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,” Mills told theheart.org | Medscape Cardiology.

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 theheart.org | Medscape Cardiology. “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, 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,” 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,” 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 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, 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 Hansen.

“Chicken and Egg”

“There is a strong link between the brain and the heart, and we know that cardiovascular risk factors have an overlap with dementia risk factors — especially vascular dementia,” said Mills. 

She explained that an atherogenic lipid profile results in narrowing of the arteries, with less blood reaching the brain. “This can lead to stress in the brain, which drives inflammation and pathology.”

But cholesterol itself plays an important role in inflammation, Hansen said. In the periphery, it is “part of an integral response to tissue damage and infection”.

In the brain, once cholesterol is synthesized by the astrocytes, it is transported to neurons via the apolipoprotein E (APOE) protein, which plays a role in brain cholesterol homeostasis, Mills explained. Those with the ε4 allele of APOE (APOE4) tend to have faultier transport and storage of lipids in the brain, relative to the other APOE variants.

It’s known that individuals with APOE4 are particularly vulnerable to late-onset AD, Hansen observed. By contrast, APOE2 has a more protective effect. “Most people have APOE3, which is ‘in between,'” he said.

When there is neuronal uptake of “invading cholesterol,” not only is amyloid produced but also neuroinflammatory cytokines, further driving inflammation. A vicious cycle ensues: Cholesterol induces cytokine release; and cytokine release, in turn, induces cholesterol synthesis — which “suggests an autocatalytic function of cholesterol in the escalation of inflammation,” Hansen suggested. He noted that permeability of the BBB also allows inflammatory cytokines from elsewhere in the body to invade the brain, further driving inflammation.

Mills elaborated: “We know that generally, in dementia, there appear to be some changes in cholesterol metabolism in the brain, but it’s a chicken-and-egg question. We know that as the disease progresses, neurons are dying and getting remodeled. Do these changes have to do with the degenerative process, or are the changes in the cholesterol metabolism actually driving the degenerative disease process? It’s probably a combination, but it’s unclear at this point.”

Lipids in Plasma vs CSF

Mills explained that HDL particles in the brain differ from those in the periphery. “In the CNS, you have ‘HDL-like particles,’ which are similar in size and composition [to HDL in the periphery] but aren’t the same particles.” The brain itself generates HDL-like lipoproteins, which are produced by astrocytes and other glial cells and found in cerebrospinal fluid (CSF).

Dyslipidemia in the periphery can be a marker for cardiovascular pathology. In the brain, “it can be an indication that there is active damage going on, depending on which compartment you’re looking at.”

She noted that plasma lipid levels and brain CSF lipid levels are “very different.” Research suggests that HDL in the CSF exhibits similar heterogeneity to plasma HDL, but these CSF lipoproteins present at 100-fold lower concentrations, compared to plasma HDL and have unique combinations of protein subpopulations. Lipidomic analysis studies show that these compartments “get very different readings, in terms of the predominant lipid disease state, and they are regulated differently from the way lipids in the periphery are regulated.” 

In the brain, the cholesterol “needs to get shuttled from glial cells to neurons,” so defects in the transport process can disrupt overall brain homeostasis, said Mills. But since the brain system is separate from the peripheral system, measuring plasma lipids is more likely to point to cardiovascular risks, while changes reflected in CSF lipids are “more indicative of alteration in lipid homeostasis in the brain.”

HDL and Triglycerides: A Complicated Story

Mills noted that HDL in the periphery is “very complicated,” and the idea that HDL, as a measure on its own, is “necessarily ‘good’ isn’t particularly informative.” Rather, HDL is “extremely heterogeneous, very diverse, has different lipid compositions, different classes, and different modifications.” For example, like oxidized LDL, oxidized HDL is also “bad,” preventing the HDL from having protective functions.

Similarly, the apolipoproteins associated with HDL can affect the function of the HDL. “Our understanding of the HDL-like particles in the CNS is limited, but we do understand the APOE4 link,” Mills said. “It seems that the HDL-like particles containing APOE2 or APOE3 are larger and are more effective at transferring the lipids and cholesterol linked to them relative to APOE4-containing particles.” 

Because HDL is more complex than simply being “good,” measuring HDL doesn’t “give you the full story,” said Mills. She speculates that this may be why there are studies suggesting that high levels of HDL might not have protective benefits and might even be detrimental. This makes it difficult to look at population studies, where the different subclasses of HDL are not necessarily captured in depth. 

Mills pointed to another confounding factor, which is that much of the risk for the development of AD appears to be related to the interaction of HDL, LDL, and triglycerides. “When you look at each of these individually, you get a lot of heterogeneity, and it’s unclear what’s driving what,” she said.

An advantage of observational studies is that they give information about which of these markers are associated with trends and disease risks in specific groups vs others. 

