What is Alzheimer’s disease?
Alzheimer’s disease is a form of dementia, a general term for neurological conditions that impair cognitive function to the point of interfering with daily life. It is the most common type of dementia (representing around 60 to 80% of cases) and involves a progressive degeneration of brain cells. Alzheimer’s disease is currently the fifth-leading cause of death for people older than 65 in the United States.[1]
What are the main signs and symptoms of Alzheimer’s disease?
One of the first symptoms of Alzheimer’s disease is memory loss, especially short-term memory loss of events and experiences.[8] Other common symptoms of early disease include worsening verbal memory, impaired problem solving, lack of motivation, depression, and sleep disorders. In later stages of disease, memory and cognition continue to worsen, causing disability and deterioration of motor function, in addition to significant personality changes, behavioral changes, and psychosis.
How is Alzheimer’s disease diagnosed?
A conclusive diagnosis of Alzheimer’s disease requires examining the brain during autopsy, meaning it can’t be definitively diagnosed in someone while they are alive. Still, doctors can make a possible or probable diagnosis based on signs and symptoms, performance on neurocognitive tests (which assess things like memory, concentration, and reasoning), and the exclusion of other causes. More recently developed diagnostic methods using positron-emission tomography (PET) scans can identify amyloid accumulation (an indicator of Alzheimer’s disease) with a high level of accuracy, but this testing remains limited and is used primarily for research purposes at this time.[9]
What are some of the main medical treatments for Alzheimer’s disease?
No medical treatments have been clearly shown to stop or slow the underlying disease process in Alzheimer’s disease. However, some medication can improve symptoms. Acetylcholinesterase inhibitors can improve cognitive function in people with Alzheimer’s disease, although the effect is generally small.[14]
In 2020, a medication called aducanumab was approved by the FDA for the treatment of Alzheimer’s disease based on its effects on amyloid plaques, but whether aducanumab improves clinical outcomes (e.g., cognitive function) remains to be established.[15]
How could diet affect Alzheimer’s disease?
A few dietary patterns are associated with a lower risk of Alzheimer’s disease. This protective relationship is perhaps most commonly found with a Mediterranean diet. Among individual components of the diet, a higher intake of saturated fat and sugar has been linked to a higher risk of Alzheimer’s disease, while a higher intake of fish has been linked to a lower risk. Unfortunately, these findings derive from observational studies, meaning cause and effect can’t be determined with certainty, and there are almost no randomized clinical trials (RCTs) looking at the effect of diet on the incidence of Alzheimer’s disease.
Have any supplements been studied for Alzheimer’s disease?
RCTs have assessed a number of supplements’ effects on established Alzheimer’s disease. This includes omega-3s, B vitamins (usually folic acid, vitamin B6, and vitamin B12), vitamin D, vitamin E, ginkgo biloba, L-alpha glycerylphosphorylcholine (alpha-GPC), Lion's mane mushroom, ginseng, saffron, and sodium oligomannate. For the most part, these supplements have only conflicting or limited evidence in favor of their use.
Are there any other treatments for Alzheimer’s disease?
Exercise and physical activity have been shown to benefit physical function, functional independence, and neuropsychiatric symptoms in people with Alzheimer’s disease; aerobic exercise specifically improved cognitive function.
Several trials have investigated the effect of a keto diet and ketogenic supplements (e.g., medium-chain triglycerides) on Alzheimer’s disease and observed improvements — sometimes, but not always — in quality of life and cognitive function.
Music therapy centers around listening to music to stimulate the brain. Some research suggests the intervention can improve mood and cognitive function among people who have Alzheimer’s disease.
What causes Alzheimer’s disease?
The underlying cause of Alzheimer’s is not known for certain, but several explanations have been proposed. The most common theory is that Alzheimer’s disease is the result of neurodegeneration and a buildup of misfolded proteins called amyloid plaques and tau tangles, which are almost always found in the brains of people with the disease. However, this “amyloid hypothesis” has been criticized. Regardless of the cause of Alzheimer’s disease, various modifiable and nonmodifiable factors, including aging, depression, diabetes, and genetics, have been linked to a higher risk of the disease.
