The number of people diagnosed with dementia is falling, with almost 25% fewer people diagnosed with the disease than expected over the past 20 years.
Researchers have found that the prevalence of dementia has dropped over the last two decades, despite the population living to increasingly older ages.
The study, led by Carol Brayne, of the Cambridge Institute of Public Health (CIPH), University of Cambridge, looked at health and cognitive function in older people in Cambridgeshire, Nottingham and Newcastle.
The first part of the study took place between 1989 and 1994, while the second was between 2008 and 2011.
Researchers randomly chose people aged 65 and over and interviewed them about their lifestyles, including health, socioeconomic factors, medication and social care. A section of the sample group were then medically assessed to determine if they had dementia.
In the first study, the prevalence of people with dementia was 8.3% of the population.
Over the last two decades there have been significant changes in the older population, with more people living for longer. When applying the figure from the first study to the current population, the team estimated that 884,000 people would have dementia in the second study.
However, their findings showed that this was a huge overestimation, with just 6.5% of the population suffering from dementia in the second study.
In total, 670,000 people surveyed had dementia in the second study, which is almost 25% less than expected.
Fiona Matthews, from the University of Cambridge, said: "Changing diagnostic criteria mean that comparing newer estimates of dementia with older estimates is usually not possible, but CFAS is one of very few studies which was designed to allow us to track changing rates of dementia prevalence over time."
While rates have fallen, the authors note that the number of people with dementia living in care homes has increased from 56% to 70%. They say that dementia estimates are needed to plan for the provision of care, as current evidence used by the government is now out of date.
Brayne said: "This study provides compelling evidence of a reduction in the prevalence of dementia in the older population over two decades. Whether or not these gains for the current older population will be borne out in future generations would seem to depend on whether further improvements in primary prevention and effective health care for conditions which increase dementia risk can be achieved."
Dr Richard Horton, editor-in-chief of The Lancet said, "A reduction in prevalence of dementia in the older population is an important and welcome finding. But it is not a signal for the government to deprioritise investment in dementia care and research. Dementia remains a substantial challenge for those affected, their families, the NHS, and the Treasury.
"We need to understand better why the prevalence of dementia has fallen, and what that means for prevention and treatment services. Sadly, dementia care and research are too often neglected and underfunded in the UK."