A new study suggests that so-called good cholesterol (HDL) may not be as healthy as experts once thought.
A new study published Wednesday in the journal Neurology suggests that high or low levels of high-density lipoprotein (HDL) cholesterol may increase the risk of dementia in older adults. This is further evidence that keeping HDL cholesterol within limits is important for cardiovascular and brain health.
"The relationship between HDL cholesterol and dementia is more complex than we previously thought," said lead study author Erin Ferguson, a graduate student studying epidemiology at the University of California, San Francisco. "Although the magnitude of these relationships is relatively small, they are significant."
The results show a link between HDL cholesterol and dementia, but they do not prove that low or high lipid levels directly cause dementia.
The study, conducted by the National Institute on Aging and the National Institutes of Health, included more than 184,000 adults with an average age of 70. None of them had dementia at the start of the study. The researchers used a combination of surveys and electronic data from the Kaiser Permanente health plan in Northern California to track cholesterol levels, health behaviors and whether someone had dementia for about 13 years. During this time, just over 25,000 people developed dementia.
The Centers for Disease Control and Prevention recommends that people keep total cholesterol levels at about 150 mg/dL, or milligrams per deciliter of blood, and low-density lipoprotein (LDL) levels below 100 mg/dL. Low-density lipoprotein, or low-density lipoprotein (LDL) cholesterol, has long been known for its often fatal effects on the cardiovascular system.
Study participants were divided into three groups based on HDL cholesterol levels, adjusting for other risk factors for dementia, including high blood pressure, heart disease, diabetes and frequency of alcohol consumption.
What is a healthy HDL level?
The mean HDL cholesterol level in the study was 53.7 mg/dL, compared with 40 mg/dL in men and 50 mg/dL in women.
According to the study, people whose cholesterol levels fell significantly below these numbers were more likely to develop dementia.
People whose levels were at least 65 mg/dL (the highest of the three groups) were 15% more likely to develop dementia. Those with the lowest levels, between 11 and 41 mg/dL, had a 7% increased risk compared with the average group.
The researchers found no link between bad cholesterol and the risk of dementia.
Dr. Howard Weintraub, clinical director of the NYU Langone Heart Center for the Prevention of Cardiovascular Disease in New York City, said he was surprised by the results.
"When people have HDL levels of 90 or 100 mg/dL, it's associated with dementia. But not these lower numbers — 63 mg/dL or more," said Weintraub, who was not involved in the study.
Experts have long recognized that not all HDL cholesterol is created equal. How the body uses it and where it is stored—in the brain or elsewhere in the body—makes a difference.
How to improve HDL?
Healthy habits, including exercise, allow HDL cholesterol to be converted into HDL particles. "This allows HDL to perform several important recommended tasks, including removing LDL cholesterol from the arteries and transporting it to the liver, where the body disposes of it," says Dr. Hussein Yassin, an endocrinologist at Keck Medicine. , University of Southern California. Which specializes in how changes in fat metabolism affect a person's risk of developing Alzheimer's disease.
"The function is actually in the molecules, not the cholesterol," he said. "Merely increasing HDL levels does not increase its function."
High levels of HDL cholesterol can cause hardening of the arteries and veins. This effect on the cardiovascular system is the main cause of high HDL cholesterol, which can increase the risk of cognitive impairment; Stroke is one of the biggest risk factors for dementia. In the brain, HDL cholesterol works very differently.
"It works by different rules and has different pathways than regular cholesterol," Yassin said. Systemic cholesterol is cholesterol stored in other parts of the body outside the brain.
He added that the presence of too much HDL cholesterol in the brain can cause inflammation, which prompts the brain to produce amyloid, which are abnormal deposits that damage organs and tissues.
Another recent study found a link between genes associated with high HDL cholesterol and genes that predispose a person to dementia.
Together, the studies point to the fact that levels of good cholesterol (HDL) are complex and an area of research that needs to be better understood.
Weintraub noted that there is not yet enough evidence that HDL cholesterol plays an important role in dementia risk.
"This paper suggests that HDL may be a modifiable risk factor even later in life," Ferguson said. "But I'm not saying people should worry about it yet."
This article was originally published on NBCNews.com.