"Good cholesterol" or high-density lipoprotein (HDL) is known for its heart health benefits. But can too much of it cause health issues? Researchers say extreme levels of HDL can raise the risk of dementia.

Too much or too little HDL can slightly increase the risk of dementia, according to a new study, published in the medical journal Neurology. The study does not show a causative link to dementia.

Cholesterol is a waxy, fat-like substance made by the liver essential for functions such as hormone synthesis and digestion. Cholesterol travels through two kinds of lipoproteins in the blood: LDL (low-density lipoprotein) and HDL.

High levels of LDL are associated with the risk of heart disease as it builds up plaques and clogs the arteries, leading to heart attacks and stroke. HDL, however, flushes out cholesterol through the liver and is therefore considered to be heart-healthy.

The research team studied associations of HDL cholesterol and LDL cholesterol with dementia in a group of 184,000 participants, who did not have dementia at the beginning of the study. The average age of the participants was 70, and they were followed up for 17 years. Their health behaviors were assessed using surveys, and cholesterol levels were measured during routine health checkups.

During the study, 25,214 people developed dementia. The average HDL level of the participants was 53.7 milligrams per deciliter. Healthy levels of HDL are 40 mg/dL for males and above 50 mg/dL for females. Participants were divided into five groups based on their HDL levels.

"People with the highest levels of HDL cholesterol had a 15% higher rate of dementia compared to those in the middle group. Those with the lowest levels had a 7% higher rate of dementia compared to those in the middle group," the researchers said in a news release.

They took into account other factors such as alcohol use, high blood pressure, cardiovascular disease and diabetes.

Researchers only found a slight association between LDL and dementia.

"Previous studies on this topic have been inconclusive and this study is especially informative because of the large number of participants and long follow-up. This information allowed us to study the links with dementia across the range of cholesterol levels and achieve precise estimates even for people with cholesterol levels that are quite high or quite low," study author Maria Glymour, from Boston University, said in the news release.

The study has limitations as the survey was done on voluntary participants, which may not represent the larger population.

"The relationship between HDL cholesterol and dementia is more complex than we previously thought. While the magnitude of this relationship is relatively small, it's important," said Erin Ferguson, a lead author of the study from the University of California San Francisco.