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Having high levels of so-called “good” cholesterol could increase the risk of dementia, research suggests.
Scientists tracked rates of this memory-robbing condition among 185,000 people who were followed for nearly a decade.
Volunteers with the highest HDL cholesterol (considered good) were 15 percent more likely to suffer from dementia than those with average readings.
Meanwhile, participants with the lowest levels faced a 7 percent higher risk.
High-density lipoprotein (HDL) cholesterol is considered good because of the large amount of evidence showing that it protects the heart.
In the image: a cholesterol plaque in an artery.
It removes cholesterol from the arteries and delivers it to the liver to be broken down and removed from the body, reducing the risk of heart attacks and strokes.
But research has since begun to challenge the traditional view that more is better when it comes to HDL levels.
Some have shown that circulating too much 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 a cause, found no link between dementia and levels of low-density lipoproteins (LDL), the bad kind.
The academics acknowledged their findings were “unexpected”, saying they show there is a “complex” link between HDL cholesterol and dementia.
The researchers recruited 184,367 participants age 70, on average, at the start of the study who did not have dementia.
For two years, they completed surveys about their behaviors and had their cholesterol measured regularly. Her health records were tracked for nine years, on average.
Findings published in the journal Neurology show that 25,214 developed dementia over the course of the study.
HDL cholesterol levels were 53.7 milligrams per deciliter (mg/dL) on average.
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 between 11 and 41 mg/dL.
The results took into account other factors that could affect dementia risk, such as alcohol consumption, blood pressure, cardiovascular disease and diabetes.
The team did not suggest any potential mechanism to explain their findings.
Study author Professor Maria Glymour said: “The elevation in dementia risk with both high and low HDL cholesterol levels was unexpected.”
“But these increases are small and their clinical significance is uncertain.
“In contrast, we found no association between LDL cholesterol and dementia risk in the overall study cohort.
“Our results add to the evidence that HDL cholesterol has similarly complex associations with dementia, as well as 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 bodily functions, including digestion, the production of vitamin D and hormones.
However, too much can accumulate in the blood due to the consumption of fatty foods, lack of exercise, being overweight, smoking and alcohol consumption.
Having high LDL cholesterol does not trigger any symptoms, but it can cause clogged blood vessels 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 having high cholesterol in midlife is linked to an increased risk of developing dementia.