What are the risk factors for Alzheimer's disease?
There are a number of risk factors for the development of Alzheimer's disease. They include:
Age
The risk of developing Alzheimer's disease increases with increasing age up to about age 90. Researchers estimate that as many as 50% of those over 85 have the disease.
Genes and Heredity
Having a family history of Alzheimer's disease increases your risk of developing it too. Those with one parent with the disease have a 1.5 times greater chance of developing it, and those with two afflicted parents have a 5 times higher than average risk.
 In a few families with members who develop the rare hereditary form of early onset Alzheimer's disease, scientists have identified abnormalities in certain genes. The most important of these genes are called presenilins, which produce a protein normally involved in amyloid generation. However, when the presenilin genes are mutated, amyloid production is accelerated, leading to early onset of Alzheimer's pathology.
The gene for apolipoprotein E, which is involved in cholesterol metabolism, comes in several different forms. These include ApoE2, ApoE3, and ApoE4. Those who inherit the ApoE4 version have a higher risk of developing Alzheimer's disease. However, ApoE4 is "neither necessary nor sufficient" for developing Alzheimer's disease, meaning that those who do not have ApoE4 may still get Alzheimer's disease and those that do have ApoE4 may not end up developing dementia. ApoE2 appears to be protective, reducing Alzheimer's risk. (See our Longevity Assurance Genes Center.)
In January 2003, researchers published evidence that another gene related to cholesterol metabolism, CYP46, may also play a role in Alzheimer's risk. The study, published in the Archives of Neurology, showed that people with the TT variant of the CYP46 gene have increased levels of the beta-amyloid protein in brain tissue and cerebrospinal fluid, as well as a higher risk of developing Alzheimer's. Study participants with both the APOE4 and CYP46-TT gene variants were nearly 10 times as likely as those without either gene to develop the disease.
Virtually all persons born with Down's syndrome develop Alzheimer's disease if they live into their fourth decade of life. Interestingly, mothers who give birth before the age of 35 to children with Down's syndrome have a higher than average risk of developing Alzheimer's disease themselves.
Gender
Women are more likely to develop Alzheimer's disease than men. However, this may be related to the fact that women tend to live longer than men and are more likely to live into their 80s and 90s when the disease becomes more common.
Ethnicity
The risk of developing Alzheimer's disease is as much as four times higher for African Americans as for Caucasians. A study published in the Journal of the American Medical Association recently compared the risk of Alzheimer's disease among African Americans and Africans from Nigeria, and found that the risk was nearly twice as high for African Americans as for the Nigerians studied. The risk for Latinos is twice that of whites. People of Asian and Native American descent are at lower risk. It is unclear whether these differences in risk are due to genetic heritage, health, or social or cultural differences between ethnic groups. What is clear, however, is that Alzheimer's disease affects all ethnic groups worldwide.
Intellectual Ability
One provocative study of older nuns suggested that intellectual ability plays a role in the risk for developing Alzheimer's disease. The Nun Study, based in Wisconsin, looked at more than 100 nuns over the age of 75 (testing the 93 living subjects and reviewing autopsy results for the 14 who had died). The investigators reviewed autobiographies that the nuns had written nearly six decades earlier, and found that those women whose writing contained less complexity of ideas and simpler grammar were more likely to develop Alzheimer's disease than those whose autobiographies suggested greater intellect.
A number of studies have found a link between higher education and a reduced risk of Alzheimer's disease. For more information, see our page on lifestyle and habits that might delay or prevent Alzheimer's disease.
Head injury
A study done at Duke University reviewing the medical records of World War II veterans revealed that their risk of developing Alzheimer's disease was increased if they had suffered serious head injury in young adulthood. The risk of Alzheimer's disease or dementia was twice as high for those with a history of moderate head injury (loss of consciousness or amnesia for 30 minutes to 24 hours) and four times as high for those with a history of severe head injury (loss of consciousness for more than 24 hours).
Depression
Being depressed may increase your risk of developing Alzheimer's disease, even if the depression was earlier in adulthood. A study of nearly 2,000 subjects with Alzheimer's and their non-demented family members found that depression symptoms correlated strongly with the development of Alzheimer's within a year. Depression symptoms that occurred more than one year--and even more than 25 years--before the onset of Alzheimer's were also associated with increased risk, although the association was weaker.
In the elderly, the symptoms of depression can mimic the symptoms of Alzheimer's disease. It is important that depression be ruled out when obtaining a diagnosis for an elderly person displaying symptoms of dementia.
Cardiovascular Health
Scientists are finding that cardiovascular disease and Alzheimer's disease may share some pathways of development, as heart disease risk factors such as elevated blood pressure, homocysteine levels, and cholesterol seem to increase the risk of developing Alzheimer's disease.
Homocysteine is an amino acid linked to heart disease. A deficiency of either vitamin B12 or folate can lead to higher homocysteine levels in the blood, and homocysteine has damaging effects on the central nervous system. Even borderline low levels of vitamin B12 or folate have been found to increase the risk of developing Alzheimer's disease. Persons with elevated homocysteine levels are two to three time more likely to develop Alzheimer's disease than those with normal levels. It is unclear if homocysteine contributes directly to Alzheimer's disease or is an "innocent bystander" of another process contributing to the development of dementia.
High blood pressure and high cholesterol have also been linked to an increased risk of developing Alzheimer's. Studies are now underway to investigate whether medications that control these factors could reduce a person's risk of Alzheimer's disease.
Research indicates that damage to the blood vessels that deliver blood, oxygen and nutrients to the brain can increase the risk of Alzheimer's disease. In addition to high blood pressure and blocked arteries caused by excessive cholesterol, other causes of that damage include atrial fibrillation (an abnormal heart rhythm that increases the risk of strokes caused by blood clots lodging in the circulation of the brain) and diabetes. All of these increase the risk of stroke, and stroke and Alzheimer's disease occur in the same people quite often. A growing amount of research indications that strokes are a significant risk factor for the development of Alzheimer's disease, even if the stroke is so small as to be nearly without symptoms.
Treating or preventing the conditions that cause stroke is prudent not only to reduce the risk of stroke, but to possibly reduce the risk of developing Alzheimer's disease as well.
Updated and reviewed: April 2007 | 
