BOSTON, Feb. 11 (UPI) -- The risk for developing dementia has dropped about 20 percent per decade since 1975, researchers report in a new study, despite longtime predictions for large increases over the next few decades.
Researchers at Boston University found incidence of dementia among participants in a long-term study declined steadily and continuously over time, though they are unsure why the numbers have gone down.
A 2015 study in Britain found similar results showing the number of dementia diagnoses in Europe has stabilized during the last 20 years -- and researchers involved with the University of Cambridge study also could not explain why expected increases hadn't materialized.
Suggested explanations include differences in blood pressure and cardiovascular health treatment, both of which are thought to increase the risk for Alzheimer's disease, the most common form of dementia, as well as others.
Predictions suggest the number of Americans with Alzheimer's disease will increase by 40 percent by 2025, from 5.1 million to 7.1 million, and that it will triple to about 13.8 million people by 2050.
"Currently there are no effective treatments to prevent or cure dementia," Dr. Sudha Seshadri, a professor of neurology at Boston University's School of Medicine, said in a press release. "However, our study offers hope that some of the dementia cases might be preventable -- or at least delayed -- through primary [keep the disease process from starting] or secondary [keep it from progressing to clinically obvious dementia] prevention. Effective prevention could diminish in some measure the projected explosion in the number of persons affected with the disease in the next few decades."
For the study, published in the New England Journal of Medicine, researchers analyzed data on 5,205 participants in the Framingham Heart Study collected since 1975, when the study started tracking cognitive assessments.
In the 4 decades since the data started to be collected, the researchers found five-year age- and sex-adjusted hazard rates for developing dementia have continuously dropped: In the late 1970s and early 1980s, 3.6 per 100 people developed some type of dementia; 2.8 per 100 people had dementia in the late 1980s and early 1990s; 2.2 per 100 people had dementia in the late 1990s and early 2000s; and 2.0 per 100 people had it during the late 2000s and early 2010s.
The declines are credited, at least somewhat, to improved treatment of cardiovascular treatment and blood pressure treatment, each of which is associated with dementia. The study also focused on incidence, so causes were not factored into the analysis, researchers said.
"What I would say is we need to take that message to redouble our efforts, not to become complacent. We are doing something right," Seshadri told the Washington Post. "So if we understand what we are doing right, we can perhaps activate it."
Some researchers point out, however, that the Framingham study, which started tracking residents of Framingham, Mass., in 1948, is mostly white and covers a population based on a single location, which means it is not very representative.
Experts say they continue to expect an explosion in dementia diagnoses as baby boomers begin to reach age 70 this year, and the Boston University study shows incidence in a particular population -- which is not enough understand why the number of people with dementia has decreased so much over time.
"This is an important study, don't get me wrong. But there's a difference between what we call incidence and prevalence. Incidence is what was studied in this paper -- the number of people who develop something over a set period of time," Keith Fargo, director of Scientific Programs & Outreach at the Alzheimer's Association, told CBS News. "But it points to, in this well-studied group of people, that we're seeing decreases in their heart health risk factors -- cholesterol, smoking, hypertension. Those things were getting better. At the same time you see this decreased incidence of dementia. You can't tell whether it's causative."