‘Good’ Cholesterol May Not Be So Heart-Healthy After All, Study Finds

Researchers say having high HDL does not protect against heart disease.

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This article originally appeared on Health.com.

If you care about heart health (and really, who doesn’t?), you better sit down. The so-called “good” cholesterol, HDL, may not be so great for your health after all.

A large new study of people in the general population found that those who have the highest HDL cholesterol aren’t less likely to have heart attacks or strokes; and that while people with low levels of HDL are more likely to have those health troubles, it’s likely due to lifestyle habits and other risk factors. The study, released Monday in the Journal of the American College of Cardiology, tracked more than 631,000 Canadians, ages 40 and up, for about five years.

The findings are surprising, since it was thought that boosting HDL levels in addition to lowering “bad” LDL was best for a healthy heart and plaque-free arteries. HDL, which stands for high-density lipoprotein, is a fat-carrying protein known to ferry cholesterol out of artery walls.

But this isn’t the first time doubt has been cast on the idea of HDL as “good” cholesterol. In August, a study on 1.7 million U.S. veterans also found that people with high HDL had increased rates of death from all causes. Meanwhile, clinical trials for drugs that raise HDL have failed to significantly improve health for participants. Now, these studies may hold a clue as to why.

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The researchers found that low levels of HDL were bad (which was known), but that the highest levels were bad for your health too—which is new. People in the study with low HDL tended to be obese, to smoke, and to have poor diet and exercise habits. And they were more likely to have low incomes, and high triglycerides (another important aspect of total cholesterol).

Compared to people with HDL in the middle range, those with low levels (less than 50 mg/dl in women and less than 40 mg/dl in men) were more likely to die over the course of the study. That was true for heart- or stroke-related deaths and other causes, such as cancer. These link remained even after the authors controlled for unhealthy lifestyle factors, although the authors say they couldn’t factor out every potential contributor. (Most experts recommend an HDL level over 40 mg/dL and “the higher, the better.”)

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The biggest surprise came when they looked at people with high HDL. The risk of heart- or stroke-related death didn’t drop for people with the highest HDL levels versus those in the middle. And those with the highest HDL levels (greater than 70 mg/dl in men and greater than 90 mg/dl in women) actually had an increased risk of death from non-heart or stroke-related causes.

The reason for this increased risk is unknown, the authors say. Previous research suggests that alcohol could play a role, but here, the link remained even after the researchers controlled for heavy drinking.

Overall, the authors concluded that HDL may not be the best measure of heart disease risk—and that interventions to raise HDL may not, in fact, reduce that risk as once hoped. Rather, their findings suggest that HDL “is a marker of poor general health” and changing the number itself, say with medication, may not make any difference.

“The link between good cholesterol and heart disease is complex,” said lead author Dennis T. Ko, MD, associate professor at the Institute for Clinical Evaluative Sciences in Toronto, in a press release. “But it seems certain that there is a connection between people with low good cholesterol levels and other well-known risk factors for heart disease such as poor diet and exercise habits and other medical conditions.”

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“Focusing on raising HDL is likely not going to help these patients,” he added, “but these findings show that one of the best interventions in treating and preventing heart disease continues to be lifestyle changes.”

So, instead of achieving a specific HDL, you should focus on quitting smoking, exercising and having a healthy weight—all factors with ironclad evidence that they help your heart.

The authors acknowledge that their study had limitations, including an inability to look at different subclasses and particle sizes of HDL molecules. An editorial published with the study agrees that further research in these areas could potentially show new ways in which HDL actually does relate to heart disease.