NEW YORK — Despite fears that dementia rates were going to skyrocket as the population grows older and fatter and has more diabetes and high blood pressure, a large nationally representative survey has found the reverse.
Dementia is on the wane in the United States, and when people do get dementia, they tend to be older than before.
Previous studies found the same trend but involved much smaller and less diverse populations, including the mostly white population of Framingham, Mass., and residents of a few areas in England and Wales.
The new study found that the dementia rate in Americans 65 and older fell by 24 percent over 12 years, to 8.8 percent in 2012 from 11.6 percent in 2000. That trend is “statistically significant and impressive,” said Samuel Preston, a demographer at the University of Pennsylvania who was not associated with the study.
In 2000, people received a diagnosis of dementia at an average age of 80.7; in 2012, the average age was 82.4.
The study, published online Monday by the journal JAMA Internal Medicine, was led by researchers at the University of Michigan in Ann Arbor. It included 21,000 Americans 65 and older across all races, education, and income levels who participate in the Health and Retirement Study, which regularly surveys people and follows them as they age.
The National Institute on Aging funded the work but was not involved with the data collection, analysis, or interpretation.
“The dementia rate is not immutable,” said Dr. Richard Hodes, director of the National Institute on Aging. “It can change.”
And that “is very good news,” said John Haaga, director of the institute’s division of behavioral and social research. It means, he said, that “roughly a million and a half people aged 65 and older who do not have dementia now would have had it if the rate in 2000 had been in place.”
Keith Fargo, director of scientific programs and outreach at the Alzheimer’s Association, said the group had been encouraged by some of the previous research showing a decline but had also been “a little bit nervous” about drawing conclusions because the populations in the earlier studies were so homogeneous.
Now, he said of the new data, “here is a nationally representative study. It’s wonderful news.”
An estimated 4 million to 5 million Americans develop dementia each year. It remains the most expensive disease in America — a study funded by the National Institute on Aging estimated that in 2010 it cost up to $215 billion a year to care for dementia patients, surpassing heart disease ($102 billion) and cancer ($77 billion).
To assess dementia, participants in the new study were asked, among other requests, to recall 10 nouns immediately and after a delay, to serially subtract seven from 100, and to count backward from 20. The test was based on extensive research indicating it was a good measure of memory and thinking skills.
Participants also were asked about their education levels, income, and health.
The decline occurred despite an increase in diabetes — its prevalence among older Americans surged to 21 percent in 2012 from 9 percent in 1990.
The study found diabetes increased the risk of dementia by 39 percent.
More older people today also have cardiovascular risk factors — high levels of blood pressure, blood sugar, and cholesterol — which increase the risk of dementia. But more are taking medications for those conditions, so perhaps better control of those risk factors played a role in the decline.
The findings about obesity were especially puzzling. Compared with people of normal weight, overweight people and obese people had a 30 percent lower risk of dementia, the study found. Underweight people had a risk 2.5 times as great.
Yet the obesity picture is muddled because other studies have found that obesity in middle age increases dementia risk in old age.
Then there is the education question. On average, older Americans in 2012 had one more year of schooling than older Americans in 2000. And years of education were associated with decreased dementia risk in this study, as in many others.
It is still not clear exactly why education would reduce the risk of dementia. There is the cognitive reserve hypothesis: that education changes developing brains in a good way, making them more resistant to dementia, and that people with more education have brains that are better able to compensate for dementia damage.
But education also is linked to more wealth. People with more education often live in different environments than those who have less schooling, and they tend to have better health overall. They also are less likely to smoke.
As for black Americans, the dementia risk was higher, but some possible reasons — less education, less wealth, more cardiovascular risk factors — did not fully explain the difference. One possibility is that they received a poorer quality of education, so each year of it offered less protection from dementia, said the study’s lead author, Dr. Kenneth Langa.
In the end, much of what is happening with dementia rates defies explanation, said Langa, a professor of medicine at the University of Michigan who also works in the Veterans Affairs health care system.