What Causes Alzheimer's Disease?

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Scientists don't yet fully understand what causes Alzheimer's disease in most people. The causes probably include a combination of age-related changes in the brain, along with genetic, environmental, and lifestyle factors. The importance of any one of these factors in increasing or decreasing the risk of Alzheimer's disease may differ from person to person.

Alzheimer's disease is a progressive brain disease. It is characterized by changes in the brain—including amyloid plaques and neurofibrillary, or tau, tangles—that result in loss of neurons and their connections. These and other changes affect a person’s ability to remember and think and, eventually, to live independently.

Aging and Alzheimer's risk

Older age does not cause Alzheimer’s, but it is the most important known risk factor for the disease. The number of people with Alzheimer’s disease doubles about every 5 years beyond age 65. About one-third of all people age 85 and older may have Alzheimer's disease.

Scientists are learning how age-related changes in the brain may harm neurons and affect other types of brain cells to contribute to Alzheimer’s damage. These age-related changes include atrophy (shrinking) of certain parts of the brain, inflammation, vascular damage, production of unstable molecules called free radicals, and breakdown of energy production within cells.

However, age is only one risk factor for Alzheimer’s disease. Many people live into their 90s and beyond without ever developing dementia.

Clinical trials on Alzheimer's disease

Volunteers are needed for clinical trials that are exploring why some older adults get Alzheimer’s disease. By joining one of these studies, you may learn more about the relationship between aging and Alzheimer’s and contribute useful information to help other older adults in the future.

Genetics of Alzheimer's disease

If My Parent Has Alzheimer's, Will I Have it Too? Infographic. Click to open page and access pdf — Share this infographic and help spread the word about Alzheimer's genetics.

Many people worry about developing Alzheimer’s disease, especially if a family member has had it. Having a family history of the disease does not mean for sure that you’ll have it, too. But it may mean you are more likely to develop it.

People’s genes, which are inherited from their biological parents, can affect how likely they are to develop Alzheimer’s disease. Genetic risk factors are changes or differences in genes that can influence the chance of getting a disease. These risk factors are the reason some diseases run in families.

There are two types of Alzheimer's—early-onset and late-onset. Both types have a genetic component.

Some Differences Between Late-Onset and Early-Onset Alzheimer's Disease
Late-Onset Alzheimer's	Early-Onset Alzheimer's
Signs first appear in a person's mid-60s	Signs first appear between a person's 30s and mid-60s
Most common type	Very rare
May involve a gene called APOE ɛ4	Usually caused by gene changes passed down from parent to child

Late-onset Alzheimer's disease

Most people with Alzheimer's have late-onset Alzheimer's disease, in which symptoms become apparent in their mid-60s. Researchers have not found a specific gene that directly causes the late-onset form of the disease. However, one genetic risk factor—having one form, or allele, of the apolipoprotein E (APOE) gene on chromosome 19—does increase a person's risk. APOE ɛ4 is called a risk-factor gene because it increases a person's risk of developing the disease. However, inheriting an APOE ɛ4 allele does not mean that a person will definitely develop Alzheimer's. Some people with an APOE ɛ4 allele never get the disease, and others who develop Alzheimer's do not have any APOE ɛ4 alleles.

Early-onset Alzheimer's disease

Early-onset Alzheimer's disease occurs between a person's 30s to mid-60s and represents less than 10 percent of all people with Alzheimer's. Some cases are caused by an inherited change in one of three genes. For other cases, research shows that other genetic components are involved. Researchers are working to identify additional genetic risk variants for early-onset Alzheimer's disease.

Health, environmental, and lifestyle factors that may contribute to Alzheimer's disease

Research suggests that a host of factors beyond genetics may play a role in the development and course of Alzheimer's disease. There is a great deal of interest, for example, in the relationship between cognitive decline and vascular conditions such as heart disease, stroke, and high blood pressure, as well as metabolic diseases, such as diabetes and obesity. Ongoing research will help us understand whether and how reducing risk factors for these conditions may also reduce the risk of Alzheimer's.

A nutritious diet, physical activity, social engagement, sleep, and mentally stimulating pursuits have all been associated with helping people stay healthy as they age. These factors might also help reduce the risk of cognitive decline and Alzheimer's disease. Clinical trials are testing some of these possibilities.

Early-life factors may also play a role. For example, studies have linked higher levels of education with a decreased risk of dementia. There are also differences in dementia risk among racial groups and sexes—all of which are being studied to better understand the causes of Alzheimer’s disease and to develop effective treatments and preventions for all people.

For more information about causes of Alzheimer's disease

NIA Alzheimer’s and related Dementias Education and Referral (ADEAR) Center
800-438-4380
adear@nia.nih.gov
www.nia.nih.gov/alzheimers
The NIA ADEAR Center offers information and free print publications about Alzheimer’s and related dementias for families, caregivers, and health professionals. ADEAR Center staff answer telephone, email, and written requests and make referrals to local and national resources.

Alzheimers.gov
www.alzheimers.gov
Explore the Alzheimers.gov website for information and resources on Alzheimer’s and related dementias from across the federal government.

National Human Genome Research Institute
www.genome.gov/health

MedlinePlus
National Library of Medicine
www.medlineplus.gov

This content is provided by the NIH National Institute on Aging (NIA). NIA scientists and other experts review this content to ensure it is accurate and up to date.

Content reviewed: December 24, 2019