Despite years of being told that the lower your LDL cholesterol the better, is it possible that levels that are too low might harm you?
Yes, say researchers who now report that women who have excessively low LDL ("bad") cholesterol levels may face a higher risk for a bleeding stroke.
The finding runs counter to government guidelines that state people should strive to keep their LDL levels below 100 milligrams per deciliter (mg/dL).
But after tracking nearly 28,000 women aged 45 and up for two decades, investigators determined that women with LDL levels of 70 mg/dL or lower were more than twice as likely to experience a bleeding ("hemorrhagic") stroke than those with LDL levels in the range of 100 to 130 mg/dL.
A similar dynamic was seen with blood fats known as triglycerides. Women with the lowest fasting triglyceride levels (74 mg/dL or less) also saw their bleeding stroke risk double when compared to women with the highest levels (156 mg/dL and up).
Still, one heart expert urged caution in interpreting the results.
Dr. Gregg Fonarow is co-director of the preventative cardiology program at the University of California, Los Angeles. He stressed that the increased risks "are small in absolute terms," and there is a far greater risk that a patient with high LDL levels might experience a blood clot, an "ischemic" (clot-caused) stroke or a heart attack.
Prior research indicates that "there is not significant increases in hemorrhagic stroke, and the risk of ischemic stroke is significantly reduced," Fonarow noted.
How to explain the latest finding?
"The full mechanisms by which very low levels of LDL cholesterol or low triglycerides increase the risk of hemorrhagic stroke are not known yet," said study author Pamela Rist. She is an assistant professor of medicine at Harvard Medical School and Brigham and Women's Hospital in Boston.
But Rist pointed to the theory that low LDL and triglycerides might weaken blood vessel walls. And that could raise the risk for a bleeding stroke, though the study did not prove a cause-and-effect relationship.
Though far less common than ischemic strokes, bleeding strokes -- which occur when a blood vessel bursts and bleeds into the brain -- are trickier to treat and often more deadly.
Rist said prior studies suggest that the same risk might also affect men, though her team did not explore that possibility.
Rist's team also did not explore the link between very low LDL levels and the risk for experiencing an ischemic stroke, which accounts for 87 percent of all strokes, according to the American Stroke Association.
The findings were reported online April 10 in the journal Neurology.
All of the women in the study had been part of the Women's Health Initiative, a long-term national study that tracked both HDL ("good") and LDL cholesterol levels, as well as triglycerides. Levels were measured just once, at the beginning of the study.
Interestingly, no increased risk was seen with variations in either total cholesterol (LDL and HDL levels combined) or HDL levels alone, the researchers reported.
Rist pointed out that, unlike prior work that explored the role of statins in stroke risk, most women in this study were not taking statins. That means that the cholesterol-lowering drugs likely played no role in driving down LDL levels.
Nevertheless, "these women may have an increased risk of experiencing a hemorrhagic stroke," said Rist. "Therefore, it is important to manage other risk factors for hemorrhagic stroke, such as hypertension [high blood pressure] and smoking."
There's more on cholesterol at the U.S. National Heart, Lung, and Blood Institute.