Having high levels of so-called ‘good’ cholesterol may increase the risk of dementia, research suggests.
Scientists tracked the frequency of memory theft among 185,000 people who were followed for almost a decade.
Volunteers with the highest HDL cholesterol – considered the good kind – were 15 percent more likely to develop dementia than volunteers with average levels.
Meanwhile, participants with the lowest levels faced a 7 percent increased risk.
High-density lipoprotein cholesterol (HDL) is considered good because of the abundance of evidence showing that it protects the heart.
Pictured: a cholesterol plaque in the artery
It removes cholesterol from the blood vessels and transports it to the liver where it is broken down and removed from the body, reducing the risk of heart attacks and strokes.
But since then, research has begun to challenge the traditional view that more is better when it comes to HDL levels.
Some have shown that too much circulating in the blood can contribute to inflammation.
Others suggest that HDL clogs arteries when levels are too high, which can reduce blood flow to the brain and increase the risk of dementia.
The new Boston University study, which is purely observational and cannot prove cause, found no link between dementia and levels of low-density lipoprotein (LDL) – the bad kind.
Academics acknowledged their findings were ‘unexpected’ and said they show there is a ‘complex’ link between HDL cholesterol and dementia.
Researchers recruited 184,367 participants with an average age of 70 who did not have dementia at the start of the study.
For two years, they completed surveys about their behavior and had their cholesterol measured regularly. Their health data was kept for an average of nine years.
Findings published in the journal Neurology show that 25,214 people developed dementia over the course of the study.
HDL cholesterol levels averaged 53.7 milligrams per deciliter (mg/dl).
Levels above 40 mg/dl for men and 50 mg/dl for women are considered healthy.
Participants with the highest levels were above 65md/dL. At the other end of the scale, the lowest levels were grouped at 11 to 41 mg/dL.
The results took into account other factors that can influence the risk of dementia, such as alcohol consumption, blood pressure, cardiovascular disease and diabetes.
The team did not suggest any possible mechanism to explain their findings.
Study author Professor Maria Glymour said: ‘The increase in dementia risk with both high and low levels of HDL cholesterol was unexpected.
‘But these increases are small and their clinical significance is uncertain.
‘In contrast, we found no association between LDL cholesterol and the risk of dementia in the overall study cohort.
‘Our results add to the evidence that HDL cholesterol has as complex associations with dementia as it does with heart disease and cancer.’
The researchers noted that those who participated in the study were volunteers, meaning they may not be representative of the population.
Cholesterol is a fatty, waxy substance found in the blood that is vital for body functions including digestion, making vitamin D and hormones.
However, too much can build up in the blood due to eating fatty foods, insufficient exercise, being overweight, smoking and drinking alcohol.
High LDL cholesterol does not cause any symptoms, but it can lead to clogged arteries and increase the risk of heart problems and stroke.
Meanwhile, HDL cholesterol has been shown to protect against heart disease, with the NHS stating that levels above 18 mg/dl for men and 21.6 mg/dl for women are healthy.
Previous studies have suggested that high cholesterol in middle age is linked to a higher risk of developing dementia.