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Frequently asked questions
Alzheimer’s disease is a form of dementia, a general term for neurological conditions that impair cognitive function to the point of interfering with daily life. It is the most common type of dementia (representing around 60 to 80% of cases) and involves a progressive degeneration of brain cells. Alzheimer’s disease is currently the fifth-leading cause of death for people older than 65 in the United States.[1]
Alzheimer’s disease is a neurological disorder involving cognitive and functional impairments that almost always worsen with time. The disease is typically first identified by memory loss and forgetfulness, with episodic (personal experiences and events) and semantic (facts and words) memory usually impaired first. Short-term memory also becomes limited, with a smaller capacity and shorter duration. Procedural memory — how to perform functions and tasks — is not affected until much later.[2][3]
As the disease progresses and different areas of the brain degenerate, additional symptoms develop, including mood changes, difficulty with language, trouble performing basic activities, and, in later stages, an impaired ability to swallow and breathe.
Prior to developing Alzheimer’s disease, individuals first develop a less extreme disorder called mild cognitive impairment (MCI). Although MCI is common during older age (a time when cognitive function declines to some extent), with Alzheimer’s disease, memory problems exceed what is expected based on age. Despite MCI being a risk factor, people who develop the disorder don’t usually progress to Alzheimer’s disease[4], and there does not seem to be evidence that screening older adults for early MCI is beneficial.[5]
Once a person is diagnosed with Alzheimer’s disease, the progression of cognitive decline is about a 3-point reduction per year on the 30-point Mini–Mental State Examination (MMSE).[6][7]
In short, we don’t know as much as we would like. Genetic mutations and exposure to risk factors (e.g., cardiometabolic disorders) may slowly start to put changes into motion from birth until 40 year of age, while symptoms may not be observed until after 60 years of age. This gives a vague window from 20–60 years of age for the modification of misfolding and/or aggregation of amyloid-beta and/or tau proteins that may begin producing plaques and tangles that trigger oxidative stress and inflammation, ultimately leading to cell death and neurodegeneration. There is likely much more nuance to this process, including the influence of diet[19] and lifestyle[20] that can prevent and/or treat these characteristic modifications of AD, but more research is needed to completely understand neurodegenerative disease.
One of the first symptoms of Alzheimer’s disease is memory loss, especially short-term memory loss of events and experiences.[8] Other common symptoms of early disease include worsening verbal memory, impaired problem solving, lack of motivation, depression, and sleep disorders. In later stages of disease, memory and cognition continue to worsen, causing disability and deterioration of motor function, in addition to significant personality changes, behavioral changes, and psychosis.
A conclusive diagnosis of Alzheimer’s disease requires examining the brain during autopsy, meaning it can’t be definitively diagnosed in someone while they are alive. Still, doctors can make a possible or probable diagnosis based on signs and symptoms, performance on neurocognitive tests (which assess things like memory, concentration, and reasoning), and the exclusion of other causes. More recently developed diagnostic methods using positron-emission tomography (PET) scans can identify amyloid accumulation (an indicator of Alzheimer’s disease) with a high level of accuracy, but this testing remains limited and is used primarily for research purposes at this time.[9]
All forms of dementia — Alzheimer’s disease included — share overarching diagnostic criteria. According to the fifth edition of the Diagnostic and Statistical Manual of Mental Disorders (DSM-5), a diagnosis of dementia (which they classify as a “major neurocognitive disorder”) requires meeting the following criteria (using a person’s medical history and cognitive assessment):
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An impairment in one or more cognitive domains (e.g., executive function, language, memory.)
