Abnormally high levels of HDL-cholesterol, commonly known as 'good cholesterol', may also increase the risk of dementia in older adults, according to a study.

Researchers from Monash University in Australia explained that the very high levels of HDL-C were uncommon and not diet related, but more likely to reflect a metabolic disorder.

"While we know HDL cholesterol is important for cardiovascular health, this study suggests that we need further research to understand the role of very high HDL cholesterol in the context of brain health," said Monira Hussain, a research student at Monash's School of Public Health and Preventive Medicine.

"It may be beneficial to consider very high HDL cholesterol levels in prediction algorithms for dementia risk," she added, calling for more research.

The findings, published in The Lancet Regional Health – Western Pacific, may help doctors to recognise a group of older patients potentially at risk of dementia, particularly those aged 75 and older.

Over an average of 6.3 years, participants with very high HDL-C (more than 80 mg/dL or less 2.07 mmol/L) were observed to have a 27 per cent higher risk of dementia compared to participants with optimal HDL-C levels, while those aged 75 years and older also showed a 42 per cent increased risk compared to those with optimal levels.

Very high HDL-C levels were categorised as 80 mg/dL (more than 2.07 mmol/L) or above. The optimal level of HDL-C of 40 to 60 mg/dL (1.03-1.55 mmol/L) for men and 50 to 60 mg/dL (1.55-2.07 mmol/L) for women was generally beneficial for heart health.

Among 18,668 participants included in this analysis, 2,709 had very high HDL-C at study entry, with 38 incidents of dementia in those aged less than 75 years with very high levels, and 101 in those aged 75 and more with very high levels.