Scientists do not yet fully understand what causes AD. There probably is not one single cause, but several factors that affect each person differently. Age is the most important known risk factor for AD. The number of people with the disease doubles every 5 years beyond age 65.
Family history is another risk factor. Scientists believe that genetics may play a role in many AD cases. For example, early-onset familial AD, a rare form of this disease that usually occurs between the ages of 30 and 60, is inherited.
The more common form of AD is known as late-onset. It occurs primarily in people in their late 70’s, and no obvious inheritance pattern is seen. However, several other risk factor genes may interact with non-genetic factors to cause the disease. The only risk factor gene identified so far for late-onset AD is a gene that makes one form of a protein called apolipoprotein E (ApoE). Everyone has ApoE, which helps carry cholesterol in the blood. Only about 15 percent of people have the form of ApoE (apoE4) that significantly increases the risk of AD. It is likely that other genes may also increase the risk of AD or protect against
it, but they remain to be discovered.
Scientists still need to learn a lot more about what causes AD. In addition to genetics and ApoE, they are studying education, diet and environment to
understand the role they might play in the development of this disease. Scientists are finding increasing evidence that some of the risk factors for heart disease and stroke, such as high blood pressure, diabetes and high cholesterol may also increase the risk of AD. Evidence supporting physical, mental and social activities as protective factors against AD is also increasing.
