What Are the Causes and Risk Factors of Alzheimer’s Disease?

Alzheimer’s disease occurs when neurons (nerve cells) in the brain stop functioning, lose connections with other brain cells, and eventually die.

Scientists don’t really know what starts the process, but in investigating Alzheimer’s disease causes, they have long focused on toxic changes in the brain: the development of amyloid plaques (abnormal protein deposits) between neurons and neurofibrillary tangles (twisted strands of a protein called tau) inside neurons.

While Alzheimer’s is not a normal part of aging, one of the main risk factors for the disease is age: The majority of those with this type of dementia are 65 and older.

One in 10 people age 65 and older has Alzheimer’s dementia. After 65, the risk doubles every five years. Nearly one-third of people who are 85 and older have Alzheimer’s. (1)

Family history is a strong Alzheimer’s disease risk factor, meaning that if you have a first-degree relative such as a parent or sibling with the disease you are more likely to develop it yourself.

But that doesn’t mean that Alzheimer’s disease is hereditary. Researchers believe that genes do not directly cause Alzheimer’s except in an estimated 1 percent of all cases involving gene mutations (defects) passed directly from parent to child.

If you’ve inherited one of three gene mutations from either your mother or father, you have a very high probability of developing a highly uncommon form of Alzheimer’s called familial early-onset.

This is not a common condition: Scientists have been able to identify these mutations in only 500 families around the world.

For the vast majority of people, certain genes increase the risk of Alzheimer’s disease but do not guarantee it.

Many people with these genes will never develop Alzheimer’s, because lifestyle and environment also play a role.

Scientists are studying the complex interactions between all these factors to understand Alzheimer’s disease causes and to learn whether it’s possible to keep them at bay. (2)

A Gene That Can Affect Alzheimer’s Risk

Scientists have identified more than 20 genes that appear to influence the development of Alzheimer’s disease.

With one exception, these genes are not considered to be significant risk factors, either because they are rare or because their effect is too limited.

The gene that matters most is apolipoprotein E (APOE). There are three different APOE forms, called alleles:

• e2 is an uncommon, protective form of the gene.
• e3, the most prevalent form, is believed to neither decrease nor increase risk.
• e4 raises Alzheimer’s risk and also makes it more likely the disease will develop at an earlier age.

Still, some people with APOE-e4 never develop Alzheimer’s, while others who develop Alzheimer’s don’t have the gene. (3)

Vascular problems — those related to blood vessels, such as beta-amyloid deposits in brain arteries, ministrokes, and hardening of the arteries — may be a cause of Alzheimer’s disease as well as a result of it. 

Damaged arteries harm the brain by reducing the flow of blood, depriving brain cells of oxygen and essential nutrients like glucose; preventing the elimination of toxic beta-amyloid and tau proteins; and leading to damaging inflammation.

Researchers are working to identify exactly how and why this happens with the goal of interfering with this cycle. (4)

A study published in 2017 in the JAMA, following 322 subjects for over 20 years, found a relationship between vascular risk factors in middle age — obesity, smoking, high blood pressure, diabetes, and high cholesterol — and amyloid plaques in the brain. (5)

The researchers found that subjects who had one risk factor had an 88 percent increased risk for elevated levels of amyloid plaques. People with two or more risk factors had almost triple the risk.

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Over the past three decades, scientists have identified a link between moderate and severe traumatic brain injury and an increased risk of Alzheimer’s disease or another dementia developing years later.

These types of traumatic brain injury are most commonly caused by falls, being struck by an object, and motor vehicle crashes. All involve impact to the head that disrupts normal brain function.

One study found that older adults with a history of moderate traumatic head injury had more than double the risk of developing Alzheimer’s of those without such injury. Those who had experienced severe traumatic head injury had a 4.5 times greater risk.

Some studies have found an association between severe traumatic head injuries and elevated levels of the protein beta-amyloid in the brain, a characteristic of Alzheimer’s disease.

More research is needed to better understand the relationship between traumatic brain injury and dementia. (6)

RELATED: Living Well With Alzheimer’s Disease: Good Self-Care Makes a Difference

Does Plastic Cause Alzheimer’s?

Small pieces of plastic called microplastics have become ubiquitous in water, food, and air. On the basis of the limited information currently available, the World Health Organization (WHO) has concluded that microplastics in drinking water don’t appear to pose a health risk at current levels, but more research is needed. (7)

Does Aluminum Cause Alzheimer’s?

This fear goes back to the ’60s and ’70s, when people began avoiding products containing aluminum such as beverage cans, cooking pots, antacids, and antiperspirants. But researchers have not found any evidence linking aluminum exposure and Alzheimer’s. (8)

Do Flu Shots Cause Alzheimer’s?

This question reflects another myth about Alzheimer’s, put forth by an American doctor whose license was suspended by the South Carolina Board of Medical Examiners. In fact, mainstream medical studies have found an association between flu shots and other vaccinations and overall health, including a reduced risk of Alzheimer’s disease. (8)

Do Any Foods Cause Alzheimer’s?

Researchers do not believe that any particular food causes Alzheimer’s. But studies suggest that a style of eating dubbed the MIND diet, which emphasizes natural plant-based foods while limiting red meat, saturated fat, and sweets, may lower a person’s risk of developing the disease. (9)

Do Any Shampoos Cause Alzheimer’s?

This question appears to date back to 2004, when a research study found that a preservative in many shampoos called methylisothiazolinone (MIT) restricted the growth of neurons taken from the brains of rat embryos. (10)

But scientists have found no direct evidence that MIT can raise Alzheimer’s risk. And a final report published in 2010 in the International Journal of Toxicology determined that MIT is safe in the low concentrations typically used in personal care products like shampoo. (11)

Are There Ways to Prevent Alzheimer’s?

Researchers have found encouraging evidence that healthy habits can protect the brain and reduce the risk of Alzheimer’s disease.

For instance, a study published in JAMA in July 2019 found that even people who were genetically predisposed to develop Alzheimer’s lowered their risk by 32 percent by pursuing a healthy lifestyle that involved:

• Not smoking
• Exercising regularly
• Eating a healthy diet
• Consuming alcohol only moderately (12)