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A new study finds that the prevalence of dementia has fallen sharply in recent years, most likely as a result of Americans' rising educational levels and better heart health, which are both closely related to brain health.

Dementia rates in people over age 65 fell from 11.6% in 2000 to 8.8% in 2012, a decline of 24%, according to a study of more than 21,000 people across the US published in JAMA Internal Medicine.

"It's definitely good news," Dr Kenneth Langa, a professor of internal medicine at the University of Michigan and a coauthor of the new study told CNN.

"Even without a cure for Alzheimer's disease or a new medication, there are things that we can do socially and medically and behaviorally that can significantly reduce the risk."

The decline in dementia rates translates to about one million fewer Americans suffering from the condition, said John Haaga, director of behavioral and social research at the National Institute on Aging, part of the National Institutes of Health, which funded the new study.

According to the Columbus Dispatch, previous studies found the same trend but involved much smaller and less diverse populations like the mostly white population of Framingham, Massachusetts, and residents of a few areas in England and Wales.

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."

According to the Dispatch, 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).

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