The relationship between lipid levels and the development of dementia is an evolving but confusing landscape.
“This is an incredibly complex area, and there really isn’t a clear consensus on this subject because different lipid classes reflect different things,” according to Betsy Mills, PhD, assistant director of aging and Alzheimer’s prevention at the Alzheimer’s Drug Discovery Foundation.
Some studies suggest that excessive lipid levels may increase the risk of developing dementia and Alzheimer’s disease (AD). Others imply that elevated low-density lipoprotein (LDL) cholesterol or even triglycerides may offer some protection against subsequent dementia whereas higher levels of high-density lipoprotein (HDL) cholesterol, hitherto thought to be protective, may have a deleterious effect.
“It depends on what lipids you’re measuring, what you’re using to measure those lipids, what age the person is, and multiple other factors,” Dr. Mills told this news organization.
Teasing out the variables and potential mechanisms for the association between lipids and dementia risk necessitates understanding the role that lipids play in the healthy brain, the negative impact of brain lipid dysregulation, and the interplay between cholesterol in the central nervous system (CNS) and the cholesterol in the rest of the body.
Beyond Amyloid
The role of lipids in AD risk has historically been “overlooked,” says Scott Hansen, PhD, associate professor, Department of Molecular Medicine, Herbert Wertheim UF Scripps Institute for Biomedical Innovation and Technology, Florida.
“The common narrative is that amyloid is the culprit in AD and certainly that’s the case in familial AD,” he told this news organization. “It’s been assumed that because amyloid deposits are also found in the brains of people with late-onset AD — which is the vast majority of cases — amyloid is the cause, but that’s not clear at all.”
The “limited clinical success” of aducanumab, its “extremely small efficacy” — despite its obvious success in eradicating the amyloid plaques — suggests there’s “much more to the story than amyloid.”
He and a growing community of scientists recognize the role of inflammation and lipids. “The major finding of my lab is that cholesterol actually drives the synthesis of amyloid via inflammation. In other words, amyloid is downstream of cholesterol. Cholesterol drives the inflammation, and the inflammation drives amyloid,” he said.
‘Lipid Invasion Model’
Because the brain is an incredibly lipid-rich organ, Dr. Mills said that “any dysregulation in lipid homeostasis will impact the brain because cholesterol is needed for the myelin sheaths, cell membranes, and other functions.”
A healthy brain relies upon healthy lipid regulation, and “since the first description of AD over 100 years ago, the disease has been associated with altered lipids in the brain,” Dr. Hansen noted.
He cited the “ lipid invasion model” as a way of understanding brain lipid dysregulation. This hypothesis posits that AD is driven by external lipids that enter the brain as a result of damage to the blood-brain barrier (BBB).
“Cholesterol in the brain and cholesterol in the periphery — meaning, in the rest of the body, outside the brain — are separate,” Dr. Hansen explained. “The brain produces its own cholesterol and keeps tight control of it.”
Under normal circumstances, cholesterol from the diet doesn’t enter the brain. “Each pool of cholesterol — in the brain and in the periphery — has its own distinct regulatory mechanisms, target cells, and transport mechanisms.”
When the BBB has been compromised, it becomes permeable, allowing LDL cholesterol to enter the brain, said Dr. Hansen. Then the brain’s own lipoproteins transport the invading cholesterol, allowing it to be taken up by neurons. In turn, this causes neuronal amyloid levels to rise, ultimately leading to the creation of amyloid-b plaques. It also plays a role in tau phosphorylation. Both are key features of AD pathology.
Elevated levels of cholesterol and other lipids have been found in amyloid plaques, Dr. Hansen noted. Moreover, studies of brains of patients with AD have pointed to BBB damage.
And the risk factors for AD overlap with the risk factors for damage to the BBB (such as, aging, brain trauma, hypertension, stress, sleep deprivation, smoking, excess alcohol, obesity, diabetes, and APOE4 genotype), according to the lipid invasion model paper cited by Dr. Hansen.