Alzheimer's disease risk factors
What are risk factors?
For many diseases, particularly infectious ones, it is possible to show a single cause. For example, a virus is known to be the cause of measles. However, for chronic disorders (long-lasting conditions), there is still much about the causes that is unknown. It is thought that multiple causes working together are more likely to cause certain conditions such as a heart attack.
In their search for answers, scientists look for factors which are more common in people who develop a particular disease. The presence of these 'risk factors' is associated with an increased chance that the disease will develop.
Risk factors are characteristics of a person (e.g. blood group) or environmental (e.g. sunlight) which appear to have some relationship to the development of a disease. Other examples include exposures to a substance or product, family background or work history. Thus, a person who smokes is more likely to develop lung cancer than someone who does not smoke.
How are risk factors determined?
Two types of studies are used to determine risk factors. One approach is to monitor a group of healthy people over a long period of time and compare those who develop a disease with those who do not. Lifestyle factors (such as diet, smoking, etc.) as well as family and work histories are examined and analysed in the disease and non-disease groups.
In the second approach, people who already have the condition or disease under investigation, such as Alzheimer's disease, are compared with people who are otherwise similar, but do not have the disease. Information is gathered on personal and family characteristics, as well as past exposures which may have occurred through lifestyle and work. Risk factors are those characteristics and particular exposures which are significantly more frequent in the diseased than the non-diseased group.
It is important to remember that risk factors are not causes. No single study can verify a link between a disease and a specific factor. Repeated investigations are necessary before a link can be established.
What are the risk factors for Alzheimer's disease?
Age and family history have been established in research studies throughout the world as the 2 major risk factors in Alzheimer's disease.
- Age: The risk of developing Alzheimer's disease appears to double every 5 years survived beyond age 65, when about one in 100 people have Alzheimer's. This increases to 25 in 100 over the age of 85.
- Family history of early onset alzheimer's disease: familial Alzheimer's disease (FAD): People who have two generations or more of first degree relatives (parent or sibling) with early onset (age 65 or under) Alzheimer's disease have a greater chance of developing the disease than those with no family history.
It is important to remember that this rare form of the disease, Familial Alzheimer's Disease FAD) only accounts for 1 per cent of all cases and is known to be entirely inherited. FAD is passed directly from one generation to another through a dominant inheritance pattern. This means that if a parent has FAD each child has a 50 per cent chance of inheriting FAD.
- Genetic risk factors in late onset Alzheimer's disease: In late onset (over the age of 65) Alzheimer's disease certain genes appear to influence a person's susceptibility to developing the disease.
The most important gene discovered to date is the Apolipoprotein E gene, which is found on chromosome 19. This gene occurs in three forms in humans: types 2, 3, and 4. Every person in the world carries two Apolipoprotein genes: they can be the same type (2,2; 3,3; or 4,4) or a mixture of two types (2,3; 2,4; 3,4). What has been found is that people with at least one type 4 (especially those with two, i.e., 4,4) are at increased risk of developing Alzheimer's disease earlier in life than those with the other types of Apolipoprotein E. People with type 2 (especially 2,2) appear to be protected against developing Alzheimer's disease, until much later in life.
Scientists do not understand why this is so, and there is much speculation and research underway to find out why.
Note:
Unlike the case with chromosomal mutations in familial Alzheimer's disease, the type of Apolipoprotein does NOT mean definitely Alzheimer's disease will or will not occur. Indeed, it is known that some people can reach 90 with type 4 and not develop dementia, whereas others with type 2 can develop dementia much earlier in life. What this means is that the type of Apolipoprotein a person has is not enough alone to cause Alzheimer's disease. Other risk factors must occur for the Apolipoprotein type to have an effect. There may also be protective factors (see below). - Down syndrome: Most people with Down syndrome who live long enough to late adulthood, develop Alzheimer's disease pathology.
Research on risk factors and other possible 'protective factors'
A number of risk factors for Alzheimer's disease are being examined, including: the medical histories of individuals, alcohol and tobacco consumption and exposure to occupational and environmental substances. There are various areas being studied which may prove to be protective factors. A few specific areas of investigation include the following.
- Education (a possible protective factor). Several studies have shown that individuals with a higher education have a lower risk of developing the disease than those with less education. This may also apply to people who keep themselves mentally active throughout their lives. Education as a protective factor requires more study, as scientists must sort out whether it is education itself that makes a difference or other factors related to it (e.g. natural intelligence, income level, ability to be tested). It should be emphasised that many people with a low level of education do not develop Alzheimer's disease.
- Anti-inflammatory drugs (a possible protective factor). People with arthritis appear to have less chance of developing Alzheimer's disease than those without it. Scientists are exploring what factors might explain this result, including the role of anti-inflammatory drugs used to treat arthritis.
- Head injury (a risk factor). Individuals who have sustained a head injury (defined as causing loss of consciousness for at least 15 minutes) have a small but statistically significant greater chance of developing Alzheimer's disease than those who have not had a head injury.
- Female sex (a slight risk factor). Women have a slightly greater chance of developing Alzheimer's disease than men, even when allowance is made for their longer life span. Scientists are investigating whether hormone replacement therapy reduces this risk.
Last Reviewed: 09 March 2001
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