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A decline in cognitive ability compared with past levels of functioning
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An impairment in the ability to function independently in daily life
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Symptoms not occurring as a result of delirium
In addition to a dementia diagnosis, conclusively determining whether a person has Alzheimer’s disease requires an autopsy to see if amyloid plaques are present in the brain. This limits diagnosis to after death, which is not ideal. To help diagnose a person while still alive, other types of dementia can be ruled out, thereby increasing the likelihood that Alzheimer’s disease is present. As a result, Alzheimer’s has been referred to as a “disease of exclusion”.
Some other common types of dementia are Lewy body dementia, vascular dementia, and frontotemporal (frontal and temporal lobe) dementia. In general, they vary somewhat in their presentation.
| Type | Pathology | Percentage of all dementia | Characteristics |
|---|---|---|---|
| Alzheimer’s disease | Amyloid plaques and neurofibrillary tangles in the brain | 60–80% | Gradual onset, Progressive course, Multiple cognitive deficits, including memory impairment |
| Vascular dementia | Marker of cardiovascular disease (CVD) event (stroke, blockage in arteries) | 20% | Abrupt onset, fluctuating course, history of CVD event, focal deficits (abnormal gait, muscle weakness, etc.) |
| Lewy body dementia | Alpha-synuclein aggregates in the brain | 5–15% | Fluctuations in cognition, hallucinations, difficulty walking |
| Frontotemporal dementia | Degeneration of frontal and temporal lobes | 3% (people >65 years old) and 10% (people <65 years old) | Difficulty with language (aphasia), executive dysfunction, visuospatial function usually unaffected |
Finally, because Alzheimer’s disease nearly universally features a buildup of amyloid plaques (covered in more detail in the next section), PET scans can be used to differentiate Alzheimer’s disease from other dementia types.[13] Typical radiolabeled compounds are infused into a person’s bloodstream to run these tests. Following infusion, these compounds will make their way into the brain. They have a high affinity for amyloid, meaning the compounds will temporarily accumulate in the brain if more amyloid is present or flow out of the brain if little amyloid is present. When the brain is then imaged using a PET scan, the compounds are visible and serve as a proxy for how much amyloid is present, allowing for a more conclusive diagnosis of Alzheimer’s disease. The use of these PET scans is not widespread, though, due in part to the limited clinical utility (definitive Alzheimer’s diagnosis doesn’t typically alter the treatment plan).
No medical treatments have been clearly shown to stop or slow the underlying disease process in Alzheimer’s disease. However, some medication can improve symptoms. Acetylcholinesterase inhibitors can improve cognitive function in people with Alzheimer’s disease, although the effect is generally small.[14]
In 2020, a medication called aducanumab was approved by the FDA for the treatment of Alzheimer’s disease based on its effects on amyloid plaques, but whether aducanumab improves clinical outcomes (e.g., cognitive function) remains to be established.[15]
A few dietary patterns are associated with a lower risk of Alzheimer’s disease. This protective relationship is perhaps most commonly found with a Mediterranean diet. Among individual components of the diet, a higher intake of saturated fat and sugar has been linked to a higher risk of Alzheimer’s disease, while a higher intake of fish has been linked to a lower risk. Unfortunately, these findings derive from observational studies, meaning cause and effect can’t be determined with certainty, and there are almost no randomized clinical trials (RCTs) looking at the effect of diet on the incidence of Alzheimer’s disease.
In a cohort study[16] of 37,689 participants from the U.S. Women’s Health Initiative — Dietary Modification Trial (WHI-DM), lactose was the only sugar out of six subtypes that was significantly associated with AD risk. This suggests that dairy intake may increase AD risk, but other studies have found the opposite[17] or no effect[18].
Several dietary patterns and foods have been associated with a lower risk of Alzheimer’s disease. Currently, this evidence is entirely observational, as no clinical trials have assessed the effect of dietary patterns or foods (supplements excluded) on Alzheimer’s disease risk.
Mediterranean-style diets
Mediterranean-style diets are characterized by a higher intake of fish, fruits and vegetables, legumes, nuts, whole grains, and olive oil; limited intake of meat and dairy; and moderate intake of alcohol (often wine specifically).
