What Causes Alzheimer's Disease And Who Gets It
Although certain basic lifestyle changes can help delay the onset of the disease in some people
, the exact cause of Alzheimer's disease isn't yet understood. Alzheimer's is a progressive neurodegenerative brain disorder: Normal brain cell function is gradually destroyed, leading to irreversible declines in memory, cognition, and behavior. But what causes things to go awry remains unknown. It may be that Alzheimer's has several causes or that the interplay between genetic makeup and certain risk factors determines who's affected.
The following questions are based on the known risk factors for Alzheimer's. See how many apply to your family (or to you, to assess your risk of eventually developing the disease).
At higher risk: A family history or known genetic predisposition
Why family history matters: People with a family history of Alzheimer's are more likely to develop the disease. The risk is thought to rise with each relative who had it.
It's unknown, though, exactly how much of this association is due to genetic factors and how much is due to shared lifestyle factors. Most experts believe that some combination of the two is responsible. Even when an immediate family member has the disease, however, your increased risk is only slightly higher than if your family had no history of dementia.
Up to 80 percent of Alzheimer's risk may have a genetic component, according to a 2006 study of more than 12,000 Swedish twin sets -- a greater influence than was previously thought. But having a relative with the disease does not doom a person to a similar fate; even among identical twins, when one male twin had it, almost half of the time the other twin did not. (Among female twins, the other twin developed Alzheimer's 60 percent of the time, a difference researchers attributed to the fact that women generally live longer than men.) If Alzheimer's were solely genetic, both twins would have developed the disease, and at about the same time.
So far, only two types of genetic tests for Alzheimer's exist, and neither of these blood tests is currently recommended for routine use.
* One kind of genetic test identifies a person's risk of developing Alzheimer's disease but doesn't guarantee whether or not he'll develop the condition. Everyone inherits a form of the apolipoprotein (APOE) gene from each parent. Apolipoprotein helps carry cholesterol in the blood. Its three most common forms are APOE-e2, APOE-e3, and APOE-e4. Those who have two copies of APOE-e4 seem to be at the highest risk of getting Alzheimer's and of developing symptoms earlier in life. Having one copy of APOE-e4 also elevates the risk. Only about 15 percent of people carry the APOE-e4 form.
It's thought that APOE is only one of many genes involved in the disease process. For instance, while it's considered a strong risk factor, the APOE-e4 gene shows up in only about 40 percent of all people with Alzheimer's disease. Identifying other genes that may be involved is a focus of ongoing research. The National Institute on Aging is conducting an Alzheimer's Disease Genetics Study, which is currently recruiting sibling pairs. These pairs must both have developed Alzheimer's after age 60 and must have a third family member with or without the disease who's willing to undergo cognitive tests and blood sampling.
* A second type of existing genetic test for Alzheimer's disease can predict with certainty who develops one rare form of the disease. This is early-onset familial Alzheimer's disease, which strikes between the ages of 30 and 65 and stretches through multiple generations. It accounts for less than 5 percent of all cases.
At higher risk: Those with type 2 diabetes
Why diabetes matters: People with type 2 diabetes have at least double the risk of developing Alzheimer's compared with people without diabetes. The risk was 65 percent higher for diabetics in 2006 data from the ongoing
# High blood sugar may cause vascular problems. It's known that diabetes can cause cardiovascular problems and strokes; reduced blood flow to the brain may cause small-vessel damage there that leads to vascular dementia (a type of dementia that can appear with Alzheimer's).
# Insulin resistance leads to inflammation that can damage the brain. Insulin resistance occurs in diabetics when their cells can't use insulin properly to move glucose from the blood to be used for cell energy. The pancreas then makes extra insulin to compensate, which builds up in the blood and creates inflammation, which damages brain cells.
# Insulin resistance in the brain disrupts the proper formation of neuron connections. The autopsied brains of people with Alzheimer's always show amyloid plaques, clumps of the protein beta amyloid, which ruin brain cell connections. It's been discovered that this substance destroys the brain cells' receptors for insulin, which is used by the brain to make new memories. This results in dementia and memory loss.
As evidence of a clear biochemical link between the two conditions grows more compelling, some are calling Alzheimer's disease "type 3 diabetes."
There's also a likely genetic link between diabetes and Alzheimer's. People with diabetes who also have the APOE-e4 gene (the type that places a carrier at highest risk for Alzheimer's) were two-and-a-half times more likely to develop the disease, according to a 2008 National Institute on Aging study.
# B vitamins: People who are folate (B-9) deficient may run triple the risk of developing dementia, according to recent South Korean data. Previous research showed vitamin B-12 to be protective.
# Vitamin E: Those who consume the highest dietary amounts of this antioxidant have a lower incidence of Alzheimer's. Vitamin E supplements have not been shown to have the same protective effect.
# Vitamin C: Another antioxidant, vitamin C, seems to have a protective effect in certain people, though possibly only in dietary form. Antioxidants counter the effects of oxidative stress, which is linked to nerve cell damage and death. Over-the-counter vitamin C supplements did not reduce Alzheimer's risk in a recent study of 2,969 people 65 and older.
At higher risk: Social isolation and loneliness
Why social stimulation matters: Isolation is a risk factor for developing dementia, probably because there's less opportunity for mental stimulation. But a surprising 2007 study also found that people who were around others but felt lonely (emotional isolation) were also at higher risk for Alzheimer's. This sense of loneliness wasn't a result of the disease (as is often the case with depression) but a clear precursor.
Alzheimers RiskAlzheimers Development Questionsby: Caring Editorial Team
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