The percent of older US adults with dementia, including Alzheimer’s disease, declined from 11.6 percent in 2000 to 8.8 percent in 2012, a decrease of nearly a quarter, scientists reported on Monday.

Why it matters:

It had been thought that the baby boomers’ march toward old age would triple the number of Alzheimer’s patients by 2050. These new numbers not only portend a lesser burden on the health care system (and families) but also suggest that something has changed over the generations — and identifying that change could drive down dementia rates even further.

You’ll want to know:

That’s a significant decline: If the rate of dementia in 2012 had been what it was in 2000, “there would be well more than 1 million additional people with dementia,” said John Haaga, director of the National Institute on Aging’s behavioral and social research, who was not involved in the study. As it is, an estimated 5 million Americans 65 and older are afflicted with Alzheimer’s or other dementia.

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The nitty-gritty:

Researchers led by Dr. Kenneth Langa of the University of Michigan analyzed data on more than 10,500 Health and Retirement Study participants aged 65 or older in 2000 and 2012.

The percent of seniors with dementia fell to 8.8 percent in 2012; accounting for the greater proportion of those who were 85 years or older, the decline was even greater: to 8.6 percent, the team reported in JAMA Internal Medicine.

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One possible factor is education. The older adults in the 2012 group in the new study had, on average, about one year more education than the 2000 group. More education can produce greater cognitive reserve, in which people have enough backup synapses and neurons that losing some to Alzheimer’s still leaves them short of dementia. But the researchers found this didn’t explain the entire decline.

Curiously, being overweight or obese was associated with a decreased risk of dementia. Carrying excess pounds generally raises the risk of diabetes and heart disease, which are thought to increase the risk of dementia, but “late-life obesity may be protective,” wrote commentary authors Ozioma C. Okonkwo and Dr. Sanjay Asthana of the University of Wisconsin School of Medicine and Public Health. That may be especially true when people receive effective treatments for diabetes and heart disease, which also became more common with later generations.

What they’re saying:

“This is great news,” as Haaga put it. A commentary accompanying the JAMA Internal Medicine paper points out that the national data jibe with other evidence: Earlier this year, the long-running Framingham study found that the risk of dementia in old age fell by about 20 percent every decade between 1977 and 2008.

That study came with an asterisk, however, because the Framingham participants come from one small geographic area (near Boston) and skew white and well-off. The new data are based on large numbers of people (10,546 adults in 2000 and 10,511 in 2012) and are nationally representative, Haaga pointed out, “including a good representation of people with less education and at the lower end of the income scale.”

The new data “reinforce the Framingham findings that age-specific risk of dementia may be decreasing,” said Dr. Sudha Seshadri, a professor of neurology at Boston University School of Medicine and senior investigator for the Framingham study.

The bottom line:

The falling rates of dementia suggest there are modifiable factors, such as education and cardiovascular health, that reduce your chances of developing Alzheimer’s.