per liter for women.
Prof. Nordestgaard spoke to Medical News Today about the significance of the findings, saying, "Until now everybody has believed that the higher the HDL cholesterol the better."
"But we now show that individuals with extremely high HDL cholesterol have a higher mortality rate than those with average levels. People and doctors should stop thinking that HDL cholesterol is 'good' cholesterol and that it may be advantageous to raise HDL cholesterol."
Prof. Børge Nordestgaard
"Rather," Prof. Nordestgaard continued, "they should focus on reducing LDL and remnant cholesterol (=cholesterol in triglyceride-rich lipoproteins) with lifestyle changes or drugs."
He cautions that as the study was observational, it cannot explain causality. But he suggests that future studies should try "to understand why people with the very highest HDL cholesterol have increased mortality."