Causes and Risk Factors
Scientists have identified factors that increase the risk of Alzheimer's. The most important risk factors—age, family history and heredity—can't be changed, but emerging evidence suggests there may be other factors we can influence.
The greatest known risk factor for Alzheimer's is advancing age. Most individuals with the disease are age 65 or older. The likelihood of developing Alzheimer's doubles about every five years after age 65. After age 85, the risk reaches nearly 50 percent. One of the greatest mysteries of Alzheimer's disease is why risk rises so dramatically as we grow older.
Another strong risk factor is family history. Those who have a parent, brother, sister or child with Alzheimer's are more likely to develop the disease. The risk increases if more than one family member has the illness. When diseases tend to run in families, either heredity (genetics) or environmental factors, or both, may play a role.
Scientists know genes are involved in Alzheimer's. There are two types of genes that can play a role in affecting whether a person develops a disease - risk genes and deterministic genes. Alzheimer's genes have been found in both categories.
Risk genes increase the likelihood of developing a disease, but do not guarantee it will happen. Scientists have so far identified several risk genes implicated in Alzheimer's disease. The risk gene with the strongest influence is called apolipoprotein E-e4 (APOE-e4). Scientists estimate that APOE-e4 may be a factor in 20 to 25 percent of Alzheimer's cases.
APOE-e4 is one of three common forms of the APOE gene; the others are APOE-e2 and APOE-e3. Everyone inherits a copy of some form of APOE from each parent. Those who inherit APOE-e4 from one parent have an increased risk of Alzheimer's. Those who inherit APOE-e4 from both parents have an even higher risk, but not a certainty. Scientists are not yet certain how APOE-e4 increases risk. In addition to raising risk, APOE-e4 may tend to make Alzheimer's symptoms appear at a younger age than usual.
Deterministic genes directly cause a disease, guaranteeing that anyone who inherits them will develop the disorder. Scientists have discovered variations that directly cause Alzheimer's disease in the genes coding three proteins: amyloid precursor protein (APP), presenilin-1 (PS-1) and presenilin-2 (PS-2).
When Alzheimer's disease is caused by these deterministic variations, it is called "autosomal dominant Alzheimer's disease (ADAD)" or "familial Alzheimer's disease," and many family members in multiple generations are affected. Symptoms nearly always develop before age 60, and may appear as early as a person's 30s or 40s. Deterministic Alzheimer's variations have been found in only a few hundred extended families worldwide. True familial Alzheimer's accounts for less than 1 percent of cases.
What you can do now: Factors you may be able to influence
Most experts believe that the majority of Alzheimer's occurs as a result of complex interactions among genes and other risk factors. Age, family history and heredity are risk factors we can't change. Now, research is beginning to reveal clues about other risk factors we may be able to influence through lifestyle and effective management of other health conditions.
Head trauma: There may be a strong link between serious head injury and future risk of Alzheimer's, especially when trauma occurs repeatedly or involves loss of consciousness. Protect your brain by buckling your seat belt, wearing your helmet when participating in sports, and "fall-proofing" your home.
Heart-head connection: Growing evidence links brain health to heart health. Your brain is nourished by one of your body's richest networks of blood vessels. Every heartbeat pumps about 20 to 25 percent of your blood to your head, where brain cells use at least 20 percent of the food and oxygen your blood carries.
The risk of developing Alzheimer's or vascular dementia appears to be increased by many conditions that damage the heart or blood vessels. These include high blood pressure, heart disease, stroke, diabetes and high cholesterol. Work with your doctor to monitor your heart health and treat any problems that arise.
Studies of donated brain tissue provide additional evidence for the heart-head connection. These studies suggest that plaques and tangles are more likely to cause Alzheimer's symptoms if strokes or damage to the brain's blood vessels are also present.
General healthy aging: Other lines of evidence suggest that strategies for overall healthy aging may help keep your brain as well as your body fit. These strategies may even offer some protection against developing Alzheimer's or related disorders. Try to keep your weight within recommended guidelines, avoid tobacco and excess alcohol, stay socially connected, and exercise both your body and mind.