The proportion of US adults with dementia has declined in 16 years

The proportion of individuals affected by dementia in the US is declining, researchers have found out. This was particularly the case among people with higher levels of education.

The study, published in JAMA Internal Medicine, is not the first to highlight this trend – something which may sound surprising given that the global population is ageing, raising concerns that this will lead to a rise in the number of people affected by age-related diseases, such as Alzheimer's.

In the UK, research based on data from the Cognitive Function and Ageing Studies reported reductions in age-specific incidence of dementia, especially among men. A paper published earlier this year in the New England Journal of Medicine revealed that the incidence of dementia declined over the course of three decades among elderly participants of the US Framingham Heart Study.

"Our results, based on in-depth interviews with seniors and their caregivers, 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," says Kenneth Langa, the lead author of this new study, from the University of Michigan Medical school.

"There seems to be a decline in the proportion of people that have have dementia at any given age. However, what has caused this decline and in which groups of people the decline is seen the most are still questions to investigate", Dr James Pickett, Head of Research at Alzheimer's Society, told IBTimes UK.

The new study identifies education as a potential cause of the decline in the number of US dementia cases.

Improvements despite obesity and diabetes

The scientists used data from the long-term Health and Retirement Study to evaluate the prevalence of dementia among a nationally representative sample of more than 21,000 people aged 65 or over. They wanted to compare changes in dementia prevalence from 2000 to 2012.

They observed that 11.6% of participants met the criteria for dementia in 2000, while in 2012, only 8.8% did. This was quite an improvement, especially since rates of diabetes, obesity and high blood pressure – conditions that are believed to increase the risk of dementia by interfering with brain blood flow – have been high.

"This might not be entirely surprising because diabetes and obesity are a relatively recent problem. It may be that we won't see their impact on dementia for another generation, and that the current generation of of seniors is fitter than future older generations will be. There is no reason why rates of dementia could not go back up again when the generations burdened by these health problems reach old age", Pickett commented.

The role of education

Looking at the data, the researchers also point out that from 2000 to 2012, the average number of years of education rose from 12 to 13 years. It is therefore possible that education can to a certain degree protect people against dementia.

"This study suggests that steps throughout your life can have an impact on your chances of developing dementia. Things you in your childhood, like going to school, can delay or stop development of dementia in later life. We need to celebrate good news, and this is one. Dementia is not an inevitable part of ageing and that some things that people can do throughout their lives and what policies governments take can have an impact. But there are still challenges ahead", Pickett said.

With the number of elderly people set to increase in the coming decades, investigating in greater depth the role that education plays will be crucial. This includes investigating the 'cognitive reserve theory' – the idea is that there is no apparent direct link between the degree of brain pathology and clinical manifestations of the damage and that some people are better equipped to tolerate plaques and tangle build-up in their brains, potentially due to differences in education.

Because education is quite a complex notion and cannot simply be summed up just by looking at the numbers of years in school, future studies may want to focus on other factors to unpick exactly what in 'education' protects people – factors such a IQ or bilingualism could be interesting to look at some more.

The study also highlights the need to tackle other issues such as the need for long-term care at home and in institutions. Even if the incidence of dementia continues to decrease, smaller families with fewer members to act as caregivers, could mean challenges to care for patients remain important.