“For example, higher levels of triglycerides are associated with cardiovascular risk more in women, relative to men,” she said. And the triglyceride-to-HDL ratio seems “particularly robust” as a measure of cardiovascular health and risk. 

The interpretation of associations with triglycerides can be “tricky” and “confusing” because results differ so much between studies, she said. “There are differences between middle age and older age, which have to do with age-related changes in metabolism and lipid metabolism and not necessarily that the markers are indicating something different,” she said.

Some research has suggested that triglycerides may have a protective effect against dementia, noted Uma Naidoo, MD, director of nutritional and lifestyle psychiatry, Massachusetts General Hospital, and director of nutritional psychiatry at MGH Academy.

This may be because the brain “runs mostly on energy from burning triglycerides,” suggested Naidoo, who is the author of the books Calm Your Mind with Food and This Is Your Brain on Food.

Additionally, having higher levels of triglycerides may be linked with having overall healthier behaviors, Naidoo told theheart.org | Medscape Cardiology.

Mills commented that in middle-aged individuals, high levels of LDL-C and triglycerides are “often indicative of more atherogenic particles and risk to cardiovascular health, which is a generally negative trajectory. But in older individuals, things become more complicated because there are differences in terms of clearance of some of these particles, tissue clearance and distribution and nutrient status. So for older individuals, it seems that fluctuations in either direction—either too high or too low—tend to be more informative that some overall dysregulation is going on the system.” 

She emphasized that, in this “emerging area, looking at only one or two studies is confusing. But if you look at the spectrum of studies, you can see a pattern, which is that the regulation gets ‘off,’ as people age.” 

The Potential Role of Statins

Mills speculated that there may be “neuroprotective benefits for some of the statins which appear to be related to cardiovascular benefits. But at this point, we don’t have any clear data whether statins actually directly impact brain cholesterol, since it’s a separate pool.”

They could help “by increasing blood flow and reducing narrowing of the arteries, but any direct impact on the brain is still under investigation.”

Hansen pointed to research suggesting statins taken at midlife appear to be cardioprotective and may be protective of brain health as well, whereas statins initiated in older age do not appear to have these benefits.

He speculated that one reason statins seem less helpful when initiated later in life is that the BBB has already been damaged by systemic inflammation in the periphery, and the neuroinflammatory process resulting in neuronal destruction is already underway. “I think statins aren’t going to fix that problem, so although lowering cholesterol can be helpful in some respects, it might be too late to affect cognition because the nerves have already died and won’t grow back.”

Can Dietary Approaches Help?

Naidoo said that when looking at neurologic and psychiatric disease, “it’s important to think about the ‘long game’ — how can we improve our blood and cardiovascular health earlier in life to help potentiate healthy aging?”

From a nutritional psychiatry standpoint, Naidoo focuses on nourishing the gut microbiome and decreasing inflammation. “A healthy and balanced microbiome supports cognition, while the composition of gut bacteria is actually drastically different in patients with neurological diseases, such as AD.” 

She recommends a nutrient-dense, anti-inflammatory diet including probiotic-rich foods (such as kimchi, sauerkraut, plain yogurt, and miso). Moreover, “the quality and structure of our fatty acids may be relevant as well: Increasing our intake of polyunsaturated fatty acids and avoiding processed fats like trans fats and hydrogenated oils may benefit our overall brain health.”

Naidoo recommends extra-virgin olive oil as a source of healthy fat. Its consumption is linked to lower incidence of AD by way of encouraging autophagy, which she calls “our own process of “cellular cleanup.'”

Naidoo believes that clinicians’ guidance to patients should “focus on healthy nutrition and other lifestyle practices, such as exercise, outdoor time, good sleep, and stress reduction.” 

Mills notes the importance of omega-3 fatty acids, such as docosahexaenoic acid (DHA) , for brain health. “DHA is a major lipid component of neuronal membranes,” she said. “Because of inefficiencies in metabolism with APOE4, people tend to metabolize more of the lipids on the membranes themselves, so they have higher lipid membrane turnover and a greater need to supplement. Supplementing particularly through diet, with foods such as fatty fish rich in omega-3, can help boost the levels to help keep neuronal membranes intact.”

What This Means for the Clinician

“At this point, we see all of these associations between lipids and dementia, but we haven’t worked out exactly what it means on the individual level for an individual patient,” said Mills. Certainly, the picture is complex, and the understanding is growing and shifting. “The clinical applications remain unclear.”

One potential clinical take-home is that clinicians might consider tracking lipid levels over time. “If you follow a patient and see an increase or decrease [in lipid levels], that can be informative.” Looking at ratios of lipids might be more useful than looking only at a change in a single measure. “If you see trends in a variety of measures that track with one another, it might be more of a sign that something is potentially wrong.” 

Whether the patient should first try a lifestyle intervention or might need medication is a “personalized clinical decision, depending on the individual, their risk factors, and how their levels are going,” said Mills.