Will you ever get Alzheimer's disease? The genes you've inherited carry most of the risk, an identical-twin study shows.

It's surprising news. While people clearly inherit Alzheimer's disease risk, most researchers have given equal blame to "environmental factors." These factors may include things we encounter, things we do or don't do, or diseases we develop.

But an international study of nearly 12,000 Swedish twin pairs — a fourth of them identical twins — now finds that some 80 percent of Alzheimer's risk is genetic. University of Southern California psychologist Margaret Gatz, PhD, and colleagues report the findings in the February issue of Archives of General Psychiatry.

"It appears that genetic influences outweigh environmental influences in relative importance for Alzheimer's risk," Gatz tells WebMD.

What this means is that close relatives of people with Alzheimer's disease are at much higher risk of getting the disease than people without such a relative. What it does not mean is that if you have such a relative, you're doomed to get Alzheimer's.

"'Genetic' does not mean cast in stone," Gatz tells WebMD. "In no way is having a relative with Alzheimer's disease — even a genetically identical twin — a guarantee that a person is going to get Alzheimer's disease."

When One Twin Has Alzheimer's

Gatz's team screened for dementia among twins over age 65 in the Swedish Twin Registry. They then tested each twin with dementia for Alzheimer's disease. Extensive medical and lifestyle information was available for each study participant.

The researchers found that genetic inheritance explained about 80 percent of Alzheimer's risk. Environmental factors not shared among twins explained the other 20 percent, as might be expected.

But even among identical twins — who share the same genetic makeup — Alzheimer's disease in one twin did not mean the other twin inevitably got the disease.

Among male identical twins, when one brother had Alzheimer's disease, the other developed the disease 45 percent of the time.

Among female identical twins, when one sister had Alzheimer's disease, the other developed the disease 60 percent of the time.

The difference between men and women, Gatz says, is simply that women live longer and thus have a better chance of surviving long enough to get Alzheimer's disease.

If Alzheimer's disease was strictly genetic, one identical twin should get the disease about the same time as the other. But when both identical twins had Alzheimer's, there was as much as a 16-year difference in age of onset.

That, Gatz says, clearly shows that there must be a strong interplay between a person's genes and a person's environment.

"For instance, if twins share a gene that is a more risk-promoting gene with regard to how they process fats, then eating more fats would be more dangerous for them," she says. "If one twin ate very few fats, her risk would be much lower. We are sure that kind of thing is going on. That is why it is hard to talk about genetic risks independently of environmental risks."

Regular Exercise May Delay Alzheimer's

How to Avoid Alzheimer's

William H. Thies, PhD, vice president for medical and scientific affairs at the Alzheimer’s Association, says researchers have a good idea about what some of the environmental risks may be.

"We've long recognized that education has an impact on reducing the likelihood of Alzheimer's disease," Thies tells WebMD. "The other more lately evolving ideas fit mostly around heart disease risk factors. Alzheimer's risk is linked to high blood pressure, high cholesterol, high blood sugar, a high-fat diet, a sedentary lifestyle, and the lack of social connectivity."

We can't pick our parents, so there's not much we can do about genetic risks. But Thies points out that there are several things one can do to cut your risk of Alzheimer's disease — especially if the disease runs in your family:

—Stay mentally active.

—Be socially involved.

—Get physical exercise.

—Eat a brain-healthy diet.

Genes aren't destiny, Gatz says. If you think you or a loved one may be getting Alzheimer's disease, it's time to act.

"The strong message to people is if there is any kind of concern about dementia, it is important to get a good professional evaluation," she says. "There can be reversible causes of cognitive change. Or doctors may be able to offer either psychological or medical interventions that can slow processes of cognitive change."

Resources for Alzheimer’s Patients and Families

By Daniel J. DeNoon, reviewed by Louise Chang, MD

SOURCES: Gatz, M. Archives of General Psychiatry, February 2006; vol 663: pp 168-174. Margaret Gatz, PhD, professor of psychology, University of Southern California, Los Angeles. William H. Thies, PhD, vice president for medical and scientific affairs, Alzheimer’s Association, Chicago.