A 2021 systematic review and meta-analysis found observational evidence that adherence to a Mediterranean diet was associated with lower risks of both mild cognitive impairment and Alzheimer’s disease.[66]
The MIND (Mediterranean-DASH Intervention for Neurodegenerative Delay) diet incorporates elements of the Mediterranean-style and DASH (Dietary Approaches to Stop Hypertension) diets. It favors berries, beans, vegetables, leafy greens, whole grains, and olive oil; includes moderate fish, poultry, and wine (one glass per day); and limits sweets, cheese, butter, red meat, and fried foods. The MIND diet has been the subject of less research than the Mediterranean diet, but at least one cohort study found that higher adherence to the MIND diet was associated with a lower risk of Alzheimer’s disease.[67]
A ketogenic diet shows some evidence of benefit for people with Alzheimer’s disease, which is covered in more detail in a later section. There is currently no evidence that a ketogenic diet can prevent Alzheimer’s disease.
A prospective cohort study published in 2021 found that a higher intake of sugar-containing beverages was associated with a higher risk of Alzheimer’s disease.[68] Similarly, a 2021 study covered here found that a diet high in sugar was associated with greater amyloid accumulation in two areas of the brain: the posterior cingulate gyrus and precuneus.
Fish Intake has been associated with a lower risk of Alzheimer’s disease. A 2014 meta-analysis of six prospective cohort studies found that a 100-gram increase in weekly fish intake was associated with an 11% lower risk of Alzheimer’s disease.[69] A 2018 meta-analysis of five prospective cohort studies likewise found an association between higher fish intake and a lower risk of Alzheimer’s disease among people with a genetic predisposition to the disease.[70]
Although a possible candidate for this protective association are the omega-3 fatty acids found in seafood, this does not appear to be the case. In the 2014 meta-analysis, a higher intake of long-chain omega-3s was not associated with a lower Alzheimer’s disease.
A 2018 meta-analysis of five prospective cohort studies found no association between coffee consumption of up to 5 cups per day and risk of Alzheimer’s disease.[71] However, at least one study found that drinking more than six cups of coffee per day was associated with a higher risk of Alzheimer’s disease.[72] It’s worth noting that disturbed sleep appears to increase brain levels of amyloid beta,[64] and a 2017 meta-analysis found that having sleep problems was associated with a higher risk of Alzheimer’s disease.[65] Given that caffeine is one factor that can negatively impact sleep, being mindful of caffeine intake (from coffee and other sources) may be important in reducing Alzheimer’s risk.
Observational evidence has frequently found that light-to-moderate alcohol consumption is associated with a lower risk of Alzheimer’s disease compared with no alcohol consumption.[73][74] One concern raised with this data, however, is that these findings could be the result of confounding factors (e.g., people who drink in moderation may be more likely to have other healthy habits).[75] This would also explain why genetic evidence largely hasn’t found a beneficial link between alcohol and Alzheimer’s disease.[76][74]
Meanwhile, one study, utilizing data from seven cohort studies, found that a history of drinking to the point of losing consciousness (i.e., passing out) was associated with roughly double the risk of Alzheimer’s disease.[77]
Alzheimer’s disease commonly features a reduced ability to generate energy from glucose, and this deficit may largely affect cognitive function. So how might this be remedied? Enter ketones. Some evidence suggests that although glucose metabolism may be impaired, the brain’s ability to utilize ketone bodies for energy is relatively unaffected, at least in the early stages of Alzheimer’s disease.[97] Given this, a ketogenic diet or ketogenic supplements may improve neuronal energy production and produce clinical benefits.
One RCT looked at the effect of a ketogenic supplement called AC-1202 (which consists mostly of caprylic acid) on cognitive function among 152 people with mild to moderate Alzheimer’s disease.[98] For 90 days participants took either AC-1202 or a placebo, with cognitive function assessed using two tests: the ADAS-Cog and MMSE. ADAS-Cog scores had improved at day 45, but not at day 90, and there was no difference in MMSE scores between groups at any point. Interestingly, a subgroup analysis found that ADAS-Cog scores were improved at days 45 and 90 in non-carriers of APOE4 but not improved at either time point in carriers of the gene.
