Joan Presky worries about dementia. Her mother lived with Alzheimer’s disease for 14 years, the last seven in a memory-care residence, and her maternal grandfather developed dementia, too.
“I’m 100 percent convinced that this is in my future,” said Ms. Presky, 70, a retired attorney in Thornton, Colo.
Last year, she spent almost a full day with a neuropsychologist, undergoing an extensive evaluation. The results indicated that her short-term memory was fine — which she found “shocking and comforting” — and that she tested average or above in every cognitive category but one.
She’s not reassured. “I saw what Alzheimer’s was like,” she said of her mother’s long decline. “The memory of what she went through is profound for me.”
The prospect of dementia, which encompasses Alzheimer’s disease and a number of other cognitive disorders, so frightens Americans that a recent study projecting steep increases in cases over the next three decades drew enormous public attention.
The researchers’ findings, published in January in Nature Medicine, even showed up as a joke on the Weekend Update segment of “Saturday Night Live.”
“Dementia is a devastating condition, and it’s very much related to the oldest ages,” said Dr. Josef Coresh, director of the Optimal Aging Institute at NYU Langone Health and the senior author of the study. “The globe is getting older.”
Now the findings are being challenged by other dementia researchers who say that while increases are coming, they will be far smaller than Dr. Coresh and his co-authors predicted.
Using data from about 15,000 Americans over age 55, collected at four research clinics around the country from 1987 through 2020, Dr. Coresh’s team projected a lifetime dementia risk much higher than previous studies had: 42 percent, though most of that risk didn’t emerge until after age 85.
The higher lifetime number probably reflected the study’s reliance on a more diverse sample than earlier researchers had used, Dr. Coresh said, and more dementia cases identified through in-depth questionnaires, regular phone calls, medical records and death certificates.
The researchers applied their risk calculations to the U.S. population and estimated that the number of people who would develop dementia each year would roughly double, to about a million by 2060, from 514,000 in 2020.
Eric Stallard, an actuary and co-director of the Biodemography of Aging Research Unit at Duke University, read the study and thought the team “seemed very competent at their analysis” of individual risk.
But when it came to the projection that cases would double, which assumed that the incidence of dementia would remain stable over the next 40 years, “I don’t believe it,” Mr. Stallard said.
“The notion that the number of people with dementia will double over the next 25, 30 or 35 years due to the aging of baby boomers is widespread, it’s pervasive — and it’s wrong,” he added.
He and two other Duke researchers recently published a commentary in JAMA pointing out that the age-specific prevalence of dementia in this country had steadily declined for 40 years.
“If your risks are lower than your parents’ risks and this trend continues, you won’t see the doubling or tripling of dementia that’s been projected,” said Dr. Murali Doraiswamy, director of the Neurocognitive Disorders Program at Duke and a co-author of the JAMA article.
To be clear, experts agree that the number of people with dementia will climb in coming decades, simply because the disorder rises so steeply with age and the number of older adults in the United States will increase.
But Mr. Stallard estimates that the increase will be more like 10 to 25 percent by 2050. “It will still be a significant challenge for the health system in the U.S.,” he said.
The Duke group relied on its own long-term study of people over age 65, with more than 21,000 respondents in 1984 and about 16,000 in 2004, plus later data from the national Health and Retirement Study and the National Health and Aging Trends Study.
Their analysis found that among 85- to 89-year-olds, for instance, the proportion with dementia was about 23 percent in the cohort born in 1905. In those born 10 years later, the figure had dropped to about 18 percent.
By the time Americans born in 1935 reached their late 80s, about 11 percent had dementia; the projection for those born from 1945 to 1949 is now about 8 percent.
To Dr. Coresh, whose primary interest was in individual risk, the assumption that past declines would continue at about the current rate “would be great, but is quite an optimistic, dramatic decrease,” he said in an email.
Yet in another longitudinal study of older adults in England and China, published in Nature Aging last year, “we also found these quite marked improvements in more recently born cohorts,” said the lead author, Dr. John Beard, a medical epidemiologist at the Mailman School of Public Health at Columbia University.
“You would expect the increase in the absolute numbers of people with dementia in the U.S. will be less than we feared,” Dr. Beard said.
What has led to the decrease in dementia, also seen in several European countries? Often cited explanations include rising education levels, reduced smoking and improved treatment for high blood pressure and high cholesterol.
The Lancet Commission on dementia, intervention and care has developed a list of 14 modifiable risk factors, including greater use of hearing aids and reduced air pollution, that could still lead to greater declines.
Yet the reverse could also happen. If earlier and more widespread testing increases the number of dementia diagnoses, or if the definition of dementia broadens, rates will increase, Dr. Doraiswamy noted. Increasing life expectancy would have the same effect.
Obesity and diabetes, more common in recent decades, could lead to more dementia, but much-touted new drugs that reduce them could blunt that trend — if people can get them.
“None of this is inevitable,” said Dr. Gill Livingston, a psychiatrist at University College London who leads the Lancet Commission. “It depends on what we do.”
Public health policy makes a major difference, she noted, and, “The U.S. is in a time where policy is changing enormously.”
Dementia rates might rise, for example, “if people have less access to health care, so they are less likely to get their blood pressure treated and their high cholesterol treated,” Dr. Livingston said.
Slashed Medicaid coverage could lead to that result. So could a rollback of environmental policies, “if air pollution increases because of fossil fuels,” she added.
Already, dementia afflicts some American populations far more than others, researchers point out. Older women and Black people face greater risk, along with those who carry the APOE4 gene associated with Alzheimer’s disease.
Health disparities could mean that “affluent people will see lower rates of dementia” because of the new diabetes and obesity drugs, Dr. Doraiswamy said. “People who can’t afford them and whose conditions are not well-managed will see rates go up.”
The debate about how many older adults will develop dementia in coming decades, and about how individuals, families, government and the health care system should respond, will likely continue.
So will Ms. Presky’s fears.
For now, she enrolls in lifelong learning classes, takes walks and yoga classes despite orthopedic problems, listens to podcasts and reads a lot of history and fiction. She and her husband take in theater in New York and Phish concerts on the West Coast and will soon be heading to London and Paris.
Still, her advance directive contains many provisions about dementia. “I remain pessimistic,” she said, noting that her mother was diagnosed at 77. “I have seven more years before I meet her fate.”
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