The biggest risk factor for Alzheimer’s disease is increased age. The likelihood of developing Alzheimer’s disease doubles every 5.5 years from 65 to 85 years of age. Whereas only 1%-2% of individuals 70 years of age have Alzheimer’s disease, in some studies around 40% of individuals 85 years of age have Alzheimer’s disease. Nonetheless, at least half of people who live past the 95 years of age do not have Alzheimer’s disease.
Common forms of certain genes increase the risk of developing Alzheimer’s disease, but do not invariably cause Alzheimer’s disease. The best-studied “risk” gene is the one that encodes apolipoprotein E (apoE). The apoE gene has three different forms (alleles) — apoE2, apoE3, and apoE4. The apoE4 form of the gene has been associated with increased risk of Alzheimer’s disease in most (but not all) populations studied. The frequency of the apoE4 version of the gene in the general population varies, but is always less than 30% and frequently 8%-15%. Persons with one copy of the E4 gene usually have about a two to three fold increased risk of developing Alzheimer’s disease. Persons with two copies of the E4 gene (usually around 1% of the population) have about a nine-fold increase in risk. Nonetheless, even persons with two copies of the E4 gene don’t always get Alzheimer’s disease. At least one copy of the E4 gene is found in 40% of patients with sporadic or late-onset Alzheimer’s disease.
This means that in majority of patients with Alzheimer’s disease, no genetic risk factor has yet been found. Most experts do not recommend that adult children of patients with Alzheimer’s disease should have genetic testing for the apoE4 gene since there is no treatment for Alzheimer’s disease. When medical treatments that prevent or decrease the risk of developing Alzheimer’s disease become available, genetic testing may be recommended for adult children of patients with Alzheimer’s disease so that they may be treated.
Many, but not all, studies have found that women have a higher risk for Alzheimer’s disease than men. It is certainly true that women live longer than men, but age alone does not seem to explain the increased frequency in women. The apparent increased frequency of Alzheimer’s disease in women has led to considerable research about the role of estrogen in Alzheimer’s disease. Recent studies suggest that estrogen should not be prescribed to post-menopausal women for the purpose of decreasing the risk of Alzheimer’s disease. Nonetheless, the role of estrogen in Alzheimer’s disease remains an area of research focus.
Some studies have found that Alzheimer’s disease occurs more often among people who suffered significant traumatic head injuries earlier in life, particularly among those with the apoE 4 gene.
In addition, many, but not all studies, have demonstrated that persons with limited formal education – usually less than eight years – are at increased risk for Alzheimer’s disease. It is not known whether this reflects a decreased “cognitive reserve” or other factors associated with a lower educational level.