A crossover RCT published in 2021 examined the effect of a ketogenic diet on 26 people with Alzheimer’s disease.[99] Participants were assigned to follow either a ketogenic diet (with a target of 6% of calories from carbohydrates) or low-fat diet for 12 weeks, later switching to the other diet after a 10-week washout period. Compared with the low-fat diet, the ketogenic diet improved both the ability to perform daily activities and quality of life. The ketogenic diet did not improve cognitive function in the overall analysis, but an improvement was seen when compliance was factored in (including only assessing participants who achieved a beta-hydroxybutyrate (a ketone body) level of at least 0.6 millimoles per liter).
RCTs have assessed a number of supplements’ effects on established Alzheimer’s disease. This includes omega-3s, B vitamins (usually folic acid, vitamin B6, and vitamin B12), vitamin D, vitamin E, ginkgo biloba, L-alpha glycerylphosphorylcholine (alpha-GPC), Lion's mane mushroom, ginseng, saffron, and sodium oligomannate. For the most part, these supplements have only conflicting or limited evidence in favor of their use.
B vitamins
High levels of the amino acid homocysteine have been associated with a higher risk of Alzheimer’s disease.[78] Because folic-acid, vitamin-b6, and vitamin-b12 play a central role in the metabolism of homocysteine, these vitamins have been extensively tested in clinical trials for their ability to improve cognitive function after the onset of Alzheimer’s disease, with the evidence as a whole indicating little-to-no benefit.
Vitamin D
A large body of evidence suggests the immune system facilitates Alzheimer’s disease.[79] Given this, vitamin D, which can influence immune function, may have an effect on the disease. One clinical trial supports a beneficial effect of vitamin D supplementation in people with Alzheimer’s disease. Mendelian randomization trials have also found that having genetically higher vitamin D levels seems to lower the risk of developing Alzheimer’s disease.[80][81]
Vitamin E
Oxidative stress, including oxidative damage to neuronal proteins and DNA, seems to occur during the development of Alzheimer’s disease.[82][83] If this process contributes to the development and progression of Alzheimer’s disease, a viable intervention could be supplementing with vitamin E, a potent antioxidant. A few clinical trials have tested the effect of a very high dose of vitamin E on people with Alzheimer’s disease, with some mixed evidence of benefit, but vitamin E supplementation does not seem to prevent dementia or Alzheimer’s disease in at-risk people.
The human brain contains a large quantity of the LCn3 docosahexaenoic acid (DHA),[84] making it a reasonable hypothesis that supplementing with DHA and its precursor, eicosapentaenoic acid (EPA), could benefit brain health. Additionally, experiments on rodent models of Alzheimer’s disease have provided support for a beneficial effect of LCn3s.[85][86] Despite this, clinical trials looking at the effect of LCn3s among people with Alzheimer’s disease suggest no benefit. More research is needed to determine whether supplementation with omega-3 fatty acids can help prevent Alzheimer’s disease.
Acetylcholine is a neurotransmitter involved in cognitive function, and reductions in its signaling contribute to some of the symptoms of Alzheimer’s disease. As a result, people with the disease are frequently prescribed medications called acetylcholinesterase inhibitors (AChEIs), which inhibit the breakdown of acetylcholine, thereby increasing its levels in synapses. AChEIs can modestly benefit symptoms of Alzheimer’s disease but do not slow progression of the disease itself.[87]
L-Alpha glycerylphosphorylcholine, or alpha-GPC, is a choline-containing compound that is believed to better deliver choline to the brain, where it can be used for acetylcholine synthesis (acetylcholine is made from choline and a molecule called acetyl-CoA). Preliminary evidence from a few clinical trials suggests alpha-GPC is beneficial for people with Alzheimer’s disease.
