What is typically referred to as "good cholesterol" is now facing closer scrutiny from researchers. A new study has found a link between a higher risk of infectious diseases and high and low levels of this type of cholesterol alike.

High-density lipoprotein (HDL) cholesterol, also known as "good cholesterol," has thus far been called as such because high levels of it help to "flush" the cholesterol out of the system by carrying it to the liver.

Therefore, high levels of HDL cholesterol reportedly help to lower the risk of heart disease.

Recently, however, scientists have started to question how good HDL cholesterol actually is for health, and one study even found a worrying association between high levels of HDL cholesterol and a higher mortality risk.

Now, researchers working at Copenhagen University Hospital and the University of Copenhagen—both in Denmark—who were led by Professor Børge Nordestgaard have noted that both high and low levels of HDL cholesterol may put our health in danger.

Their study, whose results have now been reported in the European Heart Journal, reveals that high, as well as low, HDL cholesterol is strongly linked to a higher risk of hospitalization due to infectious diseases.

Even more worryingly, it is also tied to a higher risk of death caused by infectious diseases.

"Numerous studies in animals and cells," says study co-author Christian Medom Madsen, "indicate that HDL is of importance for the function of the immune system and thereby the susceptibility to infectious disease, but this study is the first to examine if HDL is associated with the risk of infectious disease among individuals from the general population."

Both high and low HDL levels pose risk

The scientists analyzed the health data of 97,166 people who were enrolled in the Copenhagen General Population Study, as well as that of an additional 9,387 people who participated in the Copenhagen City Heart Study.

All the participants were assessed for HDL cholesterol levels at baseline, and they were followed up for over 6 years, while their health developments were tracked in national health registers.

It was found that 21% of the people who presented the lowest concentrations of HDL cholesterol—as well as 8% of those with the highest levels of this type of cholesterol—had an increased risk of developing infectious diseases, such as gastroenteritis or pneumonia.

Compared with a control group of individuals with normal HDL cholesterol levels, those with very low concentrations of good cholesterol had a 75% higher risk of infectious diseases.

As for those with very high HDL cholesterol levels, they had a 43% higher risk of picking up an infectious disease than their peers from the control sample.

These results surprised and worried the researchers, especially because they also noted that the people with a heightened risk of infectious disease had a similarly high risk of premature death.

"Surprisingly, we found that individuals with both low and high HDL cholesterol had high risk of hospitalization with an infectious disease."

–Professor Børge Nordestgaard

"Perhaps more importantly," he adds, "these same groups of individuals had high risk of dying from infectious disease."

Despite these results, the researchers caution that a clear, causal relationship cannot, for now, be established between high or low HDL cholesterol and predisposition to such diseases.

That is because the current study only noted an association between the two, without looking into any possible underlying mechanisms.

Still, the correlations are strong enough for the study authors to hypothesize that, after a deeper scrutiny, a causational relationship may in fact become apparent.

Therefore, according to the researchers, further investigations should now focus on understanding exactly how HDL cholesterol actually influences the efficacy of the immune system.

"Our findings indicate that, in the future," concludes Professor Nordestgaard, "research into the role and function of HDL should not narrowly focus on cardiovascular disease, but rather focus on the role of HDL in other disease areas, such as infectious disease."