What causes Alzheimer’s?

Scientists don’t yet fully understand what causes Alzheimer’s disease, but it is clear that it develops as the result of a complex series of events that take place in the brain over a long period of time. It is likely that the causes include genetic, environmental, and lifestyle factors. Because people differ in their genetic make-up and lifestyle, the importance of these factors for preventing or delaying AD differs from person to person.

The Basics of Alzheimer’s Disease

Scientists are conducting studies to learn more about plaques, tangles, and other features of Alzheimer’s disease.  New technology has made it possible for researchers to visualize plaques by imaging the brains of living individuals. Researchers are also exploring the very earliest phases of the disease process. Findings from such studies will help researchers and clinicians better understand the causes of AD.

One of the great mysteries of Alzheimer’s disease is why it largely strikes older adults. Research on how the brain changes normally with age is shedding light on this question. For example, scientists are learning how age-related changes in the brain may harm neurons and contribute to AD damage. These age-related changes include inflammation and the production of unstable molecules called free radicals.


In a very few families, individuals develop AD in their 30s, 40s, and 50s. These individuals have a mutation, or permanent change, in one of three genes that they inherited from a parent. We know that these gene mutations cause AD in “early-onset” familial cases.

Most people with Alzheimer’s, however, have the “late-onset” form of the disease, which develops after age 60. Many studies have linked a gene called APOE to late-onset AD. This gene has three forms,. APOEe2, APOEe3, and APOEe4.  Every individual inherits two copies of the APOE gene, one from each parent.  Carrying one or two copies of APOE e4 increases a person’s risk of getting Alzheimer’s disease.  About 40 percent of all people who develop late-onset AD carry one or two copies of APOEe4.  Carrying APOE e4, however, does not necessarily mean that a person will develop Alzheimer’s disease.  As well, a person may carry only APOE e2 and/or e3 yet develop AD.

Scientists think that other risk-factor genes exist as well. Large-scale genetic research studies are searching for other genes  that either cause or increase susceptibility to Alzheimer’s disease. For more about this area of research, see the Alzheimer’s Disease Genetics Fact Sheet.

Lifestyle Factors

A nutritious diet, exercise, social engagement, and mentally stimulating pursuits can all help people stay healthy. New research suggests that such healthy lifestyle choice also might help to reduce the risk of cognitive decline and Alzheimer’s disease. Scientists are investigating associations between cognitive decline and heart disease, high blood pressure, diabetes, and obesity as well as lifestyle practices. Understanding these relationships and testing them in clinical trials will help clarify the extent to which controlling chronic health conditions and adopting healthy lifestyle practices may lessen the incidence of Alzheimer’s disease.