Ginkgo biloba tree leaves, commonly known simply as ginkgo biloba, are often used as a dietary supplement and contain a variety of terpenoids (a type of plant metabolite) known as ginkgolides. Ginkgo biloba may increase cerebral blood flow,[88] which could benefit brain health, and ginkgolides have shown some benefit in animal models of Alzheimer’s disease.[89] Although several clinical trials indicate ginkgo biloba may be beneficial for Alzheimer’s, the data as a whole are somewhat inconsistent (with seemingly variable responses based on the test used) and often have serious shortcomings (e.g., poor distinction between Alzheimer’s disease and other dementia types). Ginkgo biloba does not seem to prevent Alzheimer’s disease.
Lion’s mane mushroom
It has been posited that Hericium erinaceus, commonly known as lion’s mane mushroom, benefits cognitive function, partly based on research suggesting it increases levels of nerve growth factor (NGF), a peptide important to the growth and survival of neurons. However, whether lion’s mane (or, more specifically, terpenoids called erinacines, found in lion’s mane) actually stimulates NGF remains a source of debate. Highly preliminary research from a few clinical trials indicates lion’s mane may be beneficial for people with or at risk of Alzheimer’s disease.
Panax ginseng
Panax ginseng (aka Asian ginseng, Chinese ginseng, Korean ginseng, or even just ginseng) is a plant whose roots are often used as a supplement. One component of panax ginseng, called gintonin, has been studied in animal models for its potential to inhibit amyloid beta toxicity[90][91], making it a possible candidate for improving — or at least slowing — cognitive decline due to Alzheimer’s disease. Preliminary evidence from a few clinical trials suggests panax ginseng is beneficial for people with the disease.
Saffron contains potentially bioactive compounds, including crocin, crocetin, and safranal, that experiments performed in rodent models suggest may be able to reduce the accumulation and toxicity of amyloid beta.[92][93][94][95] Preliminary evidence from a few clinical trials suggests saffron is beneficial for Alzheimer’s disease.
Sodium oligomannate is a supplement (sold as GV-971) made from oligosaccharides (sugars) in a species of kelp called Ecklonia kurome. Some research suggests sodium oligomannate may reduce amyloid beta deposits in the brain (possibly via effects on the microbiome),[96] and one trial by a company that sells a sodium oligomannate supplement found that it benefited cognition in people with Alzheimer’s disease.
Exercise and physical activity have been shown to benefit physical function, functional independence, and neuropsychiatric symptoms in people with Alzheimer’s disease; aerobic exercise specifically improved cognitive function.
Several trials have investigated the effect of a keto diet and ketogenic supplements (e.g., medium-chain triglycerides) on Alzheimer’s disease and observed improvements — sometimes, but not always — in quality of life and cognitive function.
Music therapy centers around listening to music to stimulate the brain. Some research suggests the intervention can improve mood and cognitive function among people who have Alzheimer’s disease.
The underlying cause of Alzheimer’s is not known for certain, but several explanations have been proposed. The most common theory is that Alzheimer’s disease is the result of neurodegeneration and a buildup of misfolded proteins called amyloid plaques and tau tangles, which are almost always found in the brains of people with the disease. However, this “amyloid hypothesis” has been criticized. Regardless of the cause of Alzheimer’s disease, various modifiable and nonmodifiable factors, including aging, depression, diabetes, and genetics, have been linked to a higher risk of the disease.
The underlying cause of Alzheimer’s disease is far from a settled subject, but examining shared brain characteristics of people with the disease may provide insights.
One of the main features of Alzheimer’s disease is the accumulation in the brain of amyloid plaques,[21] which are made of a peptide called amyloid beta and have been proposed as a major driver of Alzheimer’s disease. This “amyloid hypothesis” has been challenged,[22] however, notably because drugs targeting the amyloid pathway have not improved cognition in Alzheimer’s disease.[23]
Another primary feature of Alzheimer’s disease is the presence of neurofibrillary tangles,[24] which are made up of tau proteins that, like amyloid beta, have misfolded and aggregated in the brain. According to the “tau hypothesis”, these tangles are a cause of or contributor to Alzheimer’s disease. However, because tau proteins serve important functions in the brain, another possibility is that the tangles are simply an indicator of a loss of tau function and are not harmful themselves.
