There is growing evidence that the dementia crisis may not be as bad as first feared, say researchers.
A study suggests the proportion of elderly people developing dementia is falling in the US – backing up similar findings in the UK and Europe.
Data from 21,057 people over the age of 65 in the US showed the proportion with dementia fell from 11.6% in 2000 to 8.8% in 2012.
The findings were published in the journal JAMA Internal Medicine.
There is a suggestion that higher levels of education are protecting the brain from the disease.
One expert said the results were “incredibly important for the world”.
Similar studies in Europe, published in the Lancet Neurology last year, suggested dementia rates had fallen in the UK and among Spanish men and had stabilised in other European countries.
Prof Kenneth Langa, who conducted the latest study at the University of Michigan, said: “Our results add to a growing body of evidence that this decline in dementia risk is a real phenomenon, and that the expected future growth in the burden of dementia may not be as extensive as once thought.”
The slow decline in brain function is irreversible – there are no drugs or treatments – so finding ways of preventing the condition is hugely important.
Education has long been suspected to play a role, and the study found that while the dementia rate fell, the average time older adults had spent in school or university increased from 11.8 years in 2000 to 12.7 years in 2012.
It is possible that the mental challenge of education helps protect brain cells from dying later in life, or that once neurons start to die, education helps the rest of the brain rewire and compensate to prevent the symptoms of dementia appearing.
Good physical health is also thought to help protect the brain.
However, the study showed levels of diabetes, obesity and high blood pressure all increased between 2000 and 2012.
It is possible that better medication dampened their negative impact.
Prof Carol Brayne, who conducted the European analysis and is from the University of Cambridge, said the US study added “strong further evidence” that rates were declining in some countries.
She said education appeared to be significant and that people with higher levels of education seemed to “defer” dementia until later in life.
She told the BBC News website: “These findings are incredibly important for the world and underlie the importance of access to education.
“But it is likely to be a combination of risk factors – better health from conception, vaccinations, access to education, medical care, not smoking – that taken together will have an impact.”
Prof Brayne added that identifying what could help stave off dementia would ensure “we don’t go backwards, otherwise the gains we’ve had won’t be had by future generations”.
However, the number of people affected could still soar. The falling rate could be overwhelmed by the rising numbers of people living into old age.
Despite a falling dementia rate, the disease still became the biggest killer in England and Wales last year.
There are also fewer people dying from other diseases, as well as changes to the way deaths are being recorded that move dementia up the rankings.
Hilary Evans, the chief executive of Alzheimer’s Research UK, said the latest study was a cause for “optimism” but dementia remained the “greatest medical challenge”.
He said: “While these findings present a positive picture, we must not forget that there are still huge numbers of people living with dementia.
“This useful study adds to emerging evidence suggesting that dementia prevalence may be either declining or stabilising in parts of Western Europe and the US, but there are still many unanswered questions.
“We need to understand what is driving this apparent change in dementia risk if we are to harness this knowledge to provide crucial public health advice.”
Drs Ozioma Okonkwo and Sanjay Asthana, from the University of Wisconsin, said: “The growing consistency of reports indicating a potential decrease in its prevalence is encouraging.
“The focus now should be on better understanding the factors that underlie this trend and translating that knowledge into interventions that can reduce the risk of dementia for both individuals and society as a whole.”