What Is the Cause of Alzheimer's Disease?

    Alzheimer's disease is a degenerative neurological (brain) illness that results in dementia, or the loss of cognitive processes including remembering, thinking, and reasoning. It is more frequent in those aged 65 and over; in fact, age is the most significant recognized risk factor for Alzheimer's.

Alzheimer's disease is not caused by aging. Complex brain changes can occur years or even decades before any symptoms appear. Researchers think that this illness is caused by a mix of genetics, lifestyle, and environmental factors.

Alzheimer's disease is classified into two types: early-onset and late-onset. People with late-onset illness get symptoms at the age of 65 or older. It is, by far, the most common kind of Alzheimer's disease. Less than 10% of patients with Alzheimer's show symptoms between the ages of 30 and 60.

It is still hard to determine who would develop Alzheimer's disease. Researchers are continuously investigating risk factors and have developed various ideas about what brain alterations trigger the disease. Here's what they've come up with thus far.

Hypotheses

Alzheimer's disease causes a significant loss of brain cells (neurons) and their connections (synapses), beginning in the brain regions that govern memory. More cognitive and physical abilities are impaired when the disease spreads across the brain. The brain physically shrinks as a result of this.

There are two major explanations for Alzheimer's disease development: cholinergic and amyloid.

Hypothesis of Cholinergic Action

The cholinergic hypothesis was the first explanation for Alzheimer's disease. Cholinergic neurons serve a key part in cognition (learning and comprehension) throughout the brain. Alzheimer's disease patients have a significant loss of cholinergic neurons as well as a shortage of acetylcholine (ACh), a brain chemical messenger critical for learning, memory, and other processes. 

The only authorized Alzheimer's drugs assist to maintain the amount of ACh in the brain. These medications can temporarily alleviate cognitive problems but do not prevent long-term brain damage.

Hypothesis of Amyloid

For many years, scientists assumed that excessively high amounts of beta-amyloid (a protein that surrounds nerve cells) resulted in the formation of amyloid plaques in the brain. These plaques are considered to contribute to the cognitive decline associated with Alzheimer's. The beta-amyloid theory was founded on a seminal publication published in the journal Nature in 2006.

Medications that target beta-amyloid, on the other hand, did not help Alzheimer's symptoms. Furthermore, researchers discovered that amyloid plaques may be observed in persons who do not have Alzheimer's disease as they age.

Then, in July 2022, a piece in Science magazine called this main premise into question. One neurologist discovered that the photos in the 2006 research had been manipulated. Researchers have viewed the beta-amyloid theory with caution since then.

The findings of a clinical study of a medicine named Leqembi (lecanemab) in November 2022 provided some further evidence for the involvement of beta-amyloid in Alzheimer's disease progression. After 18 months, lecanemab decreased the quantity of amyloid in individuals' brains, resulting in a modest improvement in symptoms.

More study and testing are required to confirm the currently contested idea. Researchers are also looking for novel theories for neuron loss. 

Is Alzheimer's Disease Passed Down Through Families?

Some early-onset Alzheimer's cases are familial, which means that children may inherit specific mutations from their parents. In certain circumstances, gene mutations are to blame for the disease. If you have a family history of early-onset Alzheimer's disease, talk to your doctor about genetic testing.

Approximately 70% of Alzheimer's cases are caused by hereditary factors. However, the genetics of late-onset Alzheimer's disease are less clear: it does not appear to run in families. 

Apolipoprotein E (APOE) is the gene that has the most influence on your risk of late-onset Alzheimer's. Everyone's DNA contains some version (or variation) of APOE. If you inherit the 4 variation, you are more likely to acquire the condition. Having the very rare APOE 2 variation, on the other hand, can actually help prevent you from Alzheimer's.

Who is Affected by Alzheimer's Disease?

Some people are more prone than others to develop Alzheimer's disease. Factors that might increase risk include:

Age:

 This is the most significant risk factor; most Alzheimer's symptoms appear after the age of 65, and the percentage of persons with Alzheimer's grows with age. Alzheimer's disease affects 40% of persons over the age of 85.

Gender and biological sex: 

Almost two-thirds of Alzheimer's cases are in adults who were born female. However, women may not be at a higher risk of Alzheimer's. For example, the difference may emerge because women live longer on average than men; in the United States, the rates of women and men developing Alzheimer's disease at each age are the same. Recent study has also found sex and gender bias in previous studies.

