SUNDAY, June 19, 2005 (HealthDay News) -- A study of Swedish twins has revealed that early childhood health, the number of years of education, and whether or not someone has had a stroke may contribute to the risk of developing different dementias, including Alzheimer's disease.
Increased recognition of how environmental factors play into these late-life conditions also suggests ways they might be prevented earlier in life.
The findings were presented Sunday at the Alzheimer's Association International Conference on Prevention of Dementia, in Washington, D.C.
"Twin studies are useful in trying to minimize the genetic influence on any kind of long-term observation," William Thies, vice president of medical and scientific affairs for the Alzheimer's Association, said at a teleconference on Tuesday.
The "concordance rate" for dementia among identical twins -- meaning both twins either have dementia or both do not -- runs about 50 percent. For Alzheimer's, the rate is about 60 percent. This means that about half of dementia and 60 percent or more of Alzheimer's can be attributed to genes, with the rest tracing to environment.
The big question, of course, is what in the environment may be contributing to these conditions.
For this analysis, researchers looked at 109 pairs of identical twins who were discordant for dementia (that is, only one of the pair had dementia). All twins were part of the Swedish Twins Registry, which was established in the early 1960s and includes data on more than 15,000 fraternal and identical twins. About half of the study participants who were diagnosed with dementia had Alzheimer's, with the rest having other forms of dementia.
"Then we went back and said, 'What do we know about these twins based on when they joined the twin registry?'" said study author Margaret Gatz.
"What did they tell about themselves 25 to 30 years ago that could predict whether or not they became demented?" added Gatz, a professor of psychology at the University of Southern California and foreign adjunct professor of medical epidemiology and biostatistics at the Karolinska Institute in Stockholm, Sweden.
As it turned out, there were significant differences in three areas: The demented twin in each pair was six times more likely to have had a stroke, four times more likely to have periodontal disease by midlife (a marker for inflammation) and three times more likely to report less education.
All told, 56 percent of the dementia cases had one or more of these risk factors. No one factor, however, explained the whole picture.
"For different pairs, it was different variables that were important," Gatz said. "It suggests that all these influences are additive. These are all probably related to good brain health."
Although the difference in education was considered to be statistically significant, on average the demented twin had only half a year less of education than the healthy twin, Gatz said. There were a handful of examples where one twin had a Ph.D. and the other had only a high school degree, she said.
"This would suggest that the amount of education we get at an early age can impact when we develop Alzheimer's," Thies said. "It doesn't tell us that going back to school at a later age is beneficial."
The periodontal measure is less clear, but may relate to poor childhood health conditions in general, Gatz said.
Another analysis undertaken by Gatz and her colleagues, but not included in this report, found that for twins with Alzheimer's, as opposed to other kinds of dementia, the only factor that significantly predicted dementia was periodontal disease.
"This suggests that environment could be quite important but less important for Alzheimer's than for other dementias. This is no surprise because vascular dementia has been pointed out as the most preventable," Gatz said. "It takes us to the idea that brain development is profoundly influenced really early in life."
On the other hand, she warned, "the message about practicing good brain health is important but we shouldn't oversell how preventable Alzheimer's disease is. We have a lot more to learn."
More information
For more on genetics and Alzheimer's, visit the Alzheimer's Association.
