Recent studies from the Netherlands, Sweden and England have suggested a decline, and the new research extends this look to some other parts of the world.
THE UNITED STATES
The federally funded Framingham study tracked new dementia cases among several thousand people 60 and older in five-year periods starting in 1978, 1989, 1996 and 2006. Compared with the first period, new cases were 22 percent lower in the second one, 38 percent lower in the third and 44 percent lower in the fourth one.
The average age at which dementia was diagnosed also rose -- from 80 during the first period to 85 in the last one.
During that time, there were declines in smoking, heart disease and strokes, factors linked to dementia, and a rise in the number of people using blood pressure medicines and getting a high school diploma, which reduce the likelihood of developing the condition.
"The results bring some hope that perhaps dementia cases might be preventable, or at least delayed" by improving health and education, said the study leader, Claudia Satizabal of Boston University.
Dallas Anderson, epidemiology chief at the National Institute on Aging, agreed.
"For those who get the disease, it may come later in life, which is a good thing. Getting the disease in your 80s or 90s is a very different than getting it in your early 70s," he said.
Researchers from the German Center for Neurodegenerative Diseases say that claims data from Germany's largest public health insurance company suggest that new cases of dementia declined significantly between 2007 and 2009 in men and women.
Dementia prevalence -- the proportion of people with the disease -- also declined dramatically in women ages 74 to 85. There was a trend toward a smaller decline in men but the difference was so small researchers couldn't be sure of it.
The trends corresponded with fewer strokes and better treatment of high blood pressure, cholesterol and diabetes, and more education, they said.
An updated study of dementia prevalence by Alzheimer's Disease International in 2009 concludes that its previous estimates for the disease worldwide were too low. The group now says dementia prevalence appears to have increased from about 5 percent to about 7 percent in East Asia, and in Sub-Saharan African from between 2 percent and 4 percent to nearly 5 percent.
The estimates were revised based on studies in China and sub-Saharan Africa, and the latest United Nations population projections.
Researchers from the Universidad Icesi in Colombia used current population and other sources of information to update a 20-year-old study on dementia and determined that current projections might underestimate dementia cases by up to 50 percent.
In countries where dementia appears to be declining, the rise in obesity and diabetes threatens to undo progress.
"It may be that what we have now is a sweet spot," where people with these problems are still relatively young, said Anderson, of the National Institute on Aging. "They're not in the dementia range yet, but what's going to happen? We know they're all in the pipeline."
National Institute on Aging: http://www.nia.nih.gov/Alzheimers
Patient, family info: http://www.alzheimers.gov/
Alzheimer's Association: http://www.alz.org