It's been widely predicted that the U.S. is headed toward an explosion in cases of Alzheimer's and other types of dementia as the population ages. Yet new research suggests some positive signs: the rate of new dementia cases is actually declining, and better heart health efforts may be making a difference.
As hopeful as the results may sound, they come with a few caveats, experts say.
The findings, from an analysis of data from the long-running Framingham Heart Study, are reported in the New England Journal of Medicine. Researchers crunched numbers on the heart and brain health of 5,200 people over age 60, who've been participating in the cardiovascular health study since 1975, to determine how many had developed dementia over four decades.
The researchers found there was a progressive decline in the incidence of dementia over time, meaning that fewer new cases developed than in the past. Dementia that stemmed from vascular disease showed the most significant drop. The authors also reported a parallel improvement in cardiovascular health. In both cases, the findings were observed only in people who had a high school education or beyond.
"These findings suggest that earlier diagnosis and more effective treatment of stroke and heart disease might have contributed to a lower incidence of dementia, particularly vascular dementia," during more recent decades, the authors wrote.
They added, "Few studies can accurately track the incidence of dementia over time, and our study provides robust evidence that indicates a declining trend."
Co-author Dr. Sudha Seshadri, professor of neurology at Boston University School of Medicine and a Framingham Heart Study senior investigator, said in a statement that while there are no treatments to prevent or cure dementia, the results suggest there may be ways to lower the risk.
"Our study offers hope that some of the dementia cases might be preventable -- or at least delayed -- through primary (keep the disease process from starting) or secondary (keep it from progressing to clinically obvious dementia) prevention," Seshadri said.
However, the worldwide burden of dementia will continue to grow rapidly as the Baby Boom generation ages and average life expectancy climbs.
In 2015, the number of people in the U.S. age 65 and older with Alzheimer's disease was estimated to be 5.1 million. By 2025, that number is estimated to hit 7.1 million -- a 40 percent increase. By 2050, Alzheimer's disease may nearly triple to a projected 13.8 million, unless medical breakthroughs to prevent or cure the disease are discovered.
The World Health Organization says dementia stats worldwide are dire, too, estimating a jump from 47.5 million people with dementia in the world today to 75.6 million by 2030, and as high as 135.5 million by 2050.
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Keith Fargo, director of Scientific Programs & Outreach at the Alzheimer's Association, said the study's findings are hopeful, but it doesn't mean dementia is finally on the downswing.
"There are a whole bunch of reasons why we're still expecting this big explosion in dementia in the coming years. One of the things is that this is a relatively small population of people we're looking at. While it's a big study sample -- thousands of people over course of many years -- there are some limitations that it's Framingham only," he said, referring to the Massachusetts town where the study is based.
For one, it's an overwhelmingly Caucasian and educated population, so you can't generalize to the U.S. population. "It's not necessarily going to translate to your average person in Georgia, rural Iowa, or New York City, for example," Fargo said.
"This is an important study, don't get me wrong. But there's a difference between what we call incidence and prevalence. Incidence is what was studied in this paper -- the number of people who develop something over a set period of time. Prevalence is how many people have it altogether. Many things influence prevalence, including incidence," he explained.
This is also not an intervention study where a lifestyle change or drug was given and a randomized control trial was performed, Fargo noted.
"But it points to, in this well-studied group of people, that we're seeing decreases in their heart health risk factors -- cholesterol, smoking, hypertension. Those things were getting better. At the same time you see this decreased incidence of dementia. You can't tell whether it's causative," he said.
But as the authors pointed out, there is good evidence to show that if heart risk factors are controlled, a person is probably less likely to develop dementia as they age.
"The first boomers turn 70 this year. Even if many of them have well-controlled cardiovascular risk factors, you're still going to see a huge wave of dementia hit," Fargo said.
In an accompanying commentary in the New England Journal of Medicine, scientists from Harvard and Johns Hopkins wrote: "With this latest contribution, optimism about dementia is more justified than ever before. Even if death and taxes remain inevitable, cancer, CAD [coronary artery disease], and dementia may not. But cautious optimism should not become complacency."