MONDAY, Dec. 15, 2003 (HealthDayNews) -- Genetics play a smaller role in cases of late-onset Alzheimer's disease than has been believed.
About half of someone's susceptibility to Alzheimer's occurring after age 65 can be attributed to environmental factors, according to researchers reporting in the Dec. 15 online edition of the Annals of Neurology.
Earlier studies had suggested genes contribute up to 75 percent of susceptibility in late-onset cases of the brain-wasting disease.
"Within an individual there is an unknown mixture of genetic and environmental influences," says study co-author Margaret Gatz, a professor of psychology at the University of Southern California.
Researchers evaluated 662 pairs of twins between the ages of 52 and 98 -- 265 of them identical and 397 fraternal. The researchers followed them for an average of five years, testing periodically for memory and other cognitive functions to see if they developed Alzheimer's.
Nearly 6 percent of the participants were diagnosed with the disease during the five years. In the identical twin pairs, if one sibling developed Alzheimer's, the other twin also did 32 percent of the time. In fraternal twins, only about 9 percent of the siblings developed the disease if the twin did.
By using statistical modeling techniques, the researchers computed that half or more of an individual's susceptibility to Alzheimer's later in life is attributed to environmental, not genetic, factors.
"The study confirms the importance of genetics but also points to the limits of only looking to genetic explanations, especially among those who develop Alzheimer's disease very late in life," Gatz says.
"This study is special in that it began with twins where neither member of the twin pair had dementia, and it followed the twins to see whether one or both twins later developed Alzheimer's disease," she adds.
"What we find is that genetics plays an important role, because we find that nearly 50 percent of the variation in liability to Alzheimer's disease could be attributed to genetic variation," Gatz says.
But the study findings also point to the need to look at the role of environment factors, she adds.
Such factors, she says, may include "everything from viruses and bacteria to diet, toxins, educational achievement or life events -- and begins at conception."
An estimated 4.5 million Americans have Alzheimer's disease, according to the Alzheimer's Association.
Gatz adds that "it is premature on the basis of this study to recommend specific environmental changes. It is clear, however, that the environment refers to influences over the entire life course. Other work that we have done with the twins shows that those who have a more intellectually engaged lifestyle earlier in their lives are those who are at reduced risk of dementia later."
For that reason, it's not just seniors who should be paying attention to maintaining a healthy environment, she says, but also parents of young children. "Having a healthy brain as well as a healthy body requires attention to everything from nutrition to protection against toxins and trauma."
William Thies, a spokesman for the Alzheimer's Association, says, "The most important message of all the twin studies is that at least a portion of our risk is genetic and a portion is environment."
He notes the new study is relatively small, but it does offer some hope. "There are lots of people here [in the study] who didn't get Alzheimer's and when they did they were older. It suggests if you are 80 and healthy, you have some risks, but you shouldn't be worrying every time you forget which cabinet the glasses are in."
More information
For the 10 warning signs of Alzheimer's disease, visit the Alzheimer's Association. The association also has lots more information about the disease.
