Study Claims High Good Cholesterol Levels Linked to Greater Dementia Risk

High levels of "good" cholesterol in the blood may be associated with an increased risk of dementia, a new study suggests. However, experts are divided on the extent to which this data supports a clear link between blood cholesterol and dementia development.

Dementia is a general term for a decline in memory, language, problem-solving skills and general cognitive function. It affects roughly 5.8 million people in the United States, the majority of whom are over the age of 65, according to the U.S. Centers for Disease Control and Prevention.

Scientists have yet to develop a cure for this disease, although its symptoms can be more easily abated if it is caught early. Therefore, the search for early indicators of dementia is paramount.

By retrospectively analyzing data from a trial involving nearly 20,000 individuals over the age of 65, researchers from Australia, the U.S. and Germany were able to establish a clear link between dementia incidence and high levels of "good" cholesterol in the blood.

Cholesterol is a waxy, fatty molecule that plays an important role in hormone production, digestion, cell membrane integrity and the production of important vitamins such as vitamin D. Cholesterol travels through our bodies via blood and is carried by specialized transportation molecules called lipoproteins.

There are two main types of cholesterol-ferrying lipoprotein: high-density lipoproteins (HDL) and low-density lipoproteins (LDL). Cholesterol carried by HDL is often referred to as "good cholesterol" because the HDLs carry excess cholesterol from the rest of our bodies to our liver, preventing it from building up in our arteries.

LDL cholesterol is what we often refer to as "bad cholesterol" because it carries cholesterol to our arteries, where it can build up in the blood vessels' walls and may eventually lead to a blood clot or blockage.

HDL cholesterol, meanwhile, has been associated with a lower risk of cardiovascular disease. However, new research has linked this "good" cholesterol with several other age-associated conditions, such as macular degeneration, sepsis, fractures, and now—perhaps—dementia.

"We have known for a long time that high levels of HDL-C are beneficial for reducing the risk of heart disease and stroke," Andrew Doig, a professor of Biochemistry at the University of Manchester, who was not involved in the study, told Newsweek. "This work shows that high HDL-C may not be all good, however, if it makes dementia more likely."

In the new study published in the journal The Lancet Regional Health—Western Pacific, the team found that having more than 80mg of HDL-cholesterol in every 100 milliliters of blood was associated with a 27 percent increased risk of developing dementia during the study period, with those in the over 75 group being the most at risk.

"The effect was not large (a 27 percent increase in risk), but it was real," Doig said. "Having a large sample of nearly 20,000 people has allowed meaningful conclusions to be determined on links between high-density lipoprotein cholesterol (HDL-C ) levels and the onset of dementia in the elderly.

"The authors have accounted for numerous confounders that might be affecting their results. These include sex, country, frailty, physical activity, alcohol use, smoking, level of education, other types of cholesterol, intelligence, hypertension, diabetes, chronic kidney disease and some genetic links to dementia."

Doig was quick to point out that this association does not mean that high levels of HDL cholesterol actually cause dementia.

"There might be additional factors that affect both these things, such as a genetic link that we are currently unaware of," he said. "The study also only looked at healthy Caucasians from Australia and the USA, so might not generalize to other groups of people."

Gordon Wilcock, an emeritus professor of Geratology at the University of Oxford, agreed that it is difficult to draw any firm conclusions from these associations.

"This is an important study, but there are conflicting reports about the potential role of this type of cholesterol in relation to dementia, and I think it might turn out to be a marker for something else or not relevant at all," he told Newsweek. "I would not consider altering my lifestyle to adjust my HDL-C level until the evidence is more conclusive. Even if the relationship is confirmed it is likely to be of relevance to only a small number of people."

The observed associations may also not be statistically very significant, David Curtis, a professor at the University College London Genetics Institute told Newsweek.

"It's quite doubtful that this study is demonstrating a real effect. The main claim is that very high HDL-C is associated with increased risk of dementia, but the confidence interval for the hazard ratio very nearly includes 1, implying that the results could quite easily have occurred by chance," Curtis said.

"In fact, when they take account of APOE genotype, a known risk factor for dementia, the confidence interval actually does include 1, and hence the results are not formally statistically significant. This is a post hoc analysis, and if one looks for associations, then one is sure to find some by chance."

Even so, the study may open doors to further research into how the concentrations of certain molecules in the blood could be indicative of early-onset dementia.

"We need to be able to diagnose dementia earlier and more accurately," Doig said. "Including HDL-C levels might improve a diagnostic, though HDL-C levels on their own would not work. More research is needed to try to understand why HDL-C levels and dementia seem to be linked.

"Perhaps there is a disease pathway between the two that we currently don't know about. If so, this might point to new ways to develop drugs against dementia. This is all very speculative, but worth looking at, as we desperately need better treatments for dementia."