Ethnicity: 

Although black and Hispanic people may be at a higher risk, subsequent research, such those on sex and gender, have cast doubt on this notion. When socioeconomic considerations and health problems are included (such as the high prevalence of heart disease and diabetes in Black and Hispanic populations), the disparities in risk disappear. Other social and environmental risk factors for Alzheimer's can be influenced by structural racism.

Risk Elements

While the etiology of Alzheimer's disease is unknown, researchers have found a variety of illnesses, accidents, and other factors that can all contribute to the disease's development.

Vascular (Blood Vessel) Disorders

High blood pressure (hypertension), heart disease, and stroke are examples of these disorders. Blood artery problems (such as heart failure) can influence blood circulation to the brain, leading to brain inflammation and, eventually, Alzheimer's.

Metabolic Disorders

Type 2 diabetes, high LDL cholesterol, and obesity are examples of these. High LDL cholesterol levels and obesity, in particular, can raise your risk of Alzheimer's disease in your forties.

Depression

Depression might increase your chance of Alzheimer's disease. There aren't enough research to establish if antidepressant medication reduces this risk.  

Depression can also be an early indicator of Alzheimer's disease, as the disease can generate comparable emotional symptoms.

Loneliness and social isolation

Social isolation is defined as a lack of social engagement. Loneliness is distinct—it is a sense of being alone or disconnected, which can occur even when you are surrounded by other people. Both of these characteristics are risk factors for Alzheimer's disease. 

Fortunately, resuming social activities can assist to safeguard you. Talking to people or volunteering can help you enhance your mood, cognitive, and general well-being.

TBI (Traumatic Brain Injury)

Traumatic brain damage occurs when the head is struck or jolted. A history of TBIs (for example, among athletes) can raise the risk of Alzheimer's disease. This is especially true for serious traumas, but even minor TBI (such as a concussion) can have long-term consequences.

Heavy Alcohol Consumption

Heavy alcohol consumption is linked to brain alterations and cognitive decline, but there isn't enough evidence to conclude it causes these problems. This does not imply that you must abstain from alcohol entirely—light to moderate consumption in middle to late adulthood has been found to reduce the risk of cognitive decline and dementia.

Smoking

People who have never smoked or who have quit smoking for an extended period of time may be at a lesser risk of acquiring Alzheimer's. Even if you don't entirely quit smoking, cutting back can help lower your risk. 

Secondhand smoking can also raise your risk of Alzheimer's disease, especially if you are exposed at home.

Hearing Impairment

Alzheimer's disease is linked to hearing loss, but no one knows why or if one illness causes the other. One main theory is that hearing loss may lead to social isolation, which is a risk factor for Alzheimer's disease. Another theory is that patients with hearing loss require more cognitive resources to analyze sounds, leaving less for other cognitive activities.

Pollution in the Air

Higher levels of fine particulate matter are linked to an increased risk of Alzheimer's disease. This pollution (PM 2.5) comes from sources such as power plants, building sites, and fires. 

Other forms of pollution, such as nitrogen oxides (emitted from the combustion of fuel in automobiles and power plants) and sulfur dioxide (emitted from the combustion of fossil fuels in industrial facilities), can induce brain damage comparable to that found in Alzheimer's patients.

Physical Idleness

Being physically active lowers your chances of Alzheimer's (and all other types of dementia). One 2022 analysis looked at studies that had participants follow them for at least 20 years and discovered that physical exercise might help prevent against Alzheimer's in the long run.

Low level of mental engagement

remaining intellectually active, especially before the age of 20, is just as vital as remaining physically active. Higher education has been proven in studies to lessen the risk of dementia, but so may other cognitively demanding hobbies such as reading, speaking a second language, and performing music. A intellectually demanding work might also assist.

A Quick Recap

Alzheimer's disease causes cognitive function loss, such as memory and thinking, in persons 65 and older. Researchers do not yet understand how Alzheimer's disease develops, although it is most likely caused by a mix of hereditary and environmental factors. 

The most important risk factor for Alzheimer's disease is age. Heart illness, brain damage, and a lack of mental or physical exercise are all recognized hazards. New research are incorporating demographically varied groups to better understand who develops Alzheimer's, but there is currently no method to predict who will acquire it.

It might be frightening to live with a disease that has no known origin, but keep in mind that Alzheimer's research is continually happening. If you or a loved one is diagnosed, a healthcare expert can assist clarify what to anticipate.