Scientists don’t yet fully understand what causes AD, but it is clear that it develops, because 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 AD
Scientists are conducting studies to learn more about plaques, tangles, and other features of AD. They can now visualize plaques by imaging the brains of living individuals. They are also exploring the very earliest steps in the disease process. Findings from these studies will help them understand the causes of AD.
One of the great mysteries of AD 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, people develop AD in their 30s, 40s, and 50s. These people 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 these “early-onset” familial cases.
However, most people with AD have “late-onset” AD, which usually develops after age 60. Many studies have linked a gene called APOE to late-onset AD. This gene has several forms. One of them, APOE e4, increases a person’s risk of getting the disease. About 40 percent of all people who develop late-onset AD carry this gene. However, carrying the APOE e4 form of the gene does not necessarily mean that a person will develop AD, and people carrying no APOE e4 forms can also develop AD.
Scientists think that other risk-factor genes exist as well. Largescale genetic research studies are looking to find other genes. For more about this area of research, see the Alzheimer’s Disease Genetics Fact Sheet, available at www.nia.nih.gov/Alzheimers.
A nutritious diet, exercise, social engagement, and mentally stimulating pursuits can all help people stay healthy. New research suggests the possibility that these factors also might help to reduce the risk of cognitive decline and AD. Scientists are investigating associations between cognitive decline and heart disease, high blood pressure, diabetes, and obesity. Understanding these relationships and testing them in clinical trials will help us understand whether reducing risk factors for these diseases may help with AD as well.