A new study has found that dementia rates among people 65 and older in England and Wales have plummeted by 25 percent over the past two decades, to 6.2 percent from 8.3 percent, the strongest evidence yet of a trend some experts had hoped would materialize.
Another recent study, conducted in Denmark, found that people in their 90s who were given a standard test of mental ability in 2010 scored substantially better than people who reached their 90s a decade earlier. Nearly one-quarter of those assessed in 2010 scored at the highest level, a rate twice that of those tested in 1998. The percentage severely impaired fell to 17 percent from 22 percent.
The British study, published on Tuesday in The Lancet, and the Danish one, which was released last week, also in The Lancet, confirmed something that researchers on aging have long suspected but lacked good evidence to prove: dementia rates would fall and mental acuity improve as the population grew healthier and better educated.
Epidemiologists have found associations between various health measures and risk of dementia. Incidence is lower among those who control their blood pressure and cholesterol, for example, and it is lower among those who are better educated. Since some dementia is caused by ministrokes and other vascular damage, it made sense that as populations control these risk factors better, dementia rates might drop. But few studies were rigorous enough to put the hypothesis to a test.
Dallas Anderson, an expert on the epidemiology of dementia at the National Institute on Aging, the principal funder of dementia research in the United States, said the studies were “rigorous and are strong evidence.” He added that he expected the same trends were occurring in the United States but would need to see studies done here to confirm the trend.
“It’s terrific news,” said Dr. P. Murali Doraiswamy, an Alzheimer’s researcher at Duke University, who was not involved in the new studies. It means, he said, that the common assumption that every successive generation will have the same risk for dementia does not hold true.
The new studies offer hope amid a cascade of bad news about Alzheimer’s disease and dementia. Major clinical trials of drugs to treat Alzheimer’s have failed. And a recent analysis by the RAND Corporation — based on an assumption that dementia rates would remain steady — had concluded that the number of people with dementia would double in the next 30 years as the baby boom generation aged, and so would the costs of caring for them. But its lead author, Michael D. Hurd, a principal senior researcher at RAND, said in an interview that while his estimates of current costs were correct, the future projections could be off if the falling dementia rates found in Britain held true in the United States.
“If these trends continue, it would affect our estimates,” he said.
Dr. Marcel Olde Rikkert of Radboud University Nigmegen Medical Center in the Netherlands, who wrote an editorial to accompany the Danish study, goes further. Estimates of the risk of dementia in older people, he said, “urgently need a reset.”
But Maria Carrillo, vice president of medical and scientific relations at the Alzheimer’s Association, an advocacy group, said she was not convinced that the trends were real or that they held for the United States. She said that the British paper had a methodological flaw and that the Danish work might reflect the fact that people there were generally healthier than those in the United States.
The studies assessed dementia, which includes Alzheimer’s disease but also other conditions, like ministrokes, that can make mental functioning deteriorate. Richard Suzman, the director of the division of behavioral and social research at the National Institute on Aging, said it was not possible to know from the new studies whether Alzheimer’s was becoming more or less prevalent.
“Other forms of dementia could be going down, and Alzheimer’s could be going up, for all I know,” he said.
The British researchers, led by Dr. Carol Brayne of the Cambridge Institute of Public Health, took advantage of a large study that between 1984 and 1994 tested for dementia 7,635 people, ages 65 and older, who were selected randomly in England and Wales. The subjects lived in Cambridgeshire, Newcastle and Nottingham. Then, between 2008 and 2011, the researchers assessed a similar randomly selected group of people living in the same areas. Interviewers visited the subjects at their homes and administered a standard test of mental abilities.
“We had the same population, the same geographic area, the same methods,” Dr. Brayne said. “That was one of the appeals.”
Dr. Carrillo said she questioned the data because so many subjects had declined to be assessed: the researchers assessed 80 percent of the group it approached in the first round and 56 percent of those approached in the second. That is a reasonable concern, Dr. Brayne said, but the researchers had addressed it by analyzing the data to see if the refusals might have skewed the results, and determined they had not. They had detailed information on those who refused to respond or declined an invitation to respond, which helped in the assessment.
In the Danish study, Dr. Kaare Christensen of the University of Southern Denmark in Odense and his colleagues compared the physical health and mental functioning of two groups of older Danish people. The first consisted of 2,262 people born in 1905 and assessed when they were 93 years old. The second was composed of1,584 people born in 1915 and assessed at age 95. In addition to examining the study subjects for their physical strength and robustness, the investigators gave them a standard dementia screening test, the mini-mental exam, and a series of cognitive tests.
The investigators asked how many subjects in their 90s scored high, how many had scores indicating dementia and how many were in between. The entire curve was shifted upward among the people born in 1915, they discovered.
Dr. Christensen said his study, along with the British one, “cautiously provides a basis for optimism.”
But, he said, the studies cannot tell people how to prevent dementia in old age.
“My guess is that there is no one single magic bullet, but a series of things going in the right direction,” Dr. Christensen said.