There are two known risk factors for developing Alzheimer's Disease: age and family history. Advancing age is the number one risk factor for developing AD; the probability of being diagnosed with the illness nearly doubles every five years after the age of 65.
People who have a parent or sibling affected by Alzheimer's are two to three times more likely to develop the disease than those with no family history of AD. If more than one close relative has been affected by the disease, the risk increases even more.
Two different kinds of genes can be passed from one generation to the next that increase a person's chances of developing Alzheimer's: risk genes and deterministic genes. Risk (or susceptibility) genes increase a person's likelihood of developing Alzheimer's, but they do not guarantee a future diagnosis. In other words, even if you carry this gene, there is not a 100% likelihood that you will go on to develop the disease. The gene APOE-e4 found on chromosome 19 is the primary risk gene that is thought to be associated with AD. APOE attaches tightly to beta amyloid and seems to cause an excessive number of plaques to develop in a person's brain. Besides APOE, researchers think there may be up to a dozen more risk genes for AD.
Deterministic genes are much rarer than risk genes and are only found in a few hundred extended families around the world. If a deterministic gene is inherited, there is 100% certainty that the person will develop Alzheimer's, probably before the age of 65. Scientists have discovered three deterministic genes thus far: an abnormal amyloid precursor protein on chromosome 21 (this increases beta amyloid, which can cause excessive plaques); an abnormal protein on chromosomes called presenilin 1 (which seems to cause beta amyloid to be created); and an abnormal protein on chromosome 1 called presenilin 2.
Obviously, age and family history are not under our control. Still, we may be able to reduce our risk of developing Alzheimer's by employing healthy lifestyle habits. An increasing amount of evidence suggests that lifestyle factors such as diet/nutrition, exercise, intellectual activity, and social engagement can affect someone's Alzheimer's risk. Lifestyle factors are covered in more detail in a later section on prevention.