Cholesterol consists of low-density lipoprotein cholesterol (LDL-C) and high-density lipoprotein cholesterol (HDL-C). High levels of LDL-C cause cholesterol to be deposited on the walls of blood vessels, which can lead to cardiovascular disease. HDL-C is also known as "good cholesterol" because it protects blood vessels, prevents the deposition of LDL-C on blood vessel walls, and reduces the risk of cardiovascular disease.
The association between high levels of HDL-C and a lower risk of cardiovascular disease has been well documented. However, recent studies have shown that high levels of HDL-C are also associated with a range of adverse health conditions, such as the risk of all-cause mortality, age-related macular degeneration, sepsis, and fracture risk.
Recently, researchers from Monash University published an article in The Lancet Regional Health-Western Pacific entitled "Association of plasma high-density lipoprotein cholesterol level with risk of incident dementia: a cohort study of healthy older adults". The study found that abnormally high levels of HDL-C were associated with an increased risk of dementia in older adults.
HDL-C is a complex of lipids and apolipoproteins, and within the central nervous system, HDL transports lipids to neurons, supports the synthesis of new cell membranes, and plays an important role in restoring synaptic connectivity in neurodegenerative diseases. However, when at very high levels in plasma, the structural components and roles of HDL-C are altered and may pose health risks. For example, several recent studies have shown associations between abnormally high levels of HDL-C and all-cause mortality, age-related macular degeneration, sepsis, and fractures. However, the association between it and dementia risk is unknown.
This study aimed to determine whether high plasma levels of HDL-C are associated with an increased risk of dementia in initially healthy older adults. Of the 18,668 participants, 2,709 had very high levels of HDL-C at the start of the study, with 38 cases of dementia in participants under 75 years of age and 101 cases of dementia in participants 75 years of age and older.
The study showed that over an average of 6.3 years, participants with very high levels of HDL-C (>80 mg/dL or >2.07 mmol/L) at the start of the study had a 27% higher risk of dementia than those with optimal levels of HDL-C, and for those of them aged 75 years and older, the figure was 42%.
Dr. Monira Hussain, first author of the paper and Senior Research Fellow at Monash University's School of Public Health and Preventive Medicine, said, "While we know that HDL-C is important for cardiovascular health, this study suggests that further research is needed to understand the role of very high levels of HDL-C in brain health. These findings could help us better understand the mechanisms behind dementia, but we need further research to explain why very high levels of HDL affect dementia risk."
The very high levels of HDL-C associated with dementia risk in this study were uncommon, and this level may not be related to diet but more likely reflects a metabolic disorder, the team said. The findings may also help doctors identify a class of older adults who may be at risk for dementia, especially those aged 75 and older.