Also common in Alzheimer’s disease is glucose hypometabolism,[25] a reduction in the brain’s ability to use glucose (a type of sugar) for energy, which is why some researchers call Alzheimer’s “type 3 diabetes”.[26] This reduction in the brain’s energy production may contribute to the cognitive impairments seen with Alzheimer’s disease and to the development of the disease itself.[27]
Finally, Alzheimer’s disease is neurodegenerative: it is characterized by the death of neurons (a type of brain cell) and subsequent cerebral atrophy (shrinking of the brain).[28] This process is believed to be central to the disease process and may be the result, at lea
Aging
One of the most important risk factors for Alzheimer’s disease is older age. The prevalence of Alzheimer’s disease is around 3% between ages 65 and 74, increases to 17% between 75 and 84, and reaches 32% at 85 or older.[29]
Depression has long been linked to the disease, with a 2021 meta-analysis of 28 prospective cohort studies reporting that the presence of depression or depression symptoms was associated with a 54% higher risk of Alzheimer’s disease.[30] Additionally, many genes that increase Alzheimer’s disease risk seem to do so by increasing the risk of depression.[31]
Being female
The overall incidence of Alzheimer’s disease is higher among females than males.[32] A major reason for this could simply be that women live longer than men and, as previously noted, risk increases with age.[33] Another contributing factor could be depression, which occurs more often in women[34] and, as covered, is associated with a higher risk of Alzheimer’s disease. Among females, the use of estrogen replacement therapy is associated with a lower risk of Alzheimer’s, but this has not been confirmed in clinical trials.[35]
Head injuries
Head injuries, such as a traumatic brain injury (TBI), may increase the risk of neurodegenerative disorders. Evidence suggests people who have experienced a TBI have higher odds of developing dementia,[36] but the evidence is somewhat mixed as to whether TBIs increase the risk of Alzheimer’s disease specifically.[37][38] It’s possible that characteristics of the injury itself could determine its effect on Alzheimer’s risk, but more research is needed.
Limited cognitive reserve
Some people seem capable of experiencing a significant amount of the neuronal death and amyloid plaque accumulation that characterize Alzheimer’s disease without displaying cognitive symptoms of the disease itself.[39][40] A major hypothesis used to explain these unusual cases is cognitive reserve.
Cognitive reserve refers to the capacity of the brain to compensate for damage and maintain function, and is believed to vary from person to person. The greater a person’s cognitive reserve, the longer it is believed to take for the underlying brain changes to induce cognitive impairment. In line with this, the risk of Alzheimer’s disease is lower in people with higher levels of education,[41] a factor believed to increase cognitive reserve.
Another related concept, neuroplasticity, is a broad term for the brain’s ability to alter itself. This can involve rerouting neural pathways, in some cases as a way of replacing damaged ones. An extreme example of this was described in a case study of a 29-year-old woman who had a significant stroke that required the removal of the right hemisphere of her brain.[42] This initially resulted in impairments in aspects of her cognitive function, including visuospatial ability, organization, and problem solving, but when examined around 5 years later, her cognitive function appeared to be normal, likely due in part to neuroplasticity.
Some factors that may promote neuroplasticity are adequate sleep, physical activity, and environmental enrichment.[43]
Limited physical activity
A large number of prospective cohort studies have found that physical activity is associated with a lower risk of Alzheimer’s disease.[44][44] In one meta-analysis of prospective cohort studies, compared with little or no physical activity, high amounts of physical activity was associated with a 38% lower risk of Alzheimer’s disease and moderate amounts, with a 29% lower risk.
