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The rate of dementia among thousands of seniors steadily declined over three decades, according to a study that gives what researchers call "cautious hope" that Alzheimer's and other degenerative brain diseases need not be the bane of our ever-lengthening lives.

The rate of dementia among the study's participants fell by an average of 20% per decade, according to an article to be published Thursday in The New England Journal of Medicine, and the average age of dementia onset was pushed back from 80 to 85 years.

"Our study offers cautious hope that some cases of dementia might be preventable or at least delayed," the article concludes.

Even cautious hope is welcomed.

More than 5 million Americans are now living with some form of dementia, of which Alzheimer's disease is the most common, according to the Alzheimer's Association.

As the number of baby boomers continues to swell, that number is predicted to triple by midcentury.

In 2015, some 110,000 Wisconsinites over 65 had Alzheimer's.

Because there is no treatment or cure, all will die either with or from the disease.

The lead author of the journal article — Claudia L. Satizabal, an instructor at Boston University School of Medicine — and her co-authors analyzed data from 5,205 people 60 or older participating in the Framingham Heart Study, which started in 1948 and has been extended to the original participants' children and grandchildren.

The dementia study, which reached back to 1975, broke the ensuing years into four periods, which the researchers called "epochs."

More than 2,000 people contributed data to each epoch. Group members were overwhelmingly white. Their ages ranged from 60 to 101.

■The first epoch extended from the late 1970s into the early 1980s. The rate of dementia during that period was 3.6 people per 100. The average age of dementia onset was 80.

■The second epoch extended from the late 1980s to the early 1990s. The rate of dementia during that period was 2.8 per 100, a rate decrease of 22%. The average age of onset was 82.

■The third epoch extended from the late 1990s to the early 2000s. The rate was 2.2 per 100, a rate decrease of 38% from the first epoch. The average age of onset was 84.

■The fourth epoch extended from the late 2000s to the early 2010s. The rate was 2 per 100, a decrease of 44% from the first period. The average age of onset was 85.

Significantly, the reductions were noted only among those who had at least a high school diploma.

Researchers noted that, over the three decades of reductions, they observed a trend toward higher eduction as well as a parallel trend away from most vascular risks, with the exceptions of obesity and diabetes.

The researchers found that improvements in cardiovascular health were seen only among those with at least a high school diploma.

However, they wrote, "none of these trends completely explain the decrease in the incidence of dementia."

David S. Jones, a professor at Harvard Medical School's Department of Global Health & Social Medicine, was the lead author of a companion essay that sought to place the dementia study in perspective.

Using coronary heart disease as an example, the essay said, predicting the trajectories of chronic diseases is extremely difficult.

"Forecasts of (heart disease's) future continue to swing between narratives of triumph and catastrophe," the essay says.

"Narratives of dementia remain similarly malleable," the essay says. "Even if a dementia decline has begun, it might not last."

The essay, however, reflects the main article's cautious hope.

Heart disease and cancer were once considered inevitable scourges of longer lives. Their inevitability, however, has been undermined by preventive measures, treatments and cures.

"Many people think that we can live even longer lives — but lives compromised by dementia, vision loss, and hearing loss. Whether that fate is inevitable or whether these, too, are malleable scourges remains to be seen."