More Alzheimer's Risk Factors

1. How old is he or she?

• At lower risk: Under age 70
• At higher risk: Over age 85

Why age matters: Age is the most significant established risk factor for Alzheimer's disease. One in 7 people over age 71 has some form of dementia, and 2.4 million of these have Alzheimer's, according to the latest National Institutes of Health data. The percentage of people with Alzheimer's rises from just 2.3 percent of those ages 71 to 79 to 18 percent of those ages 80 to 89, and 29.7 percent of those 90 or older. Some estimates say as many as half of all people over age 85 have Alzheimer's disease.

The total number of people with the disease doubles with every five-year age jump after age 65.

2. Is it a he or a she?

• At lower risk: Men
• At higher risk: Women

Why gender matters: Because women live longer than men, on average, and Alzheimer's disease risk rises with age, more women than men develop it. In addition, some research indicates that a lack of estrogen after menopause may contribute to the fact that, overall, slightly more women are affected. Taking hormone-replacement therapy has not been shown to protect against Alzheimer's.

Vascular dementia is more common in men than women, probably because more men develop contributing factors such as hypertension and vascular problems.

3. Have any of his or her parents or siblings had Alzheimer's?

• At lower risk: No family history or known genetic predisposition
• 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.