Type 2 diabetes has frequently been associated with a higher risk of Alzheimer’s disease, with two meta-analyses finding an increase of about 55 to 57%.[45][46] A number of mechanisms have been suggested to explain this finding, but confirmation has remained elusive. One theory is that the brain becomes insulin resistant, and the resulting reduction in neuronal insulin signaling increases the production of amyloid beta, the main component of amyloid plaques.[47]
Of note, the relationship between diabetes and Alzheimer’s disease does not seem to be explained by differences in body weight, as the association remains after adjusting for body mass index(BMI).[45]
Some evidence suggests people with type 1 diabetes may also be at higher risk of developing dementia.[48][49] These studies did not differentiate dementia type, however, meaning more research is needed to determine whether type 1 diabetes is associated with a higher risk of Alzheimer’s disease or a different form of dementia (e.g., vascular dementia).
Elevated body weight
A 2020 meta-analysis of prospective cohort studies found that being overweight or obese at midlife was associated with a higher risk of Alzheimer’s disease.[50] One possible explanation could be that elevated body fat promotes greater insulin resistance and a higher risk of type 2 diabetes, a risk factor for Alzheimer’s disease, as discussed. Interestingly, the prior meta-analysis also found being overweight (and, in high-quality studies, obese) during later life was associated with a lower risk of Alzheimer’s disease. It’s possible this merely reflects reverse confounding, as some research suggests people begin to lose weight in the years leading up to their dementia diagnosis.[51]
Abnormal blood pressure
Meta-analyses of numerous prospective cohort studies have found that hypertension (high blood pressure) around midlife (ages 45 to 65) is associated with a higher risk of Alzheimer’s disease.[52][41] Hypertension was not found to be associated with Alzheimer’s disease in later life (≥65 years old), although in one meta-analysis, use of antihypertensive medication during this time was nonetheless associated with a lower risk.[52]
Several abnormal blood pressure patterns have also been associated with a higher risk of Alzheimer’s disease, including orthostatic hypotension (low blood pressure when rising from a sitting position);[41] higher blood pressure in the evening, relative to the daytime;[53] and increased fluctuations in blood pressure (i.e., throughout the day or from day to day).[54]
Microbes in the brain
A growing body of evidence suggests amyloid beta may function as an antimicrobial peptide, binding to bacteria and viruses and preventing infection and disease.[55]
The herpes virus (typically herpes simplex virus 1; HSV-1) is present more often in the brains of people with Alzheimer’s than those without the disease.[56] This has led to the theory that amyloid beta is produced as a protective response to the herpes virus in the brain and inadvertently promotes Alzheimer’s disease.[57]
Another microbe linked to Alzheimer’s disease, Porphyromonas gingivalis, is also associated with periodontal disease, which itself is linked to Alzheimer’s disease.[58] Compounds called gingipains, produced by this bacteria, have been observed at higher rates in the brains of people with Alzheimer’s disease and these gingipains have been shown to produce neurotoxic effects in mice.[59]
Poor sleep quality
The glymphatic system is a sort of biological drainage system that moves fluid into and out of the brain. This fluid has been shown to collect amyloid beta and clear some of it from the brain,[60] and the rate of this clearance increases dramatically during sleep.[61] As a result, it’s been theorized that inadequate sleep may promote Alzheimer’s disease by disrupting the removal of amyloid beta. However, the evidence for this hypothesis is mixed.
By and large, prospective cohort studies have not found an association between shorter sleep duration (about 6 hours or fewer per night) and a higher risk of Alzheimer’s disease, but they have found a higher risk with longer sleep duration (around 9 or more hours per night).[62] Most of these studies relied on self-reported sleep times, however, and several studies have found that people often inaccurately assess how much they sleep.[63]
Meanwhile, a number of observational studies have found that poor sleep quality is associated with a higher risk of Alzheimer’s disease, including when using objective measures of sleep efficiency (e.g., wrist actigraph, which records movement through a device worn on the wrist).[64][65] Still, it’s important to note that neurodegenerative diseases can disrupt areas of the brain involved in sleep regulation, meaning it’s possible that poor sleep could simply be a consequence of early brain changes in the years leading up to a dementia diagnosis.
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